Listeria: A foodborne pathogen that knows how to survive

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International Journal of Food Microbiology 113 (2007) 1 – 15
www.elsevier.com/locate/ijfoodmicro
Review
Listeria: A foodborne pathogen that knows how to survive
Megha Gandhi, Michael L. Chikindas ⁎
Department of Food Science, Rutgers, The State University of New Jersey, 65 Dudley Road, New Brunswick, NJ 08901, USA
Received 24 January 2006; received in revised form 19 June 2006; accepted 4 July 2006
Abstract
The foodborne pathogen Listeria is the causative agent of listeriosis, a severe disease with high hospitalization and case fatality rates. Listeria
monocytogenes can survive and grow over a wide range of environmental conditions such as refrigeration temperatures, low pH and high salt
concentration. This allows the pathogen to overcome food preservation and safety barriers, and pose a potential risk to human health. This review
focuses on the key issues such as survival of the pathogen in adverse environments, and the important adaptation and survival mechanisms such as
biofilm formation, quorum sensing and antimicrobial resistance. Studies on the development of technologies to prevent and control L.
monocytogenes contamination in foods and food processing facilities are also discussed.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Listeria monocytogenes; Low temperature; Acid stress; Osmotic stress; Biofilm; Quorum sensing; Antimicrobial resistance; Food preservation
Contents
1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mechanisms of survival under adverse environmental conditions . . .
2.1. Survival at low temperatures . . . . . . . . . . . . . . . . . .
2.1.1. Changes in membrane composition . . . . . . . . . .
2.1.2. Changes in gene expression and induction of proteins .
2.1.3. Compatible solutes as cryoprotectants . . . . . . . . .
2.1.4. Role of general stress sigma factor (σB) . . . . . . . .
2.2. Survival under acid stress . . . . . . . . . . . . . . . . . . . .
2.2.1. Induction of proteins . . . . . . . . . . . . . . . . . .
2.2.2. pH homeostasis. . . . . . . . . . . . . . . . . . . . .
2.2.3. Glutamate decarboxylase system . . . . . . . . . . . .
2.2.4. Role of general stress sigma factor (σB) . . . . . . . .
2.2.5. Two-component regulatory systems . . . . . . . . . .
2.3. Survival under osmotic stress . . . . . . . . . . . . . . . . . .
2.3.1. Induction of proteins . . . . . . . . . . . . . . . . . .
2.3.2. Compatible solutes as osmoprotectants. . . . . . . . .
2.3.3. Role of general stress sigma factor (σB) . . . . . . . .
2.3.4. Two-component regulatory systems . . . . . . . . . .
Biofilms and quorum sensing. . . . . . . . . . . . . . . . . . . . . .
3.1. Formation of biofilms . . . . . . . . . . . . . . . . . . . . . .
3.2. Characteristics of biofilms . . . . . . . . . . . . . . . . . . . .
3.3. Strain-specific biofilm-forming capacity. . . . . . . . . . . . .
⁎ Corresponding author. Tel.: +1 732 932 9611x218; fax: +1 732 932 6776.
E-mail address: [email protected] (M.L. Chikindas).
0168-1605/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ijfoodmicro.2006.07.008
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M. Gandhi, M.L. Chikindas / International Journal of Food Microbiology 113 (2007) 1–15
3.4. Role of general stress sigma factor (σB) in biofilm formation . . . .
3.5. Quorum sensing . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6. Role of quorum sensing in biofilm formation . . . . . . . . . . . .
3.7. Significance of biofilms in the food industry . . . . . . . . . . . .
3.8. Control of biofilms . . . . . . . . . . . . . . . . . . . . . . . . . .
4. Resistance to antimicrobial agents. . . . . . . . . . . . . . . . . . . . . .
4.1. Resistance to antibiotics . . . . . . . . . . . . . . . . . . . . . . .
4.2. Resistance to sanitizers and disinfectants. . . . . . . . . . . . . . .
4.3. Resistance to bacteriocins . . . . . . . . . . . . . . . . . . . . . .
5. Advanced strategies for control of food safety . . . . . . . . . . . . . . .
5.1. General stress sigma factor (σB) as the target for food preservation .
5.2. Multiple hurdle technology. . . . . . . . . . . . . . . . . . . . . .
5.3. Encapsulation technology . . . . . . . . . . . . . . . . . . . . . .
5.4. Active packaging technology. . . . . . . . . . . . . . . . . . . . .
6. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Illness caused due to the consumption of contaminated foods
has a wide economic and public health impact worldwide. The
Centers for Disease Control and Prevention (CDC) estimate that
foodborne diseases are responsible for about 76 million illnesses,
which result in 325,000 hospitalizations and 5000 deaths in the
United States each year (Mead et al., 1999). The preliminary
FoodNet data for 2004 has shown an overall decline in the incidence of infections caused by foodborne pathogens such as
Campylobacter, Yersinia, Salmonella and Listeria (Anonymous,
2005). However, it is important to realize that many of the foodborne illnesses are sporadic and may not get accounted as part of
an outbreak. Further, several factors such as consumer awareness,
disease surveillance by the local and state public health departments, varying incubation periods and severity of a disease affect
the degree of reporting of foodborne illnesses and outbreaks
(Olsen et al., 2000). The Department of Health and Human
Services in the United States has launched the “Healthy People
2010” initiative, aimed at health promotion and disease prevention
with objectives for improving the health of all people in the first
ten years of the 21st century. The primary focus areas of the
initiative include improved food safety in the United States and a
reduction in the incidence of foodborne diseases caused by
Campylobacter, E. coli O157:H7, Listeria and Salmonella (www.
cdc.gov/nchs/hphome.htm#Healthy%20People%202010, 2005).
Foodborne illnesses caused by pathogens such as Campylobacter and Salmonella are most commonly reported in the United
States (Mead et al., 1999). However, pathogens such as Listeria
monocytogenes can adapt to survive and grow in a wide range of
environmental conditions and cause listeriosis, a severe disease
with high hospitalization and case fatality rates (Mead et al., 1999).
The elderly, pregnant, newborn and immunocompromised populations are more susceptible to listeriosis. The immunocompromised
category includes people with AIDS or on immunosuppressive
drugs such as corticosteroids for cancer treatment, which reduce
their T-cell mediated immunity (Rocourt and Cossart, 1997).
Listeriosis has a long incubation time, which makes it difficult to
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identify the pathogen and trace the contaminated food. Once the
pathogen gains entry into mammalian cells by phagocytosis, they
are released from the membrane-bound vacuole and begin to
multiply. The pathogen uses actin polymerization for intracellular
movement and cell-to-cell spread infecting a vast range of host
tissues, with the liver being the main site of infection (Rocourt and
Cossart, 1997). Meningitis, septicemia and other infections of the
central nervous system are commonly seen in patients with listeriosis. In pregnant women, listeriosis may lead to spontaneous
abortion, still birth or fetal death (Rocourt and Cossart, 1997).
L. monocytogenes can be found in a wide variety of raw and
processed foods. Milk and dairy products, various meats and
meat products such as beef, pork, fermented sausages, fresh
produce such as radishes, cabbage, seafood and fish products
have all been associated with Listeria contamination (Rocourt
and Cossart, 1997). Foods such as soft cheeses, hot dogs and
seafood have been implicated in several outbreaks of human
listeriosis. Listeria is a Gram-positive pathogen, with the ability
to adapt to a wide range of conditions such as refrigeration
temperatures (2–4 °C), acidic foods, high salt foods and within
the host immune system (Rocourt and Cossart, 1997).
Even though the statistics show that the incidence of listeriosis
has declined, outbreaks and contaminated product recalls continue
to occur (www.cdc.gov/ncidod/dbmd/diseaseinfo/listeriosis_t.
htm, 2005). A multi-state outbreak of L. monocytogenes occurred
in the United States in 2002. Consumption of contaminated turkey
deli meat resulted in 46 culture-confirmed cases, seven deaths and
three fetal deaths in eight states (Anonymous, 2002). There are
several factors that have influenced the contamination of foods by
Listeria and the incidence of listeriosis. Advances in the field of
medicine continue to affect the dynamic human population, increasing the average lifespan of people and survival of immunocompromised and elderly individuals. Recent modifications in
food production and processing practices, globalization of the food
industry and the growing demand for imported and ethnic foods
are also important factors. Further, ever-changing food habits of
the consumer, with a trend towards consumption of minimally
processed, ready-to-eat convenience foods and refrigerated or
M. Gandhi, M.L. Chikindas / International Journal of Food Microbiology 113 (2007) 1–15
frozen food products have affected the incidence of listeriosis over
the past years (Rocourt and Bille, 1997).
Collaborative efforts by the government, public health agencies, food industry and consumers are necessary to determine the
risks associated with consumption of various foods, virulence
mechanisms of L. monocytogenes, incidence and epidemiology
of the disease. The United States Food and Drug Administration's
(FDA) Center for Food Safety and Applied Nutrition (CFSAN)
along with the Food Safety and Inspection Service (FSIS) and
Centers for Disease Control and Prevention (CDC) published a
quantitative assessment of relative risk to public health from the
consumption of selected categories of ready-to-eat foods that may
be contaminated with the foodborne pathogen L. monocytogenes.
The assessment provides valuable information on the predicted
risk of serious illness and death associated with consumption of
ready-to-eat foods contaminated with L. monocytogenes (www.
cfsan.fda.gov/~dms/lmr2-toc.html, 2003).
Currently, FDA has a zero-tolerance policy in place for L.
monocytogenes in ready-to-eat foods. Based on this policy, the
detection of any L. monocytogenes in the food makes the product
adulterated. The FDA is reviewing a petition made by trade
associations to change the zero-tolerance policy for L. monocytogenes in foods that do not support the growth of the organism.
The petition has requested the establishment of a regulatory limit
for L. monocytogenes of 100 CFU/g in foods that do not support
the growth of the microorganism. Based on the risk assessment
published by FDA and FSIS (www.cfsan.fda.gov/~dms/lmr2-toc.
html, 2003), and research articles (Chen et al., 2003), the petition
proposes that concentrating on the number of L. monocytogenes
present in a food rather than just its presence alone may be more
effective in improving food safety and promoting public health.
2. Mechanisms of survival under adverse environmental
conditions
2.1. Survival at low temperatures
L. monocytogenes has the ability to grow over a wide range
of temperatures (2–45 °C). Its survival and growth at refrigeration temperatures (2–4 °C) are two of the many factors that
make the control of this foodborne pathogen difficult (Rocourt
and Cossart, 1997). Since refrigeration is one of the most
common ways to increase the shelf life of foods, understanding
the mechanisms behind its survival and growth at low temperature could provide information to help develop more effective control methods for the pathogen.
2.1.1. Changes in membrane composition
The lipids in the membranes of bacterial cells are in a fluid,
crystalline state and this physical state is important to maintain
membrane fluidity and function. Changes in temperature lead to an
alteration in the membrane lipid composition to maintain the ideal
membrane fluidity required for proper enzyme activity and transport of solutes across the membrane. A high proportion of iso and
anteiso, odd-numbered, branched-chain fatty acids characterize the
cell membrane of Listeria (Annous et al., 1997). The changes that
occur in the membrane fatty acid composition of L. monocytogenes
3
in response to low temperature have been extensively studied. One
of the main changes is an increase in the proportion of C15:0 at the
expense of C17:0, when the temperature is reduced below optimum
(7 °C). Growth at low temperatures also results in an increase in the
degree of unsaturated fatty acids, which helps enhance the fluidity
of the membrane (Beales, 2004). Annous et al. (1997) also showed
that changing the growth temperature from 20 °C to 5 °C led to
fatty acid shortening (a decrease in C17:0) and a switch from iso to
anteiso branching (i-C15:0 to a-C15:0). The shortening of fatty acid
chain length decreases the carbon–carbon interaction between
neighboring chains in the cell membrane and this helps maintain
the optimum degree of membrane fluidity for growth at low
temperatures (Beales, 2004).
2.1.2. Changes in gene expression and induction of proteins
L. monocytogenes produces cold shock proteins (Csps) in
response to a temperature downshock and cold acclimation proteins (Caps) that are synthesized during balanced growth at low
temperatures (Bayles et al., 1996). The authors subjected L.
monocytogenes to a cold shock from 37 °C to 5 °C and used twodimensional gel electrophoresis to identify the proteins induced as
a result of the cold shock. About 12 proteins (Csps) were induced
in cold-shocked cultures and about four proteins (Caps that were
also identified as Csps) were produced during balanced growth at
5 °C compared to 37 °C (Bayles et al., 1996). Cold acclimation of
a pathogen is accompanied by changes in microbial gene expression. Liu et al. (2002) identified RNAs that are synthesized at
higher levels when L. monocytogenes is grown at 10 °C in
comparison to 37 °C. Increased expression of mRNA for chaperone proteases such as GroEL, ClpP and ClpB indicates that these
enzymes may be involved in the degradation of abnormal or
damaged polypeptides that arise due to growth at low temperatures (Liu et al., 2002).
2.1.3. Compatible solutes as cryoprotectants
The ability of L. monocytogenes to accumulate compatible
solutes such as glycine betaine and carnitine and the role of these
compounds as cryoprotectants have been widely studied.
Angelidis and Smith (2003) investigated the uptake and accumulation of compatible solutes in cells during growth at refrigeration temperatures. Mutants of L. monocytogenes were made to
study the role of three compatible solute systems — glycine
betaine porter I (BetL), glycine betaine porter II (Gbu) and the
carnitine transporter (OpuC). The transport of betaine into the cell
is mediated mainly through Gbu. The solute-mediated cryoprotection stimulates growth in cells subjected to cold stress. Low
level of betaine was also transported via BetL and OpuC. Major
uptake of carnitine is mediated through OpuC, with Gbu and BetL
transporting low levels and thus providing only weak cryoprotection (Angelidis and Smith, 2003). Wemekamp-Kamphuis et al.
(2004) also showed that deletions of these osmolyte transporters
reduced the growth of Listeria at low temperatures (WemekampKamphuis et al., 2004).
2.1.4. Role of general stress sigma factor (σB)
The survival of bacteria under adverse environmental conditions involves changes in the transcription of genes that is made
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possible by the association of alternative sigma factors with the
core RNA polymerase. The alternative stress sigma factor, sigmaB (σB) has been identified in Gram-positive bacteria such as
Bacillus subtilis and L. monocytogenes. The general stress sigma
factor σB is stimulated in response to temperature downshift and
the sigB mutant fails to accumulate solutes such as betaine and
carnitine in L. monocytogenes (Becker et al., 2000). This suggests
that accumulation of cryoprotectants is one of the functions of σB
during growth at low temperature.
The varied responses of L. monocytogenes for survival and
growth at low temperatures demonstrate the versatility of this
emerging pathogen to adapt to a wide range of environmental
conditions. Its ability to grow at refrigeration temperatures is of
particular concern in refrigerated foods, which are consumed
without any further processing such as soft cheeses, refrigerated
smoked seafood and refrigerated meat spreads. Foods such as
hot dogs and deli meats are also a concern, unless they are
reheated thoroughly.
2.2. Survival under acid stress
L. monocytogenes encounters a low-pH environment in acidic
foods, during gastric passage and in the phagosome of the
macrophage (Cotter and Hill, 2003). The pathogen responds to
and survives in these low-pH environments by utilizing a number
of stress adaptation mechanisms. Exposure of L. monocytogenes
to mild acidic pH of 5.5 (1 M lactic acid) induces the acid
tolerance response (ATR), wherein the cells are resistant to severe
acidic conditions (O'Driscoll et al., 1996).
2.2.1. Induction of proteins
The acid response on exposure of cells to an acidic pH involves
several changes in the cell. Phan-Thanh and Mahouin (1999)
studied the expression of proteins by exposing cells to a lethal
acidic pH (acid stress) and a non-lethal acidic pH (acid adaptation). Even though more proteins were induced in cells exposed
to the lethal pH, a majority of the proteins induced were common
to both pH conditions used in the study. The protein GroEL that
showed increased synthesis during the growth of Listeria at low
temperature was also induced under acid stress. Other proteins
induced were ATP synthase and various transcriptional regulators
(Phan-Thanh and Mahouin, 1999). Acid-adapted L. monocytogenes (pH 5.2, 2 h) had increased resistance to heat shock (52 °C),
osmotic shock (25–30% NaCl) and alcohol stress, suggesting that
acid adaptation also provides cross-protection against other stress
factors (Phan-Thanh et al., 2000). The cross-resistance of acidadapted cells to other stresses has important implications for the
food industry, particularly since foods commonly encounter sublethal acidic treatments during processing (van Schaik et al.,
1999).
2.2.2. pH homeostasis
Microorganisms maintain their intracytoplasmic pH via the
mechanism of pH homeostasis achieved by proton transport across the cell membrane. In aerobic organisms, the active transport
of H+ is coupled with electron transport in respiratory chains. On
the other hand, anaerobic bacteria carry out H+ transport via H+-
ATPase molecules using energy from ATP hydrolysis. L.
monocytogenes is a facultative anaerobic bacterium that may
use both processes for pH homeostasis (Shabala et al., 2002). The
F0F1-ATPase is a multisubunit enzyme that serves as a channel for
proton translocation across the cell membrane by utilizing ATP.
The enzyme is highly conserved and has been extensively studied
in E. coli. The F1 portion of the enzyme consists of five subunits —
α3, β3, γ, δ, ε, whereas the F0 part is made up of a, b2, c10. The
flow of protons (proton gradient) causes the rotary motion of F0
and then F1 subunit leading to the synthesis of ATP. In the reverse
reaction, ATP hydrolysis causes the F0 subunit to rotate in the
opposite direction (Yoshida et al., 2001). Cotter et al. (2000)
performed experiments using N,N′-dicyclohexylcarbodiimide
(DCCD) that inhibits proton extrusion by F0F1-ATPase to determine the role of the enzyme in ATR of Listeria. The treatment of
L. monocytogenes cells with the DCCD inhibitor before and
during acid challenge resulted in enhanced acid sensitivity of
acid-adapted cells. There was a three-log reduction in survival of
the pathogen indicating the involvement of the F0F1-ATPase in
acid adaptation of Listeria (Cotter et al., 2000).
2.2.3. Glutamate decarboxylase system
L. monocytogenes utilizes the glutamate decarboxylase (GAD)
system to survive acid stress. The GAD system is composed of
three genes gadA, gadB and gadC genes. The gadA and gadB
genes encode two glutamate decarboxylases and the gadC gene
codes for a glutamate/ϒ-aminobutyrate antiporter (Cotter et al.,
2001). It has been proposed that glutamate is taken up by the cell
via a specific transporter followed by its decarboxylation in the
cytoplasm, producing ϒ-aminobutyrate and resulting in the utilization of an intracellular proton. The ϒ-aminobutyrate is then
exported from the cell via an antiporter located in the cell membrane. The proton loss from the cell results in an increase in the pH
of the cytoplasm and the release of alkaline ϒ-aminobutyrate into
the environment raises the external pH slightly (Small and
Waterman, 1998). Cotter et al. (2001) studied the role of the GAD
system in the acid resistance of L. monocytogenes during gastric
transit by using synthetic human and porcine gastric fluid. They
found that addition of glutamate increased the survival of the wild
type strain in gastric fluid and this is of concern in foods containing
glutamate. Further, deletions of gadA, gadB and gadC genes
resulted in enhanced sensitivity of the strain to low pH. The gadAB
mutant also showed reduced survival rate in gastric fluid. This
shows that a functional GAD system is vital for the acid resistance
of L. monocytogenes and to successfully pass through the gastric
environment and infect the small intestine (Cotter et al., 2001).
2.2.4. Role of general stress sigma factor (σB)
Wiedmann et al. (1998) conducted studies to determine the
role of general stress transcription factor σB on the acid resistance
of L. monocytogenes. The sigB mutant showed a lower level of
acid resistance in stationary phase compared to the wild type.
Their findings suggested that the survival of L. monocytogenes
upon exposure to an acidic environment is dependent on the
expression of σB-dependent proteins (Wiedmann et al., 1998).
Investigating the role of σB in growth phase-dependent acid
resistance and adaptive ATR in the wild type strain and sigB
M. Gandhi, M.L. Chikindas / International Journal of Food Microbiology 113 (2007) 1–15
mutant, Ferreira et al. (2003) showed the presence of σB-dependent and σB-independent mechanisms of acid resistance through
various phases of growth. The survival and increased resistance of
log-phase L. monocytogenes cells to gastric fluid following
exposure to mild acidic conditions were partially dependent on σB
(Ferreira et al., 2003). Further evidence on the regulation of
expression of L. monocytogenes genes transcribed from σBdependent promoters was provided by Kazmierczak et al. (2003).
The alternative stress sigma factor regulates the expression of the
gadB gene involved in acid stress survival and OpuC, which is a
chill-activated transporter for carnitine. These studies shed light
on the diverse role of σB in the survival of L. monocytogenes
under acid stress conditions. In addition to the regulation of genes
for survival under acid stress conditions, the stress-responsive
factor σB also regulates virulence gene expression in this foodborne pathogen (Kazmierczak et al., 2003).
2.2.5. Two-component regulatory systems
A two-component regulatory system, consisting of lisR and
lisK, which encode the response regulator and histidine kinase
respectively has been identified in L. monocytogenes. These twocomponent signal transductions systems can sense changes in the
environment such as low pH, oxidative and ethanol stresses via a
membrane-associated histidine kinase, and the response regulator
enables the cell to respond by altering gene expression (Cotter et al.,
1999). The study by Cotter et al. (1999) has shown that LisRK
signal transduction system is involved in response to stresses such
as low pH and regulation of virulence gene expression in Listeria.
The acidic pH of many foods is one of the many factors that
help to prevent the growth of foodborne pathogens. Therefore, the
acid tolerance response observed in L. monocytogenes is of particular concern during food processing, because an exposure of the
pathogen to mild acidic conditions could confer resistance to more
severe acidic conditions. Gahan et al. (1996) used acid-adapted
and non-adapted L. monocytogenes to compare their survival in a
variety of acidic food products. A constitutive acid tolerant mutant
isolated by prolonged exposure to pH 3.5 (3 M lactic acid) was
used to investigate survival of the pathogen in acidic foods
(O'Driscoll et al., 1996). The acid-adapted L. monocytogenes and
the acid tolerant mutant showed better survival in commercial
natural yogurt and cottage cheese made in the laboratory. The acid
tolerant mutant showed enhanced resistance during the ripening of
hard cheeses such as cheddar cheese and the authors recovered a
significant number of cells after the 70-day ripening period.
Cheese manufacturers should take the survival of acid tolerant
strains of L. monocytogenes after the ripening period into consideration. The authors investigated the survival of these L.
monocytogenes strains during milk fermentation by Streptococcus thermophilus. The acid-adapted strain demonstrated enhanced
survival compared to the non-adapted culture. The acid tolerant
mutant also survived similar to the acid-adapted cells during the
first 4 h of fermentation (Gahan et al., 1996).
2.3. Survival under osmotic stress
Microorganisms can ‘sense’ and adapt to constantly changing
environments, and this is an essential part of their growth and
5
survival. The response of microorganisms to osmotic stress involves both physiological changes and variations of gene expression patterns and is called osmoadaptation (Hill et al., 2002).
The use of salt to lower the water activity is one of the methods of
food preservation used by the food industry; however, the ability
of Listeria to adapt and survive in high concentrations of salt
makes it difficult to control the pathogen in foods.
2.3.1. Induction of proteins
One of the mechanisms used by Listeria to tolerate salt stress is
a change in its gene expression leading to an increased or decreased synthesis of various proteins. Duche et al. (2002a) studied
the expression pattern of proteins using 2-D gel electrophoresis
after inducing salt stress in L. monocytogenes. About twelve
proteins induced by salt stress were identified by microsequencing and mass spectrometry. Similar to the two groups of proteins
induced in response to cold shock mentioned earlier (Csp and
Cap) (Bayles et al., 1996), Duche et al. (2002a) identified salt
shock proteins (Ssp) induced rapidly but overexpressed for a short
period and the stress acclimation proteins (Sap), that are rapidly
induced and continue to be overexpressed several hours after
conditions return to normal. Two general stress proteins (DnaK
and Ctc) were identified among the Ssps induced in L. monocytogenes. DnaK functions as a heat shock protein, stabilizing
cellular proteins. Among the eleven Saps identified, GbuA, which
functions as an osmoprotectant transporter for glycine betaine was
induced in response to salt stress. Since the Ctc protein was
induced in response to salt stress (Duche et al., 2002a,b), Gardan
et al. (2003) investigated the role of this protein in osmotic stress.
The ctc gene is involved in the resistance of L. monocytogenes to
high osmolarity in the absence of osmoprotectants such as glycine
betaine and carnitine in the medium (Gardan et al., 2003).
2.3.2. Compatible solutes as osmoprotectants
Several studies have been conducted on the role of compatible
solutes in osmoadaptation. These are highly soluble compounds
that have no net charge at physiological pH and can be accumulated at high concentrations within a cell without affecting
cellular functions. Bayles and Wilkinson (2000) showed that
glycine betaine, proline betaine, acetyl carnitine, carnitine, ϒbutyrobetaine and 3-dimethylsulphoniopropionate function as
osmoprotectants in L. monocytogenes. The presence of these
compounds resulted in an up to 2.6-fold increase in growth rate of
salt-stressed cells compared to stressed cells without any osmoprotectants. The cells take up osmolytes from the external environment as a response to osmotic stress, which helps to regain the
osmotic balance within cells (Bayles and Wilkinson, 2000).
2.3.3. Role of general stress sigma factor (σB)
The general stress sigma factor σB in L. monocytogenes is
important for the utilization of betaine and carnitine as osmoprotectants (Becker et al., 1998). Later studies by Kazmierczak
et al. (2003) identified the genes regulated by σB. The expression
of the ctc gene that was shown to contribute to the osmotic stress
response by Gardan et al. (2003) is dependent on σB in L.
monocytogenes. While the sigma factor is an important part of the
general stress response of L. monocytogenes to adverse
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environmental conditions, it should be noted that the extent to
which an organism depends on σB for its stress response varies
between serotypes (Moorhead and Dykes, 2003).
2.3.4. Two-component regulatory systems
Kallipolitis and Ingmer (2001) identified response regulators
that are a part of the two-component signal transduction system
and involved in the osmotic stress response. One of the proteins
identified was homologous to KdpE proteins that are a part of the
Kdp two-component system. The Kdp uptake system is involved
in the transport of potassium (K+ ) into L. monocytogenes cells
(Kallipolitis and Ingmer, 2001). Further studies by Brøndsted
et al. (2003) investigated the role of kdpE, which encodes the
response regulator and the downstream gene (orfX ) in adaptation
to salt stress. Their results indicate that adaptation to high osmotic
stress requires expression of both kdpE and orf X genes and this
effect depends on the potassium level in the medium. Thus, the
uptake of potassium from the environment via the Kdp system has
a protective effect on L. monocytogenes against salt stress. The
orf X gene is responsible for triggering the activation of σB
(Brøndsted et al., 2003).
In addition to individual stress factors, it is important to keep in
mind that cross-protection to environmental stresses is commonly
seen as part of the stress response of Listeria in foods. This is
crucial when deciding on food processing and preservation parameters, since exposure of the pathogen to one kind of sub-lethal
stress can confer cross-protection to other lethal stresses. For
example, making of cheese involves exposure of the product
initially to an acidic environment followed by salting, where the
product is exposed to sodium chloride (Faleiro et al., 2003).
Faleiro et al. (2003) showed that acid-adapted strains of L.
monocytogenes isolated from cheese showed enhanced survival
in salt stress (20% NaCl) compared to the non-adapted strains.
The reverse, where induction of an acid tolerance response occurs
following osmoadaptation was also investigated. The results
showed that osmoadaptation resulted in the induction of ATR and
the cells were able to survive lethal acidic conditions, but there
were strain differences in the acid response (Faleiro et al., 2003).
3. Biofilms and quorum sensing
3.1. Formation of biofilms
Microorganisms can exist in the environment either as planktonic cells or as communities in biofilms, where they are attached
to a surface and enclosed in a matrix predominantly made up of
polysaccharide material. Microbial biofilms demonstrate a decreased growth rate, variation in the genes transcribed (Donlan,
2002), higher rate of gene transfer by conjugation (Hausner and
Wuertz, 1999), increased production of exopolysaccharide
(Sutherland, 2001) and more importantly an enhanced resistance
to sanitizers, disinfectants and antimicrobial agents (Robbins
et al., 2005). Biofilms can form on a wide range of surfaces such
as medical devices, water system piping, industrial equipment, as
well as in food processing facilities (Donlan, 2002). Biofilms in
food processing environments occur on food handling surfaces or
areas where food is stored or on food processing surfaces such as
conveyer belts and stainless steel equipment (Wong, 1998; Kumar
and Anand, 1998). The formation of biofilms in food manufacturing and processing facilities is of concern because bacteria
from biofilms can be transferred to food products. L. monocytogenes is commonly isolated from food processing environments, especially in the meat (Midelet and Carpentier, 2002) and
dairy (Pritchard et al., 1995) industries. Biofilms of Listeria are of
particular concern, since they are more resistant to disinfectants
and sanitizing agents compared to planktonic cells and this makes
their elimination from food processing facilities a big challenge
(Mah and O'Toole, 2001; Lewis, 2001).
3.2. Characteristics of biofilms
Cells attached to surfaces in a biofilm differ in many aspects
from freely suspended planktonic cells. Hefford et al. (2005)
studied the differences in the physiology of cells in biofilms and
planktonic cells from the same culture by comparing their protein
expression. Out of the nineteen proteins that showed higher
expression in biofilm-grown cells, many of them were involved in
stress response, protein synthesis and several regulatory functions
of the cell (Hefford et al., 2005). Listeria can either exist in
monoculture biofilms or be a part of mixed culture biofilms with
bacteria such as Flavobacterium (Bremer et al., 2001). Bremer
et al. (2001) conducted a study to determine the level of attachment of L. monocytogenes to stainless steel surfaces in a pure
culture biofilm and a mixed culture biofilm with Flavobacterium.
They found that a significantly higher number of L. monocytogenes cells attached to stainless steel surfaces in a mixed
culture biofilm compared to the single culture biofilm. Further,
cells of L. monocytogenes from the mixed culture biofilm could
survive for a longer period of time (Bremer et al., 2001). The
results of this study are important because pathogens such as
Listeria are found with different microorganisms in food processing facilities and this increases the possibility for mixed
culture biofilms to develop. Carpentier and Chassaing (2004)
isolated twenty-nine bacterial strains from food processing environments after cleaning and disinfection, and grew them in binary
culture biofilms along with L. monocytogenes. The aim of the
study was to investigate the effect of these isolates on the biofilmforming capacity of L. monocytogenes on stainless steel coupons.
Their data showed that sixteen of the strains led to a decrease in
the biofilm colony forming units counts, whereas four of the
strains had a positive effect on the settlement of L. monocytogenes
on stainless steel surfaces (Carpentier and Chassaing, 2004).
Thus, it is important to keep in mind the impact of resident
microorganisms on the biofilm-forming capacity of L. monocytogenes in food processing facilities.
3.3. Strain-specific biofilm-forming capacity
Several studies have shown that L. monocytogenes strains vary
in their ability to adhere to surfaces and form biofilms (Chae and
Schraft, 2000; Kalmokoff et al., 2001; Borucki et al., 2003). Chae
and Schraft (2000) grew thirteen L. monocytogenes strains on
glass surfaces to study the differences in adherence of cells and
biofilm growth among various strains. There was a significant
M. Gandhi, M.L. Chikindas / International Journal of Food Microbiology 113 (2007) 1–15
difference in the adherence of various strains to the glass surface.
Biofilm growth for 24 h also showed significant differences in cell
numbers among L. monocytogenes strains (Chae and Schraft,
2000). Kalmokoff et al. (2001) investigated the differences in
adsorption, attachment and biofilm formation among L. monocytogenes isolates on stainless steel surfaces. Their results showed
no relation between the level of adsorption and serotype of the
strain. Among the isolates studied, only one strain formed a
biofilm, but there were significant differences in the adherence of
cells from various strains (Kalmokoff et al., 2001). A recent study
examined the relationship between L. monocytogenes biofilm
formation, phylogeny and persistence in the environment
(Borucki et al., 2003). They observed increased biofilm formation
in serotypes 1/2a and 1/2c and in persistent strains isolated from
milk samples compared to non-persistent strains. However, the
serotypes mentioned above with increased biofilm ability are not
commonly involved in foodborne disease outbreaks (Borucki
et al., 2003).
3.4. Role of general stress sigma factor (σB) in biofilm
formation
To investigate whether the alternative sigma factor σB that
plays an important role in the various stress responses of L.
monocytogenes affects the surface attachment of this foodborne
pathogen, Schwab et al. (2005) conducted studies that looked at
the attachment of wild type L. monocytogenes and a sigB
mutant to stainless steel. The data suggested that initial attachment of both wild type and mutant to the surface was the same;
however, the number of cells of sigB mutant attached was
significantly lower than the wild type after 48 or 72 h of
incubation (Schwab et al., 2005).
3.5. Quorum sensing
The formation of biofilms as discussed above is a complex,
step-wise process involving adsorption, attachment of cells to a
surface and differentiation. Each of these stages is controlled by a
wide range of factors such as the properties of the surface, characteristics of the organism such as the genetics and cell surface
properties and environmental parameters like temperature and pH
(Donlan, 2002). Among the many factors, cell-to-cell signaling
commonly known as quorum sensing in bacteria has been an area
of science that is extensively studied and its role in biofilm
formation has been a subject of much debate in recent years
(Kjelleberg and Molin, 2002). A review on biofilm formation and
quorum sensing discusses the two social behaviors of microorganisms and sheds some light on the relationship between the
two phenomena (Parsek and Greenberg, 2005).(Xavier and
Bassler 2003). defined quorum sensing as “a process of bacterial
cell-to-cell communication involving the production and detection of extracellular signaling molecules called autoinducers.” An
important criterion for this phenomenon to occur is the presence
of a threshold cell density, such that a sufficient quantity of the
signaling molecule can be produced (Bassler, 2002). The Grampositive bacteria typically produce oligopeptides as autoinducers,
and signaling between cells is via a two-component phosphorelay
7
system. On the other hand, acylated homoserine lactones (HSLs)
function as autoinducers in Gram-negative organisms (Winans
and Bassler, 2002). Studies have shown that this type of cell-tocell signaling can regulate virulence, bioluminescence, sporulation and biofilm formation in bacteria (Bassler, 2002).
3.6. Role of quorum sensing in biofilm formation
The discussion on quorum sensing in this paper is limited to its
role in biofilm formation and growth of pathogens in foods.
Studies on the role of quorum sensing in a biofilm environment
and its effect on the pathogenicity or antimicrobial resistance of a
biofilm have been published. Davies et al. (1998) showed that
cell-to-cell communication in bacteria via a synthetic signaling
molecule was involved in the development of biofilms of Pseudomonas aeruginosa. This organism has two cell-to-cell signaling
systems — the lasR–lasI and rhlR–rhlI systems. The authors
used the wild type strain and a double mutant that lacked both
quorum sensing systems to study the role of cell-to-cell signals in
biofilm differentiation. Both strains were similar with respect to
initial attachment and growth on the glass surface; however, the
biofilm produced by the mutant was much thinner and the cells
were more densely packed in comparison to the biofilm made by
the wild type strain. Further, treatment with the detergent sodium
dodecyl sulfate (SDS) removed the mutant biofilm much more
easily from the surface compared to the wild type biofilm (Davies
et al., 1998). The enhanced susceptibility of the mutant biofilm to
the detergent is encouraging because disabling the quorum
sensing system in a pathogen could serve as a way of controlling
biofilms in food processing environments. On one hand, several
papers have shown that signaling molecules are involved in cellto-cell communication in biofilm development, while on the
other, papers such as the one by Van Houdt et al. (2004) suggest
that N-acyl-homoserine lactone-based quorum sensing does not
play a role in biofilm formation by Gram-negative bacteria isolated from a raw vegetable processing line. However, the authors
mention that this does not rule out the possibility that quorum
sensing systems could be involved in other aspects such as biofilm
resistance (Van Houdt et al., 2004). There has been a plethora of
studies on the formation of L. monocytogenes biofilms in food
processing environments, but there have been no significant
studies investigating the presence of a quorum sensing system in
this organism. Recently, Ermolaeva et al. (2004) discussed the
presence of a diffusible, low molecular weight autorepressor that
restricts the expression of the PrfA virulence regulon in L.
monocytogenes via a quorum sensing mechanism (Ermolaeva
et al., 2004). This finding of an autorepressor that plays a role in
the virulence and pathogenicity may serve as a lead for further
studies to uncover any other quorum sensing systems that might
be present in L. monocytogenes. A good understanding of the
cell-to-cell signaling phenomenon of microorganisms such as L.
monocytogenes can be used to control the growth or virulence of
foodborne pathogens by identification of compounds that can
function as quorum sensing antagonists (Smith et al., 2004). Lu
et al. (2004) investigated if fresh produce and processed foods can
have autoinducer-2-like activity and whether food additives could
behave as autoinducers and show similar activity. The data
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M. Gandhi, M.L. Chikindas / International Journal of Food Microbiology 113 (2007) 1–15
showed maximum autoinducer-2-like activity in the frozen fish
sample, whereas some samples like turkey patties showed highest
inhibition of autoinducer-2-like activity. Studies of this kind can
help in determining factors useful for controlling the growth of
microorganisms in foods (Lu et al., 2004).
3.7. Significance of biofilms in the food industry
Biofilms are a major concern in the meat industry, where the
surviving microflora from carcasses can contaminate surfaces of
equipment and this in turn can lead to contamination of products
placed on these surfaces. Conveyer belts and stainless steel surfaces of equipment are commonly found to be contaminated even
after sanitizing treatments (Midelet and Carpentier, 2002). A
study by Midelet and Carpentier (2002) investigated the transfer
of microorganisms to beef from conveyer belt and stainless steel
surfaces, which were conditioned with meat exudates and then
contaminated with different microorganisms including L. monocytogenes to simulate a meat-processing environment. The conveyer belt surfaces were made of either polyvinylchloride (PVC)
or polyurethane, which is the material frequently used by the food
industry. L. monocytogenes attached more strongly to the polymers in comparison to other microorganisms on the surface. In
general, attachment strengths of all strains were higher for polymer surfaces than for stainless steel. The data revealed that the
number of microorganisms transferred is dependent on the density of the microflora on the food contact surfaces and their
respective attachment strengths. Under the conditions used in the
study, L. monocytogenes seemed to be the most difficult to remove by sanitizing treatments from polymer surfaces compared
to other strains used (Midelet and Carpentier, 2002).
3.8. Control of biofilms
A number of studies aimed at finding effective strategies to
eliminate biofilms from food processing environments have been
published. A study by Norwood and Gilmour (2000) tested the
efficacy of sodium hypochlorite in eliminating a steady-state
mixed culture biofilm of L. monocytogenes, Pseudomonas fragi
and Staphylococcus xylosus. Exposure to 1000 ppm free chlorine
for 20 min was required to reduce L. monocytogenes biofilm by a
two-log cycle; however, planktonic cells of all three organisms
were eliminated by an exposure to 10 ppm free chlorine for 30 s.
The paper also discusses the protective effects of microorganisms
by enhanced production of extracellular polysaccharide in multispecies biofilms, which could lead to a greater resistance to
antimicrobial agents in comparison to a monoculture biofilm
(Norwood and Gilmour, 2000). Somers and Wong (2004)
conducted studies to determine the inactivation of L. monocytogenes by two detergent and sanitizer combinations. The first
combination used a chlorinated-alkaline, low-phosphate detergent and dual peracid sanitizer and the second combination contained a solvated-alkaline environmental sanitation product and
hypochlorite sanitizer. For the study, biofilms were developed in
the presence or absence of meat residue on various materials
commonly used in the food industry such as conveyer belts,
rubber and stainless steel surfaces. Over time, the presence of
meat residue increased the biofilm-containing cell numbers. Both
the detergents brought about a significant reduction in cell
numbers. Among the sanitizers, dual peracid was not effective in
eliminating the organism significantly. The first combination was
effective only on 50% of the samples, whereas the second one was
effective on 86.1% of the samples tested (Somers and Wong,
2004). While a large proportion of research is focused on testing
various antimicrobial agents to eliminate biofilms, Chmielewski
and Frank (2004) used predictive modeling to determine the effect
of heat inactivation of L. monocytogenes in monoculture and in
mixed biofilms with Pseudomonas species and Pantoea agglomerans. Their data suggested that with proper control of time and
temperature, hot water sanitation of stainless steel surfaces could
serve as an efficient method for elimination of L. monocytogenes
biofilms (Chmielewski and Frank, 2004). Zhao et al. (2004)
studied the competitive-exclusion of L. monocytogenes by microorganisms isolated from biofilms in drains of food processing
facilities. The organisms with anti-listerial activity isolated were
tested further for their effectiveness to eliminate L. monocytogenes biofilms on stainless steel coupons. Enterococcus durans
and Lactococcus lactis were the two isolates that brought about a
reduction of more than 5 log10 CFU of L. monocytogenes/cm2
(Zhao et al., 2004). Similar studies could help develop the
competitive-exclusion strategy to effectively control L. monocytogenes biofilms.
4. Resistance to antimicrobial agents
Many of the foodborne diseases that result in diarrhea are
usually not severe and the patient recovers over a short period of
time. However, more severe and prolonged illness can occur due to
pathogens such as L. monocytogenes and usually requires antibiotic
treatment. The prevalence of antimicrobial resistance among
foodborne pathogens can be a potential problem in the treatment
of humans with antibiotics. The spread and mechanisms of
antimicrobial resistance among food-related bacteria have been
widely studied. An area of much debate is the relationship between
the use of antibiotics in animal husbandry and the development of
antibiotic resistance in human pathogenic bacteria. Antibiotics have
been used in animal husbandry to control bacterial diseases and for
growth promotion (White et al., 2002). This usage can lead to the
development of resistance to one or even multiple antibiotics in
foodborne pathogens, which could ultimately be transmitted to
human beings via the food chain. The widespread use of
antimicrobial agents such as antibiotics, sanitizers or disinfectants
in food processing or equipment cleaning and their effect on
antimicrobial resistance is being investigated. Genetic factors such
as the mobility of antibiotic resistance genes found on plasmids and
transposons can increase the transfer of antibiotic resistance
between bacteria. For example, resistance could be transferred
from environmental bacteria to human foodborne pathogens
(Sorum and L'Abee-Lund, 2002). The ability of bacteria to adapt
to adverse environmental conditions is an important factor in the
development of resistance. This is because an exposure of the
organism to a sub-lethal level of an antimicrobial agent can lead to
adaptation and development of resistance to higher levels of the
antimicrobial or even cross-resistance to other agents.
M. Gandhi, M.L. Chikindas / International Journal of Food Microbiology 113 (2007) 1–15
4.1. Resistance to antibiotics
Antimicrobial resistance in the foodborne pathogen Listeria is
emerging in recent years. Studies have shown that several species
of Listeria isolated from humans or from food production or
processing facilities are resistant to one or more antibiotics. Walsh
et al. (2001) looked at 1001 isolates of Listeria from retail foods to
determine their levels of resistance to eight antibiotics. About
10.9% of the isolates was resistant to one or more antibiotics.
Resistance to penicillin or tetracycline was the most common and
there was no resistance to the antibiotics commonly used for
treatment of listeriosis. However, this does not eliminate the
possibility that resistance to antibiotics used for listeriosis treatment such as ampicillin and gentamycin cannot be acquired, since
penicillin and ampicillin belong to the same family of beta-lactam
antibiotics (Walsh et al., 2001). On the other hand, Mayrhofer
et al. (2004) determined the antimicrobial resistance of L.
monocytogenes isolates from 304 meat samples. The study did
not reveal any resistant isolate from the samples for the antibiotics
tested. (Mayrhofer et al., 2004). Prazak et al. (2002) tested the
sensitivity of twenty-one isolates of L. monocytogenes from
cabbage, water and environmental samples to various antibiotics.
The study showed that 20 isolates (about 95%) were resistant to
two or more antibiotics. About 85% of the isolates was resistant to
penicillin and one of the strains was also resistant to gentamycin.
This study is important since it reveals the presence of multidrug
resistant strains of L. monocytogenes in food and environmental
samples (Prazak et al., 2002). The differences in results reported
by the two studies could be due to the type of samples used and
variability in the procedures used for antimicrobial sensitivity
testing.
4.2. Resistance to sanitizers and disinfectants
In addition to antibiotic resistance, the emergence and spread
of resistance among foodborne organisms to sanitizers and disinfectants used by the food industry are also becoming a concern.
Many studies are done to determine the susceptibility of Listeria
to quaternary ammonium compounds (QACs), commonly used as
disinfectants in food processing facilities. Mereghetti et al. (2000)
studied ninety-seven epidemiologically unrelated L. monocytogenes strains for their sensitivity to QACs. The strains were
isolated either from the environment, food products, animals or
humans. Seven of the isolates that were of environmental or food
origin had high MICs to the QACs tested such as benzalkonium
chloride and cetrimide. The authors concluded that the resistance
in these strains could explain the persistence of some organisms in
food processing facilities. Further, all isolates contained the mdrL
gene, which encodes an efflux pump responsible for conferring
resistance to QACs. They suggested that rotation between two
different sanitizers for cleaning of food facilities could prove
useful to prevent the development of persistent and resistant
strains (Mereghetti et al., 2000). To et al. (2002) investigated the
adaptation and development of resistance in L. monocytogenes
after exposure to sub-lethal concentrations of disinfectants used in
the food industry. Two resistant and four sensitive strains of L.
monocytogenes were grown in the presence of sub-lethal levels of
9
benzalkonium chloride, a sanitizer widely used in food processing
sites and the resistance acquired after adaptation was studied by
evaluating the use of efflux pumps by the organism and comparing the cell surface properties of resistant and sensitive strains.
The sensitive strains had an MIC that was five-fold higher and the
MIC of resistant strains doubled after the period of adaptation.
The efflux pumps were responsible for the adaptation of sensitive
strains, whereas the originally resistant strains showed a change in
their fatty acid profile after adaptation (To et al., 2002). Two
persistent and two non-persistent strains of L. monocytogenes
isolated from an ice cream and poultry plant were tested for their
resistance and adaptive response to disinfectants (Lunden et al.,
2003). The initial resistance of persistent and non-persistent
strains to disinfectants was different and the strains adapted after a
two-hour exposure to sub-lethal concentrations of disinfectants.
The strains also showed adaptation to increasing levels of the
disinfectant. However, the resistance to sodium hypochlorite disappeared in a week, whereas the resistance to QACs and tertiary
alkylamine was maintained even 28 days after exposure. L.
monocytogenes showed cross-adaptation to the same (related) or
different family (unrelated) of disinfectants. The cross-adaptation
of L. monocytogenes strains to related and unrelated disinfectants
poses a question on whether the practice of rotation of sanitizers in
the food industry suggested by Mereghetti et al. (2000) would be
really effective in controlling the development of antimicrobial
resistance (Lunden et al., 2003). Romanova et al. (2002)
investigated the sensitivity of nineteen L. monocytogenes strains
to sanitizers commonly used in the meat industry. Some of the
isolates were from a listeriosis outbreak and some from the meatprocessing facility. Five of the isolates showed a resistant phenotype to the sanitizers tested and they contained two plasmids.
Similar to the findings of Mereghetti et al. (2000), all the isolates
tested contained the mdrL gene, which encodes an efflux pump
(Romanova et al., 2002).
4.3. Resistance to bacteriocins
Antimicrobial peptides produced by bacteria such as lactic
acid bacteria are called bacteriocins. They are synthesized from
ribosomes and are effective against closely related bacteria
(Klaenhammer, 1993). Nisin is a 34-amino acid bacteriocin
produced by Lactococcus lactis strains and is approved for use
in food preservation in many countries; however, several other
bacteriocins have shown the potential for future applications in
food systems (Cleveland et al., 2001). Since its discovery and
approval for use in foods, nisin has been used as a preservative
in the dairy and meat industries to control pathogens such as L.
monocytogenes. Nisin acts on target cells by permeabilizing the
cytoplasmic membrane. The formation of pores leads to the
leakage of cytoplasmic substances from the cell (Abee et al.,
1994). However, similar to the use of antibiotics, the concern
with the use of bacteriocins is the development of resistance in
foodborne pathogens. Gravesen et al. (2002) investigated the
frequency of resistance development in L. monocytogenes to
two bacteriocins, pediocin PA-1 and nisin A, along with the
effects of strain differences and environmental conditions. The
resistance frequencies for pediocin investigated in about 20
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M. Gandhi, M.L. Chikindas / International Journal of Food Microbiology 113 (2007) 1–15
strains were approximately 10− 6, irrespective of the environmental conditions, while the frequency of resistance to nisin
was strain-specific and varied with environmental conditions
from 10− 7 to 10− 2. The paper sheds light on the development of
resistance to bacteriocins in a food system and the influence of a
number of environmental factors such as low temperature,
acidic pH and presence of sodium chloride on the frequency of
resistance development (Gravesen et al., 2002).
5. Advanced strategies for control of food safety
There has been an ongoing effort to control the foodborne
pathogen Listeria in foods and in food processing facilities.
Research performed by academia, government agencies and the
food industry is aimed at developing new and improved methods
to prevent the survival and growth of Listeria. The wide range of
efforts to achieve food safety include better monitoring and reporting of foodborne diseases by government agencies, routine
food sampling and testing, establishment of HACCP, inspection
at food processing facilities, training of food workers and general
awareness among consumers about food safety (Bryan, 2002).
Several studies are being conducted that utilize various preservation techniques for the control of Listeria in foods and most
of them aim at achieving food safety without compromising the
sensory and nutritional qualities of foods.
5.1. General stress sigma factor (σB) as the target for food
preservation
The advances in genomics have led to the identification of
genes, the proteins they make and their functions. This information is proving very useful to develop new strategies to prevent
the survival and growth of pathogens. van Schaik and Abee
(2005) discuss the use of alternative sigma factor σB for inactivation and control of Listeria in the food industry. As discussed
earlier, σB is an important regulator of the stress response in
Listeria. Once the pathogen senses an environmental stress, the
signal is relayed to σB and its activation initiates transcription of
the σB regulon. The protein products being produced are a part of
the stress response and confer protection to the cell. Since σB
plays an important role in the stress response of the organism, an
in-depth understanding on the mechanism of σB activation could
be used to develop preservation technologies, which aim towards
inactivating the sigma factor and thereby controlling the stress
response of the pathogen (van Schaik and Abee, 2005).
5.2. Multiple hurdle technology
The growing demand for fresh, minimally processed foods by
consumers has led to the need for natural food preservation
methods such as the use of antimicrobial peptides to control the
growth of foodborne pathogens, that have no adverse effects on
the consumer or the food itself. The bacteriocin nisin has been
widely used as a preservative to control the growth of pathogens
in foods. Nisin has also proved very useful as a part of hurdle
technology, where a combination of two or more treatments is
used to obtain a more effective method of food preservation
(Cleveland et al., 2001). The combined action of nisin and carbon
dioxide on L. monocytogenes cells grown at 4 °C has been
investigated (Nilsson et al., 2000). Nisin brought about a two-log
reduction in wild type L. monocytogenes cells and acted synergistically with carbon dioxide to give a four-log reduction in cell
count. Nisin had no effect on nisin-resistant cells grown in the
presence of air or carbon dioxide. Carbon dioxide increased the
lag phase of L. monocytogenes by six days and was more effective
against nisin-resistant cells compared to the wild type strain. The
presence of carbon dioxide increases the membrane permeability
and the proportion of short-chain fatty acids in the cell membrane,
which helps in the pore formation by nisin (Nilsson et al., 2000).
Modi et al. (2000) studied the combined effect of heat and nisin on
wild type and nisin-resistant L. monocytogenes cells. The heat
sensitivity of wild type and nisin-resistant strains was the same in
the absence of nisin. The synergistic effect of heat and nisin on
nisin-resistant cells caused a 3.7 log reduction in the first 7 min of
treatment. The sub-lethal heat treatment alters the membrane
permeability along with nisin that causes poration of the cell
membrane (Modi et al., 2000). Further, certain foods such as
liquid whole egg and dairy products when processed by thermal
treatments such as pasteurization result in undesirable changes in
the sensory and nutritional qualities of the product. This has led to
the development and use of non-thermal processing technologies
such as Pulsed Electric Fields (PEF) in the food industry.
Calderon-Miranda et al. (1999a,b) investigated the use of PEF and
nisin in combination to inactivate at Listeria innocua in liquid
whole egg and skim milk. The results showed an additive effect
on the inactivation of L. innocua in both foods when the pathogen
was exposed to PEF and the sensitized cells treated with nisin
(Calderon-Miranda et al., 1999a,b). Multiple hurdle technology
targets the bacterial cell in different ways resulting in better
control of the pathogen. Arques et al. (2004) studied the effectiveness of reuterin, an antimicrobial compound produced by
Lactobacillus reuteri, along with nisin against Gram-positive and
Gram-negative organisms in milk. At the concentrations tested,
reuterin exhibited bacteriostatic activity against L. monocytogenes. When reuterin was used in combination with nisin, it acted
synergistically to inhibit L. monocytogenes (Arques et al., 2004).
Bacteriocins such as nisin do not have a significant inhibitory
effect on Gram-negative organisms; however, the use of nisin
with other agents could probably serve as a method to enhance its
effectiveness against a wider range of organisms. Branen and
Davidson (2004) investigated the effect of ethylenediaminetetraacetic acid (EDTA) and lactoferrin on the antimicrobial activity
of nisin. Low levels of EDTA used in the study synergistically
enhanced the activity of nisin against L. monocytogenes. EDTA
also increased the effectiveness of nisin against E. coli, a Gramnegative organism. EDTA functions as a chelator of divalent
cations and permeabilizes the outer membrane of Gram-negative
bacteria by releasing the lipopolysaccharide (LPS). This allows
nisin to act easily on the cytoplasmic membrane. Lactoferrin
alone did not show any bacteriostatic effect against the organisms
tested, but in a combination treatment with 50% less nisin,
lactoferrin totally inhibited L. monocytogenes. The use of bacteriocins with other treatment methods to achieve food preservation
requires the use of lower concentrations of the bacteriocin, and
M. Gandhi, M.L. Chikindas / International Journal of Food Microbiology 113 (2007) 1–15
this helps to prevent the risk of development of bacteriocinresistant population of cells (Branen and Davidson, 2004). A
study by Ettayebi et al. (2000) investigated the synergistic action
of nisin and thymol against the pathogens L. monocytogenes and
B. subtilis. Thymol is an essential oil component in thyme and has
previously been shown to have antimicrobial activity (Ettayebi
et al., 2000). The results showed that nisin Z and thymol when
used alone resulted in only a partial inhibition of both pathogens.
But the two agents acted synergistically in combination treatment
and sub-inhibitory concentrations of both nisin Z and thymol were
sufficient to reduce the growth of both pathogens. Thymol alters
the bacterial membrane structure resulting in greater permeability
for nisin. This results in a higher concentration of nisin within the
bacterial cells, thus permitting the use of lower nisin concentrations when used synergistically to obtain the same level of antibacterial activity (Ettayebi et al., 2000).
5.3. Encapsulation technology
Commercial preparations of nisin such as Nisalpin (2.5% pure
nisin A) are commonly added directly to foods for preservation.
However, the loss of nisin activity over time in various food
systems has been reported (Benech et al., 2002b). The problem of
nisin degradation and loss of its activity during storage of foods
drew the attention of researchers to the technique of microencapsulation. Encapsulation technology is a method of enclosing
materials into capsules before delivery into a system. Encapsulated materials are protected from adverse effects of heat, moisture, pH changes and their activity is maintained for prolonged
periods of time (Gibbs et al., 1999). This method has been widely
used for encapsulation of drugs in the field of medicine
(Maswadeh et al., 2000) and in other areas of food science such
as delivery of flavor compounds (Zasypkin and Porzio, 2004) and
vitamins (Lee et al., 2002) into foods and microencapsulation of
probiotic bacteria (Kailasapathy, 2002). Substances such as fats,
starches, proteins and lipids are commonly used to encapsulate
materials using techniques such as spray drying, extrusion coating
and entrapment in liposomes. The release of encapsulated materials
can be initiated by various conditions such as temperature, pH or
moisture (Gibbs et al., 1999). Several recent studies have
investigated the use of encapsulation to deliver antimicrobial
agents such as nisin into food systems (Benech et al., 2002a,b;
Were et al., 2004). The use of free nisin in cheeses to inhibit the
growth of spoilage and pathogenic microorganisms results in the
inhibition of starter cultures, ultimately affecting the acidification
and flavor of the final product. Further, the use of bacteriocinproducing strains during cheese manufacture alters the quality of
the final fermented product. Studies by Benech et al. (2002b) have
investigated the use of nisin encapsulated in liposomes for delivery
into cheddar cheese and its antimicrobial activity against L.
innocua. Nisin A and nisin Z are the two natural variants of nisin,
with nisin Z being more soluble than nisin A. This is due to the
presence of asparagine at position 27 in nisin Z and histidine in
nisin A (Benech et al., 2002b). Cheddar cheese prepared with either
nisin Z encapsulated in liposomes or cheese with in situ production
of nisin Z using a nisinogenic starter was compared over six
months of ripening for the inhibitory activity on L. innocua. A
11
greater reduction in L. innocua was observed in cheese made with
nisin Z-containing liposomes compared to the cheese made with
nisin Z-producing starter. After the six-month ripening phase,
cheese made with nisin Z-liposomes had less than 10 CFU/g of L.
innocua and nearly 90% of the nisin activity was retained. On the
other hand, cheddar cheese made with the nisin-producing starter
contained 104 CFU/g of L. innocua and only 12% of the nisin
activity. The authors suggested that the encapsulation of nisin in
phospholipid vesicles resulted in its higher concentration maintained over a longer period of time and greater activity (Benech
et al., 2002b). Benech et al. (2002a) also used anti-nisin Z
antibodies and Transmission Electron Microscopy (TEM) to study
the localization of nisin Z molecules in the cheddar cheese matrix
and understand the mechanism of its inhibitory action against
pathogens. The images from TEM showed nisin to be encapsulated
within liposomes and also associated with the hydrophobic liposomal membrane. They predicted that the quick release of encapsulated nisin into the food system would provide short-term
antimicrobial activity, whereas the immobilized nisin would provide an inhibitory effect over a longer period of time due to its
slower desorption from the membrane (Benech et al., 2002b). Were
et al. (2004) evaluated the antimicrobial activity of nisin Z encapsulated in phospholipid liposomes against L. monocytogenes. The
results showed the increased ability of nisin in liposomes to inhibit
bacterial growth compared to free nisin (Were et al., 2004). The
technique of delivery of bacteriocins in an encapsulated form into
food systems for improving its stability and antimicrobial action is
very promising, but it requires further research for optimizing the
technology for use in various food products.
5.4. Active packaging technology
Contamination of foods can occur during any stage of the
manufacturing or processing phase. Despite the difficulty and
uncertainty in identifying the source of contamination in foodborne disease outbreaks, several surveillance reports have shown
that post-process contamination of foods has been a major cause
in many of the outbreaks. The sources of recontamination identified are unprocessed raw materials added to finished processed
foods, food contact surfaces and environments, defective
packaging and food handling personnel (Reij and Den Aantrekker, 2004). The review by Reij and Den Aantrekker (2004)
provides a comprehensive list of outbreaks that have been caused
due to post-process contamination of foods by various pathogens.
Outbreaks of L. monocytogenes have occurred due to contamination of butter and hot dogs in the processing environment,
contamination of cooked meat from the dicing machine and
rillettes being contaminated from a filling and packaging machine
(Reij and Den Aantrekker, 2004). To deal with the problem of
post-process contamination, research has led to the development
of active packaging, wherein materials are incorporated into the
packaging to either control the atmosphere within the package
(such as moisture content, pH, oxygen level) or inhibit the growth
of spoilage and pathogenic organisms on the food product
(Ozdemir and Floros, 2004). The growing consumer demand for
minimally processed, conveniently packaged and extended shelfstable foods has drawn much attention to the field of antimicrobial
12
M. Gandhi, M.L. Chikindas / International Journal of Food Microbiology 113 (2007) 1–15
packaging, in which edible films and coatings are used to deliver
antimicrobial agents such as lysozyme, triclosan and nisin into
food systems (Cagri et al., 2004; Cha and Chinnan, 2004). Sebti
et al. (2002) developed a biodegradable packaging by incorporating nisin and stearic acid in it, which serve as an antimicrobial
agent and moisture barrier respectively. The pH of the hydroxyl
propyl methyl cellulose (HPMC) film was adjusted to 3 to prevent
the nisin and stearic acid from interacting. The packaging showed
high inhibitory activity against L. monocytogenes and S. aureus
(Sebti et al., 2002). Mauriello et al. (2004) developed a polyethylene film activated using a bacteriocin produced by Lactobacillus curvatus 32Y. The antimicrobial packaging was
developed by the coating method and its efficacy tested against
L. monocytogenes-contaminated pork steak and ground beef.
The results showed about a one log reduction in cell numbers,
with the highest antimicrobial activity after 24 h at 4 °C
(Mauriello et al., 2004). In recent years, many studies have
investigated the use of a variety of antimicrobial edible films and
coatings for food products. For example, Nisaplin-containing
cellophane-based coating for chopped meat (Guerra et al., 2005),
antimicrobial efficacy of corn zein films containing nisin against
pathogens such as L. monocytogenes (Hoffman et al., 2001), nisin
and lauric acid impregnated soy-based films for turkey bologna
and their inhibitory activity on L. monocytogenes (Dawson et al.,
2002), inhibition of L. monocytogenes on turkey frankfurters
coated with zein films containing nisin, sodium diacetate and
sodium lactate (Lungu and Johnson, 2005).
An in-depth understanding of the material being used to
develop the film or coating and the antimicrobial agent added to
make it active is essential because the release kinetics of antimicrobial agents differ depending on the kind of packaging
material being used and the agent itself. The ideal bioactive
packaging would have the release of the antimicrobial agent at a
rate that achieves the highest inhibitory action against microorganisms in the food system. Chung et al. (2001) investigated the
effect of slow release of propyl paraben from a polymer coating
versus direct addition of propyl paraben against Saccharomyces
cerevisiae, which causes spoilage of foods. The slow release of
the agent from the carboset coating showed a slow and continuous
inhibition of S. cerevisiae. On the other hand, direct addition
resulted in cell outgrowth after a period of incubation, and the
cells were more tolerant to propyl paraben when compared to cells
from the slow release culture (Chung et al., 2001). Buonocore
et al. (2003) developed a mathematical model to describe the
release kinetics of antimicrobial agents from cross-linked polyvinyl alcohol into water. The antimicrobial agents lysozyme,
nisin and sodium benzoate were used in the study. They determined the diffusion of water molecules into the polymeric matrix
and the reverse diffusion of the antimicrobial agent from the film
into the water to develop the model (Buonocore et al., 2003).
Since the antimicrobial action of bacteriocins is highly dependent
on the mode of delivery into the food system, Chi-Zhang et al.
(2004) evaluated the efficacy of instant addition of nisin and slow
release of nisin into a broth system. The instantaneous addition is
similar to nisin formulated into foods and the slow release of
nisin mimics the release of the bacteriocin from a packaging film
to the food. Both types of delivery methods resulted in the
inhibition of L. monocytogenes; however, bacterial cells developed resistance to nisin over time, with the resistance being
higher when the cells were exposed to instant addition of nisin.
Further, excess of nisin in the system did not inhibit the pathogen
due to development of nisin resistance. The study suggests that
the combination of instantaneous and slow release of nisin into a
food system is the most efficient way to obtain the greatest
antimicrobial effectiveness (Chi-Zhang et al., 2004).
6. Conclusion
The foodborne pathogen, Listeria emerged in the late 20th
century and has been a cause of many outbreaks of listeriosis with
high case fatality rates. The economic impact due to big product
recalls, severity of the disease, hospitalization and treatment costs
has drawn the attention of researchers towards the development of
preventive measures to control the spread of L. monocytogenes.
Ongoing research is focused towards deciphering the mechanisms
behind the ability of the pathogen to survive and grow under suboptimal conditions such as low temperature, acidic pH and osmotic stress. An in-depth understanding of the physiology of the
organism, establishment of HACCP in the food industry, regulated product sampling and testing, along with better detection
and surveillance systems to report foodborne disease outbreaks is
valuable in controlling the pathogen. As mentioned earlier, the
increasing demand of consumers for minimally processed, readyto-eat foods, and globalization of the food industry where a
greater proportion of foods of ethnic origin are being imported
may contribute to an increase in the incidence of foodborne
diseases such as listeriosis. However, concerted efforts by academia, government and the food industry, as well as consumer
awareness can prove beneficial to develop innovative strategies
for the control of Listeria in foods and food processing environments and meet the consumer demand for minimally processed,
ready-to-eat foods, without any loss in sensory and nutritional
attributes.
Acknowledgement
This research was supported (in part) by the New Jersey
Agricultural Experiment Station Project #10152 through U.S.
Hatch Act funds.
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