Bacterial Cell Structure, Function and Classification

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Bacterial Cell Structure, Function and
Classification
Prokaryotes vs. Eukaryotes
Cells can be classified as prokaryotic or eukaryotic. Prokaryotic cells are exemplified by their
lack of membrane nuclei and organelles. The word prokaryote comes from Greek, "pro" meaning
"before" and "karyon" meaning nucleus. Literally, "before the nucleus". Eukaryotic cells have
membrane bound nuclei and organelles. The prefix "eu" means "true", so eukaryotic means "true
nucleus".
Differences between prokaryotes and eukaryotes
Prokaryotes
Small: 0.3-2um
Eukaryotes
Larger: 5-50um
Always unicellular
Can be unicellular or
multicellular
Nuclei?
NO
YES
Membrane-bound organelles?
NO
YES
Where found?
Everywhere
Usually in environments that
are not extreme
Size
Table 3.1
Classification and Nomenclature
In the 1700s, Carolus Linnaeus developed a taxonomic system in order to categorize living
things. His system grouped organisms with similar characteristics, and had only two kingdoms:
Animalia and Plantae. In the roughly 300 years since the development of his taxonomic scheme,
we have learned that there are many other types of organisms, and the similarities and
differences are not always obvious. For example, in Linnaeus' time, mushrooms were grouped
with plants. Today, they are in their own kingdom, Fungi, and we know that they are actually
more closely related to animals than to plants.
Today, microbiologists use the Domain system of classification, developed by Carl Woese. This
system is based on gene sequences, and groups organisms into three domains: Bacteria, Archaea
and Eukarya. The kingdom system fits within the domain system.
Fig. 3.1 The Domain System of Classification
Domains Bacteria and Archaea are both prokaryotic. It had previously been thought that archaea
were a type of bacteria, but due to the work of Woese and others, it is now known that although
they look like bacteria, Archaea are different at molecular level. In addition having a different
type of ribosome than bacteria, archaea do not have peptidoglycan in their cell walls.
The Domain Eukarya contains all of the eukaryotic kingdoms, such as Animalia, Plantae, Fungi
and Protista.
Nomenclature
Most bacteria are referred to by their genus and species names. Example: genus:
Escherichia, species: coli = Escherichia coli. The genus can be shortened to just the capitalized
first initial: E. coli. The genus and species should always be italicized, unless they are handwritten, in which case they must be underlined.
Strains are slight variations within the species. E. coli has many different strains, some of which
are harmless and many of which are harmful. E. coli Nissle 1917 (aka EcN) is a non-pathogenic,
potentially probiotic form of E. coli. E. coli O157:H7 is a pathogenic form of E. coli that is
transmitted in food and causes life-threatening infections.
Bacterial Cell Structure & Function
Bacteria are prokaryotes as they lack nuclei. Bacterial cells are contained by a cell membrane
(aka plasma membrane or cytoplasmic membrane) that is made of a phosopholipid bilayer. This
makes it similar to our cell membranes, which are also made of a phospholipid bilayer. The job
of the cell membrane is to "contain" the cell and to control what enters and leaves the cell.
Fig. 3.2 Bacterial Cell Membrane http://webcom.grtxle.com/customization/uploads/FIGURE0308.JPG
DNA
Most prokaryotes have only one chromosome that is made up of a circular molecule of DNA.
Bacteria contain far fewer genes than humans. A typical E. coli bacterium contains around 3,500
genes, where a human contains around 30,000 genes.
Bacteria may also have plasmids which are smaller circular segments of extrachromosomal
DNA. Plasmids vary in size and may contain only a handful of genes or a few hundred genes.
The genes found on plasmids vary widely and may be used to aid in bacterial survival by
providing antibiotic resistance, the ability to produce toxins or new metabolic capabilities. While
all bacteria have chromosomes, plasmids are "optional". Some bacteria have many, some a few
and some have no plasmids.
Nucleoid/Nucleoid Region
Prokaryotes do not have membrane-bound nuclei, but the area surrounding the chromosome is
called the nucleoid or nucleoid region (meaning "nucleus like"). The cytoplasm of the nucleoid
region tends to be thicker and more gelatinous than the rest of the cytoplasm
Ribosomes
Ribosomes are the site of protein of synthesis. Bacterial ribosomes are considered to be 70s and
consists of two parts: the small ribosomal subunit (30s) and the large ribosomal subunit (50s).
The “s” indicates the density of each subunit when spun at high speeds. Prokaryotic ribosomes
are smaller than eukaryotic ribosomes. Eukaryotic ribosomes are 80s consisting of a 40s and 60s
subunits.
Cytoplasm and Cytosol
The cytoplasm is the entire contents of the cell within the cell membrane, and the cytosol refers
specifically to the liquid portion of the cellular contents.
Granules
Many bacteria have granules within the cytoplasm. These granules can be made of starch, lipids,
or any substance the cell is storing for later use.
External Structures
The Cell Wall
It is vital to your success in this class to get a handle on the structure of the bacterial cell wall
now! Understanding the structure of the bacterial cell wall is necessary to understand:




bacterial replication
antibiotic susceptibility: which antibiotics work and why
the reaction of the immune system to different types of bacteria
how to identify unknown bacteria in the lab
The bacterial cell wall is made of a unique substance, peptidoglycan, which is found only in
bacteria. Peptidoglycan is made up of alternating subunits of N-acetylglucosamine (NAG) and
N-acetylmuramic acid (NAM), strung together like beads on a string. But the really important
part of peptidoglycan is that several of these chains line up in rows and then are cross-linked to
each other via adjacent NAM molecules. This cross-linking makes the peptidoglycan very
strong, essentially forming a "chain-mail armor" or mesh around the cell. This allows the cell to
maintain its shape and to resist bursting due to osmotic pressure.
Fig. 3.3 The Structure of Peptidoglycan Image by A. Swarthout
Bacterial Cell Walls Come In 2 Varieties: Gram Positive and Gram Negative
The Gram Positive cell wall is characterized by a thick layer of peptidoglycan. Up to 50% of the
total cell weight is its cell wall. The Gram positive wall also has teichoic acids and lipoteichoic
acids embedded in it. The lipoteichoic acids anchor the peptidoglycan to the cell membrane,
while the teichoic acids aid the passage of ions through the cell wall. Remember: the cell
membrane is NOT part of the cell wall! Certain genera (not all!) of Gram Positive organisms
such as Mycobacterium, contain upto 60% mycolic acid (a waxy substance) in their cell walls.
Fig. 3.4 Gram Positive Cell Wall Image by A. Swarthout
The Gram negative cell wall is characterized by a thin layer of peptidoglycan, which is then
surrounded by an outer membrane. It is important to note that the outer membrane is part of the
cell wall and is not considered "another cell membrane". The outer membrane is made up of two
layers or "leaflets". The inner leaflet is made of phospholipids (just like the cell membrane), but
the outer leaflet is made of lipopolysaccharide (LPS). LPS is made of a polysaccharide (O
antigen) plus a lipid (lipid A). Lipid A is an endotoxin and triggers inflammation, fever and
shock in victims of Gram negative infections. The outer membrane also contains porins which
are highly selective transport proteins. The space surrounding the peptidoglycan is called the
periplasm.
Fig. 3.5 Gram Negative Cell Wall Image by A. Swarthout
******Antibiotics and Peptidoglycan*******
Several antibiotics work by interfering with the synthesis of peptidoglycan. If the peptidoglycan
is not made properly, the cell wall is weak and the cell is overcome by osmotic pressure and
lyses. Because the outer membrane of Gram negative cells surrounds and protects the
peptidoglycan of those cells, antibiotics like penicillin, cephalosporin and vancomycin which
work by interfering with the synthesis of peptidoglycan may not work on Gram negative cells,
because they won’t have physical access to the peptidoglycan.
An enzyme present in human tears and sweat, lysozyme, can damage peptidoglycan and cause
lysis of bacterial cells.
Glycocalyx
The glycocalyx is a slimy layer of polysaccharides secreted by some bacteria outside their cell
walls. If the glycocalyx is thin, not highly organized and not firmly attached to the cell wall, it is
called a slime layer. Slime layers allow the bacteria that have them to glide along solid surfaces
as a mode of motility.
If the glycocalyx is thick, highly organized and firmly attached to the cell wall it is called a
capsule. Capsules help to hide bacteria from the phagocytic white blood cells of the immune
system, thus aiding in the ability of the bacteria to cause disease.
Flagella
Flagella are long filamentous structures that propel bacteria, giving them motility. Flagella can
come in a variety of arrangements, and these arrangements are characteristic of the species of
bacteria that have them.
Fig. 3.6 Structure of Flagellum http://webcom.grtxle.com/customization/uploads/FIGURE0318.JPG
Fig. 3.7 Flagellar arrangements
Pili and Fimbriae
Pili and fimbriae are protein projections from the surface of the bacterial cell. They are shorter
than flagella, more rigid and not used for motility. Pili and fimbriae are used by bacterial cells to
attach to surfaces in the environment and inside our bodies. Pili are generally longer than
fimbriae.
A specialized type of pili, the sex pilus, is used transfer DNA from one bacterial cell to another.
A Generalized Bacterial Cell
Fig. 3.8 A Generalized Bacterial Cell Shutterstock Image: Image ID:107374295
All bacterial cells have a cell membrane, a nucleoid, a chromosome, ribosomes and cytoplasm.
Most bacteria have a cell wall. The most notable exception is the genus Mycoplasma. Because
they lack cells walls, Mycoplasma are smaller than other types of bacteria, and they are
pleomorphic (they do not have a standard shape) as their shape changes to "fit" the space they
are in. In addition, the lack of a cell wall makes them intrinsically resistant to antibiotics that
work by disrupting the structure of the cell wall, such as penicillin, cephalosporin and
vancomycin.
Structures such as pili, fimbriae, flagella, plasmids and the glycocalyx are not found in all
bacterial cells, but are specific to certain types of bacteria, and their presence/absence can aid in
bacterial identification.
Special Bacterial Cell Structures
1. Endospores
Some Gram positive bacteria are capable of forming survival structures called endospores.
Endospores are formed primarily by members of the genus Clostridium and the genus Bacillus.
Endospores provide the cells with resistance to heat, chemicals, dessication, UV light and
nutrient deprivation. Cells that produce endospores do not have the spore all of the time. They
exist and function as vegetative cells when conditions are favorable, then undergo sporulation
to form endospores when conditions become unfavorable for their survival.
An endospore returns to its vegetative state by a process called germination. Germination is
triggered by physical or chemical damage to the endospores coat. The endospore’s enzymes then
break down the extra layers surrounding the endospore, water enters, and metabolism resumes.
Since one vegetative cell forms a single endospore, sporulation in bacteria is not a means of
reproduction meaning this process does not increase the number of cells. Bacterial endospores
differ from spores formed by eukaryotic fungi and algae, which detach from the parent and
develop into another organism and, therefore, represents reproduction.
When released into an environment, endospores can survive extreme heat, lack of water,
exposure to toxic chemicals (disinfectants), and radiation. For example, a 7500 year old
endospore of Thermoactinomyces vulgaris from the freezing muds of Elk Lake Minnesota
germinated after being warmed and placed in nutrient media, and 25-40 million year old
endospores found in the gut of a stingless bee entombed in amber were found in the Dominican
Republic. After being placed in nutrient media these 25-40 million year old endospores
reportedly germinated!
Endospores are important from a clinical viewpoint and in the food industry because they are
resistant to processes that normally kill vegetative cells. Such processes include heating,
desiccation (drying), use of chemicals, and UV radiation. Most cells are killed when
temperatures reach above 70 degrees Celsius, endospores however can survive boiling water for
several hours or more. Endospores of thermophilic (heat-loving) bacteria can survive in boiling
temperatures for up to 19 hours. Endospore-forming bacteria are a problem in the food industry
because they are likely to survive under-processing, and, if conditions for growth occur, some
species produce toxins and disease. Special methods for controlling endospores will be discussed
later in the semester.
Fig. 3.9 Sporulation http://webcom.grtxle.com/customization/uploads/FIGURE03-28.JPG
Fig. 3.10 Structure of An Endospore
http://webcom.grtxle.com/customization/uploads/FIGURE03-29.JPG
Notable Endospore-Forming Bacteria
Clostridium botulinum
Clostridium botulinum is a soil-dwelling, endospore-forming bacterium that is a frequent
contaminant of canned foods. If the canning process is not done correctly, endospores of C.
botulinum will survive the process and germinate in the can or jar of food, then begin to produce
and secrete botulinum toxin into the food. This neurotoxin interferes with the function of the
nervous system.
Nerve cells release acetylcholine into the synaptic cleft. The acetylcholine binds to receptors on
muscle cells and serve as a signal for muscle contraction. Botulinum toxin prevents the release of
acetylcholine from nerve cells, resulting in flaccid paralysis in the victim. Symptoms begin
twelve to 36 hours after ingestion of the contaminated food. If left untreated, vital circulatory and
respiratory muscles stop functioning, leading to death of the individual.
Treatment of botulism involves administration of preformed botulism antitoxin, which is a
special form of antibody against the neurotoxin. There is no vaccine against botulism, so the best
cure is prevention: boiling foods at 100°C for 10 minutes to inactivate the neurotoxin.
Infant botulism is the most common form of botulism in the United States. Honey and corn syrup
may contain bacterial endospores, which may germinate in the body. Older children and adults,
with fully functioning immune systems, seem to be able to fight off the effects of the toxins
released by the germinating endospores. Infants during their first year of life, and
immunocompromised adults, are at risk of infection.
Botulinum toxin is used pharmaceutically as Botox®.
Clostridium tetani
A close relative of C. botulinum is Clostridium tetani. Endospores of C. tetani may enter a
wound and then germinate in the body, where they begin to produce tetanospasmin, a
neurotoxin that binds to muscle cells causing prolonged muscle contraction. The common name
for this condition, "lock jaw" is the result of facial muscle contraction. It can also lead to
impairment of the respiratory and cardiac systems.
Tetanus immunoglobulin (TIG), an antitoxin, may be administered as a preventative measure in
response to acute injury. However, the DTaP (Diphtheria, Tetanus, and Pertussis) vaccine is also
available. It consists of a series of four shots given at age two months, four months, six months,
and 12-18 months. Booster shots are given at four to six years of age, at 11-12 years, and
recommended every 10 years thereafter
Bacillus anthracis
The endospores of Bacillus anthracis have a fine, powdery consistency that aerosolize easily,
making B. anthracis a candidate for bio-weapon development. Once these airborne spores are
inhaled, they germinate in the body and cause flu-like symptoms that eventually worsen into
severe breathing problems and shock. Left untreated, there is an 85-90% mortality rate. With
treatment, the mortality rate drops to around 45%. (Center for Disease Control and Prevention).
Vaccination is available for those as high risk of exposure and the antibiotic ciprofloxacin is
effective if administered promptly after exposure.
Clostridium difficile
Antibiotic-associated colitis, or “C. diff,” is caused by the microorganism Clostridium difficile.
This organism is found as normal fecal flora in about 3% of healthy people, but is most often
associated with nosocomial (hospital-acquired) infections in nursing homes and long-term care
facilities. Extensive antibiotic therapy disrupts normal healthy gut flora, allowing C. difficile the
opportunity to colonize the intestinal tract. Antibiotic-associated colitis results in extreme
diarrhea and abdominal pain, and is difficult to treat with antibiotics. Health care workers need to
employ both standard and contact precautions when working with C. difficile patients.
Clostridium perfringes
Gas gangrene may result from exposure to Clostridium perfringes through surgical wounds,
injury, or severe burns. Individuals with poor cardiovascular or respiratory function, such as
diabetic patients, are at greatest risk. If the endospore enters the person’s body through a wound
opening, an anaerobic or oxygen-poor environment encourages endospore germination. Within a
few days, pain at the site of injury is accompanied by gaseous, discolored, and foul-smelling
tissue. Treatment involves debridement of the wound, antibiotics, hyperbaric oxygen, and limb
amputation if the disease has progressed. There are no vaccines or antitoxins against C.
perfringes at this time.
2. Biofilms
Biofilms are complex aggregates of microbes—prokaryotes and eukaryotes—that live in
polysaccharide-encased communities. These communities may be suspended in aqueous
environments or attached to surfaces. Biofilms are responsible for many annoying conditions
such as slime in sink drains and toilets, but they are also responsible for many types of more
serious infections. Dental plaque is the result of biofilm formation. Patients who receive implants
(artificial hips or heart valves) or have in-dwelling devices such as venous or urinary catheters
may get infections caused by bacteria growing on/in these devices in the form of biofilms. These
infections are difficult to treat because the polysaccharide matrix encasing the microbes protects
them from the effects of antibiotics.
Biofilms, like microorganisms in general, may also be beneficial. They are being used in
wastewater treatment and their activities are necessary for nutrient cycling.
Biofilms can be formed by bacteria, archaea, and yeast. They can be homogeneous, meaning
they are composed of only one type of organism, or heterogeneous—composed of a mixture of
microbes.
Biofilm Formation
Biofilm formation begins when one or a few microbes attach to a surface by van der Waals
forces (weak attractions between molecules). The microbes then strengthen their attachment by
using fimbriae and pili. The attached cells send out molecular signals to recruit other cells to the
biofilm. Finally, they begin to produce the extracellular polysaccharide matrix - a sticky, slimy
substance that allows them to adhere better and allows other cells from the surrounding fluid to
adhere to the surface as well. The biofilm grows as attached cells go through binary fission and
other cells from the environment are "recruited" to join the developing biofilm.
The matrix protects the cells in the biofilm community from exposure to antimicrobial drugs,
antiseptics, and chemical disinfectants, making it difficult to treat infections caused by microbes
living in biofilms. The polysaccharide matrix also has channels running through it that the
microbes can use to share nutrients, share genetic information (like antibiotic resistance genes),
and communicate.
Fig. 3.11 IMAGE: Biofilm formation Permission needed for image—
Bacterial Cell Shapes and Arrangements
The most common bacterial shapes are coccus (plural: cocci), which are round and bacillus
(plural bacilli) which are rod shaped.
Fig. 3.12 Shapes of Bacteria Shutterstock: Image ID:162012344
Other shapes include:
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vibrio, which looks a rod with a slight bend
spirillum an “S” or spiral shaped cell
spirochete a corkscrew shaped cell with flagella for motility
Arrangements
Bacteria may exist as single cells, or in arrangements involving multiple cells. These are not
multicellular organisms, and the bacteria within them will survive on their own if removed from
the arrangement.
The arrangements are determined by the way the bacteria reproduce, specifically, the plane on
which the bacteria divide when going through cell division.
The most common bacterial arrangements
Fig. 3.13 Arrangements of Bacterial Cells Image by A. Swarthout
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