The Intricate Cellular Structure of Bacteria: Unveiling the Complexities Within Bacterial Cells
The Intricate Cellular Structure of Bacteria: Unveiling the Complexities Within Bacterial Cells
SHAPE & SIZE OF BACTERIA
Bacteria are classified by shape into three basic groups: cocci,
bacilli, and spirochetes (Figure 2–1). The cocci are round,
the bacilli are rods, and the spirochetes are spiral-shaped. Some
bacteria are variable in shape and are said to be pleomorphic
(heterogeneous shape). The ni, shape of a bacterium is determined
by its rigid cell wall. The microscopic appearance of a bacterium
is one of the most important criteria used in its identification.
In addition to their characteristic shapes, the arrangement
of bacteria is important. For example, certain cocci occur in
pairs (diplococci), some in chains (streptococci), and others
in grapelike clusters (staphylococci). These arrangements are
determined by the orientation and degree of attachment of the
bacteria at the time of cell division. The arrangement of rods
and spirochetes is medically less important and is not described
in this introductory chapter.
Bacteria range in size from about 0.2 to 5 μm (Figure 2–2).
The smallest bacteria (Mycoplasma) are about the same size as
the largest viruses (poxviruses) and are the smallest organisms
capable of existing outside a host. The longest bacteria rods are
the size of some yeasts and human red blood cells (7 μm).
STRUCTURE OF BACTERIA
The structure of a typical bacterium is illustrated in Figure 2–3,
and the important features of each component are presented in
Table 2–1.,
Cell Wall
The cell wall is the outermost component common to all bac-
teria (except Mycoplasma species, which are bounded by a cell
membrane, not a cell wall). Some bacteria have surface features
external to the cell wall, such as capsule, flagella, and pili, which
are less common components and are discussed next.
The cell wall is located external to the cytoplasmic mem-
brane and is composed of peptidoglycan (see page 6). The
peptidoglycan provides structural support and maintains the
characteristic shape of the cell.
FIGURE 2–1 Bacterial morphology. A: Cocci in clusters (e.g.,
Staphylococcus; A-1); in chains (e.g., Streptococcus; A-2); in pairs with
pointed ends (e.g., Streptococcus pneumoniae; A-3); in pairs with
kidney bean shape (e.g., Neisseria; A-4). B: Rods (bacilli): with square
ends (e.g., Bacillus; B-1); with rounded ends (e.g., Salmonella; B-2);
club-shaped (e.g., Corynebacterium; B-3); fusiform (e.g., Fusobacte-
rium; B-4); comma-shaped (e.g., Vibrio; B-5). C: Spirochetes: relaxed
coil (e.g., Borrelia; C-1); tightly coiled (e.g., Treponema; C-2).
(Reproduced with permission from Joklik WK et al. Zinsser Microbiology. 20th ed.
Originally published by Appleton & Lange. Copyright 1992, McGraw-Hill.)
Cell Walls of Gram-Positive and Gram-Negative
Bacteria
The structure, chemical composition, and thickness of the
cell wall differ in gram-positive and gram-negative bacteria
(Table 2–2, Figure 2–4A, and “Gram Stain” box).
(1) The peptidoglycan layer is much thicker in gram-positive
than in gram-negative bacteria. Many gram-positive bacteria
also have fibers of teichoic acid that protrude outside the pepti-
doglycan, whereas gram-negative bacteria do not have teichoic
acids.
(2) In contrast, the gram-negative bacteria have a complex
outer layer consisting of lipopolysaccharide, lipoprotein, and
phospholipid. Lying between the outer-membrane layer and the
cytoplasmic membrane in gram-negative bacteria is the
periplasmic space, which is the site, in some species, of
enzymes called β-lactamases that degrade penicillins and other
β-lactam drugs.
The cell wall has several other important properties:
(1) In gram-negative bacteria, it contains endotoxin, a lipo-
polysaccharide (see pages 8 and 43).
(2) Its polysaccharides and proteins are antigens that are use-
ful in laboratory identification.
(3) Its porin proteins play a role in facilitating the passage of
small, hydrophilic molecules into the cell. Porin proteins in the
outer membrane of gram-negative bacteria act as a channel to
allow the entry of essential substances such as sugars, amino
acids, vitamins, and metals as well as many antimicrobial drugs
such as penicillins.
Cell Walls of Acid-Fast Bacteria
Mycobacteria (e.g., Mycobacterium tuberculosis) have an unusual
cell wall, resulting in their inability to be Gram-stained (Figure
2–4B). These bacteria are said to be acid-fast because they
resist decolorization with acid–alcohol after being stained with
carbolfuchsin. This property is related to the high concentration
of lipids, called mycolic acids, in the cell wall of Mycobacteria.
Note that Nocardia asteroides, a bacterium that can cause
lung and brain infections in immunocompromised individu-
als, is weakly acid-fast. The meaning of the term “weakly” is
that if the acid-fast staining process uses a weaker solution of
hydrochloric acid to decolorize than that used in the stain for
Mycobacteria, then N. asteroides will not decolorize. However,
if the regular-strength hydrochloric acid is used, N. asteroides
will decolorize.
In view of their importance, three components of the cell
wall (i.e., peptidoglycan, lipopolysaccharide, and teichoic acid)
are discussed in detail here.
Peptidoglycan
Peptidoglycan is a complex, interwoven network that surrounds
the entire cell and is composed of a single covalently linked mac-
romolecule. It is found only in bacterial cell walls. It provides rigid
support for the cell, is important in maintaining the characteristic
shape of the cell, and allows the cell to withstand low osmotic
pressure. A representative segment of the peptidoglycan layer is
shown in Figure 2–5. The term peptidoglycan is derived from
the peptides and the sugars (glycan) that make up the molecule.
Synonyms for peptidoglycan are murein and mucopeptide.
Figure 2–5 illustrates the carbohydrate backbone, which
is composed of alternating N-acetylmuramic acid and N-
acetylglucosamine molecules. Attached to each of the muramic
acid molecules is a tetrapeptide consisting of both d- and
l-amino acids, the precise composition of which differs from
one bacterium to another. Two of these amino acids are wor-
thy of special mention: diaminopimelic acid, which is unique
to bacterial cell walls, and d-alanine, which is involved in the
cross-links between the tetrapeptides and in the action of peni-
cillin. Note that this tetrapeptide contains the rare d-isomers
of amino acids; most proteins contain the l-isomer. The other
important component in this network is the peptide cross-link
FIGURE 2–4 A: Cell walls of gram-positive and gram-negative bacteria. Note that the peptidoglycan in gram-positive bacteria is much
thicker than in gram-negative bacteria. Note also that only gram-negative bacteria have an outer membrane containing endotoxin (lipopolysac-
charide [LPS]) and thus have a periplasmic space where β-lactamases are found. Several important gram-positive bacteria, such as staphylococci
and streptococci, have teichoic acids. (Reproduced with permission from Ingraham JL, Maaløe O, Neidhardt FC. Growth of the Bacterial Cell. Sinauer Associates; 1983.)
B: Cell wall of Mycobacterium tuberculosis: Note the layers of mycolic acid and arabinoglycan that are present in members of the genus
Mycobacterium but not in most other genera of bacteria.
between the two tetrapeptides. The cross-links vary among spe-
cies; in Staphylococcus aureus, for example, five glycines link the
terminal d-alanine to the penultimate l-lysine.
Because peptidoglycan is present in bacteria but not in
human cells, it is a good target for antibacterial drugs. Several
of these drugs, such as penicillins, cephalosporins, and van-
comycin, inhibit the synthesis of peptidoglycan by inhibiting
the transpeptidase that makes the cross-links between the two
adjacent tetrapeptides (see Chapter 10).
Lysozyme, an enzyme present in human tears, mucus, and
saliva, can cleave the peptidoglycan backbone by breaking its
glycosyl bonds, thereby contributing to the natural resistance of
the host to microbial infection. Lysozyme-treated bacteria may
swell and rupture as a result of the entry of water into the cells,
which have a high internal osmotic pressure. However, if the
lysozyme-treated cells are in a solution with the same osmotic
pressure as that of the bacterial interior, they will survive as
spherical forms, called protoplasts, surrounded only by a cyto-
plasmic membrane.
Lipopolysaccharide
The lipopolysaccharide (LPS) of the outer membrane of the cell
wall of gram-negative bacteria is endotoxin. It is responsible for
many of the features of disease, such as fever and shock (espe-
cially hypotension), caused by these organisms (see page 43). It
is called endotoxin because it is an integral part of the cell wall,
in contrast to exotoxins, which are actively secreted from the
bacteria. The constellation of symptoms caused by the endo-
toxin of one gram-negative bacterium is similar to another, but
the severity of the symptoms can differ greatly. In contrast,
FIGURE 2–6 Endotoxin (lipopolysaccharide [LPS]) structure.
The O-antigen polysaccharide is exposed on the exterior of the cell,
whereas the lipid A faces the interior. (Reproduced with permission from
Brooks GF et al. Medical Microbiology. 19th ed. Originally published by Appleton &
Lange. Copyright 1991, McGraw-Hill.)
the symptoms caused by exotoxins of different bacteria are usu-
ally quite different.
The LPS is composed of three distinct units (Figure 2–6):
(1) A phospholipid called lipid A, which is responsible for
the toxic effects.
(2) A core polysaccharide of five sugars linked through keto-
deoxyoctulonate (KDO) to lipid A.
(3) An outer polysaccharide consisting of up to 25 repeating
units of three to five sugars. This outer polymer is the impor-
tant somatic, or O, antigen of several gram-negative bacteria
that is used to identify certain organisms in the clinical labora-
tory. Some bacteria, notably members of the genus Neisseria,
have an outer lipooligosaccharide (LOS) containing very few
repeating units of sugars.
Teichoic Acid
Teichoic acids are fibers located in the outer layer of the
gram-positive cell wall and extend from it. They are composed
of polymers of either glycerol phosphate or ribitol phosphate.
Some polymers of glycerol teichoic acid penetrate the peptido-
glycan layer and are covalently linked to the lipid in the cyto-
plasmic membrane, in which case they are called lipoteichoic
acid; others anchor to the muramic acid of the peptidoglycan.
The medical importance of teichoic acids lies in their ability to
induce inflammation and septic shock when caused by certain
gram-positive bacteria; that is, they activate the same pathways
as does endotoxin (LPS) in gram-negative bacteria. Teichoic acids
also mediate the attachment of staphylococci to mucosal cells.
Gram-negative bacteria do not have teichoic acids.
Cytoplasmic Membrane
Just inside the peptidoglycan layer of the cell wall lies the cyto-
plasmic membrane, which is composed of a phospholipid bilayer
similar in microscopic appearance to that in eukaryotic cells.
They are chemically similar, but eukaryotic membranes contain
sterols, whereas prokaryotes generally do not. The only prokary-
otes that have sterols in their membranes are members of the
genus Mycoplasma. The membrane has four important functions:
(1) active transport of molecules into the cell, (2) energy genera-
tion by oxidative phosphorylation, (3) synthesis of precursors of
the cell wall, and (4) secretion of enzymes and toxins.
Cytoplasm
The cytoplasm has two distinct areas when seen in the electron
microscope:
(1) An amorphous matrix that contains ribosomes, nutrient
granules, metabolites, and plasmids.
(2) An inner, nucleoid region composed of DNA.
Ribosomes
Bacterial ribosomes are the site of protein synthesis as in
eukaryotic cells, but they differ from eukaryotic ribosomes
in size and chemical composition. Bacterial ribosomes are
70S in size, with 50S and 30S subunits, whereas eukaryotic
ribosomes are 80S in size, with 60S and 40S subunits. The dif-
ferences in both the ribosomal RNAs and proteins constitute
the basis of the selective action of several antibiotics that inhibit
bacterial, but not human, protein synthesis (see Chapter 10).
Granules
The cytoplasm contains several different types of granules that
serve as storage areas for nutrients and stain characteristically
with certain dyes. For example, volutin is a reserve of high
energy stored in the form of polymerized metaphosphate. It
appears as a “metachromatic” granule since it stains red with
methylene blue dye instead of blue as one would expect. Meta-
chromatic granules are a characteristic feature of Corynebacte-
rium diphtheriae, the cause of diphtheria.
Nucleoid
The nucleoid is the area of the cytoplasm in which DNA is
located. The DNA of most prokaryotes is a single, circular mol-
ecule; however, there are important exceptions. For instance,
the genome of Vibrio cholerae, the causative agent of cholera, is
composed of two circular chromosomes. Borrelia burgdorferi,
the spirochete that causes Lyme disease, is composed of a linear
chromosome and multiple circular and linear plasmids (see
below). The size of bacterial genomes varies widely, with the
smallest genome containing just over 130 genes and the largest
containing approximately 11,600 genes. By contrast, human
DNA has approximately 25,000 genes.
Because the bacterial nucleoid contains no nuclear mem-
brane, no nucleolus, no mitotic spindle, and no histones, there
is little resemblance to the eukaryotic nucleus. One major dif-
ference between bacterial DNA and eukaryotic DNA is that
bacterial DNA has no introns, whereas eukaryotic DNA does.
Plasmids
Plasmids are extrachromosomal, double-stranded, circular
DNA molecules that are capable of replicating independently
of the bacterial chromosome. Although plasmids are usually
extrachromosomal, they can be integrated into the bacterial
chromosome. Plasmids occur in both gram-positive and gram-
negative bacteria, and several different types of plasmids can
exist in one cell:
(1) Transmissible plasmids can be transferred from cell to
cell by conjugation (see Chapter 4 for a discussion of conjuga-
tion). They are large (molecular weight [MW] 40–100 million),
since they contain about a dozen genes responsible for synthesis
of the sex pilus and for the enzymes required for transfer. They
are usually present in a few (1–3) copies per cell.
(2) Nontransmissible plasmids are small (MW 3–20 million)
since they do not contain the transfer genes; they are frequently
present in many (10–60) copies per cell.
Plasmids carry the genes for the following functions and
structures of medical importance:
(1) Antibiotic resistance, which is mediated by a variety of
enzymes, such as the β-lactamase of S. aureus, Escherichia coli,
and Klebsiella pneumoniae.
(2) Exotoxins, such as the enterotoxins of E. coli, anthrax
toxin of Bacillus anthracis, exfoliative toxin of S. aureus, and
tetanus toxin of Clostridium tetani.
(3) Pili (fimbriae), which mediate the adherence of bacteria
to epithelial cells.
(4) Resistance to heavy metals, such as mercury, the active
component of some antiseptics (e.g., Merthiolate and Mercuro-
chrome), and silver, which is mediated by a reductase enzyme.
(5) Resistance to ultraviolet light, which is mediated by DNA
repair enzymes.
(6) Bacteriocins, which are toxic proteins produced by
certain bacteria that are lethal for other bacteria. Two common
mechanisms of action of bacteriocins are (i) degradation of
bacterial cell membranes by producing pores in the membrane
and (ii) degradation of bacterial DNA by DNAse. Examples of
bacteriocins produced by medically important bacteria are
colicins made by E. coli and pyocins made by Pseudomonas
aeruginosa. Bacteria that produce bacteriocins have a selective
advantage in the competition for food sources over those that
do not. However, the medical importance of bacteriocins is that
they may be useful in treating infections caused by antibiotic-
resistant bacteria.
Transposons
Transposons are pieces of DNA that move readily from one
site to another either within or between the DNAs of bacteria,
plasmids, and bacteriophages. Because of their unusual ability
to move, they are nicknamed “jumping genes.” Some transpo-
sons move by replicating their DNA and inserting the new copy
into another site (replicative transposition), whereas others
are excised from the site without replicating and then inserted
into the new site (direct transposition). Transposons can code
for drug-resistant enzymes, toxins, or a variety of metabolic
enzymes and can either cause mutations in the gene into which
they insert or alter the expression of nearby genes.
FIGURE 2–7 Transposon genes. This transposon is carrying a
drug-resistance gene. IR, inverted repeat. (Reproduced with permission from
Willey JM et al. Prescott’s Principles of Microbiology. New York, NY: McGraw-Hill; 2009.)
Transposons typically have four identifiable domains. On
each end is a short DNA sequence of inverted repeats, which
are involved in the integration of the transposon into the recipi-
ent DNA. The second domain is the gene for the transposase,
which is the enzyme that mediates the excision and integration
processes. The third region is the gene for the repressor that
regulates the synthesis of both the transposase and the protein
encoded by the fourth domain, which, in many cases, is an
enzyme mediating antibiotic resistance (Figure 2–7). Note that
for simplicity, the repressor gene is not shown in Figure 2–7.
Antibiotic resistance genes are transferred from one bacte-
rium to another primarily by conjugation (see Chapter 4). This
transfer is mediated primarily by plasmids, but some transpo-
sons, called conjugative transposons, are capable of transfer-
ring antibiotic resistance as well.
In contrast to plasmids or bacterial viruses, transposons
are not capable of independent replication; they replicate as
part of the DNA in which they are integrated. More than one
transposon can be located in the DNA; for example, a plasmid
can contain several transposons carrying drug-resistant genes.
Insertion sequences are a type of transposon that have fewer
bases (800–1500 base pairs), since they do not code for their
own integration enzymes. They can cause mutations at their site
of integration and can be found in multiple copies at the ends of
larger transposon units.
Structures Outside the Cell Wall
Capsule
The capsule is a gelatinous layer covering the entire bacterium.
It is typically composed of polysaccharide. The sugar compo-
nents of the polysaccharide vary from one species of bacteria to
another and frequently determine the serologic type (serotype)
within a species. For example, there are 91 different serotypes
of Streptococcus pneumoniae, which are distinguished by the
antigenic differences of the sugars in the polysaccharide capsule.
The capsule is important for four reasons:
(1) It is a determinant of virulence of many bacteria since
it limits the ability of phagocytes to engulf the bacteria. Nega-
tive charges on the capsular polysaccharide repel the negatively
charged cell membrane of the neutrophil and prevent it from
ingesting the bacteria. Variants of encapsulated bacteria that have
lost the ability to produce a capsule are usually nonpathogenic.
(2) Specific identification of an organism can be made by using
antiserum against the capsular polysaccharide. In the presence of
the homologous antibody, the capsule will swell greatly. This swell-
ing phenomenon, which is used in the clinical laboratory to iden-
tify certain organisms, is called the Quellung reaction.
(3) Capsular polysaccharides are used as the antigens in cer-
tain vaccines because they are capable of eliciting protective
antibodies. For example, the purified capsular polysaccharides
of 23 types of S. pneumoniae are present in the current vaccine.
(4) The capsule may play a role in the adherence of bacteria
to human tissues, which is an important initial step in causing
infection.
Flagella
Flagella are long, whiplike appendages that move the bacteria
toward nutrients and other attractants, a process called chemo-
taxis. The long filament, which acts as a propeller, is composed
of many subunits of a single protein, flagellin, arranged in sev-
eral intertwined chains. The energy for movement, the proton
motive force, is provided by adenosine triphosphate (ATP),
derived from the passage of ions across the membrane.
Flagellated bacteria have a characteristic number and loca-
tion of flagella: some bacteria have one, and others have many;
in some, the flagella are located at one end, and in others, they
are all over the outer surface. Only certain bacteria have flagella.
Many rods do, but most cocci do not and are therefore nonmo-
tile. Spirochetes move by using a flagellum-like structure called
the axial filament, which wraps around the spiral-shaped cell to
produce an undulating motion.
Flagella are medically important for two reasons:
(1) Some species of motile bacteria (e.g., E. coli and Proteus
species) are common causes of urinary tract infections. Flagella
may play a role in pathogenesis by propelling the bacteria up the
urethra into the bladder.
(2) Some species of bacteria (e.g., Salmonella species) are
identified in the clinical laboratory by the use of specific anti-
bodies against flagellar proteins.
Pili (Fimbriae)
Pili are hairlike filaments that extend from the cell surface. They
are shorter and straighter than flagella and are composed of
subunits of pilin, a protein arranged in helical strands. They are
found mainly on gram-negative organisms.
Pili have two important roles:
(1) They mediate the attachment of bacteria to specific
receptors on the human cell surface, which is a necessary step
in the initiation of infection for some organisms. Mutants of
Neisseria gonorrhoeae that do not form pili are nonpathogens.
(2) A specialized kind of pilus, the sex pilus, forms the
attachment between the male (donor) and the female (recipient)
bacteria during conjugation (see Chapter 4).
FIGURE 2–8 Bacterial spores. The spore contains the entire DNA genome of the bacterium surrounded by a thick, resistant coat.
Glycocalyx (Slime Layer)
The glycocalyx is a polysaccharide coating that is secreted by
many bacteria. It covers surfaces like a film and allows the bac-
teria to adhere firmly to various structures (e.g., skin, heart
valves, prosthetic joints, and catheters). The glycocalyx is an
important component of biofilms (see page 36). The medical
importance of the glycocalyx is illustrated by the finding that it
is the glycocalyx-producing strains of P. aeruginosa that cause
respiratory tract infections in cystic fibrosis patients, and it
is the glycocalyx-producing strains of Staphylococcus epider-
midis and viridans streptococci that cause endocarditis. The
glycocalyx also mediates adherence of certain bacteria to the
surface of teeth. This plays an important role in the formation
of plaque.
Bacterial Spores
These highly resistant structures are formed in response to
adverse conditions by two genera of medically important
gram-positive rods: the genus Bacillus, which includes the
agent of anthrax, and the genus Clostridium, which includes
the agents of tetanus and botulism. Spore formation (sporula-
tion) occurs when nutrients, such as sources of carbon and
nitrogen, are depleted (Figure 2–8). The spore forms inside
the cell and contains bacterial DNA, a small amount of cyto-
plasm, cell membrane, peptidoglycan, very little water, and
most importantly, a thick, keratin-like coat that is responsible
for the remarkable resistance of the spore to heat, dehydra-
tion, radiation, and chemicals. This resistance may be medi-
ated by dipicolinic acid, a calcium ion chelator found only
in spores.
Once formed, the spore has no metabolic activity and can
remain dormant for many years. Upon exposure to water and
the appropriate nutrients, specific enzymes degrade the coat,
water and nutrients enter, and germination into a potentially
pathogenic bacterial cell occurs. Note that this differentiation
process is not a means of reproduction since one cell produces
one spore that germinates into one cell.
The medical importance of spores lies in their extraordinary
resistance to heat and chemicals. As a result of their resistance
to heat, sterilization cannot be achieved by boiling. Steam heat-
ing under pressure (autoclaving) at 121°C, for at least 15 min-
utes, is required to ensure the sterility of products for medical
use. Spores are often not seen in clinical specimens recovered
from patients infected by spore-forming organisms because the
supply of nutrients is adequate.
Table 2–4 describes the medically important features of bac-
terial spores.
