The Intricate Structure and Composition of Bacterial Cells: Unveiling the Complicated Constructions That Underlie Microbial Life
SHAPE & SIZE OF BACTERIA
Micro organism are labeled by form into three fundamental teams: cocci,
bacilli, and spirochetes (Determine 2–1). The cocci are spherical,
the bacilli are rods, and the spirochetes are spiral-shaped. Some
micro organism are variable in form and are stated to be pleomorphic
(heterogeneous form). The form of a bacterium is set
by its inflexible cell wall. The microscopic look of a bacterium
is without doubt one of the most essential standards utilized in its identification.
Along with their attribute shapes, the association
of micro organism is essential. For instance, sure cocci happen in
pairs (diplococci), some in chains (streptococci), and others
in grapelike clusters (staphylococci). These preparations are
decided by the orientation and diploma of attachment of the
micro organism on the time of cell division. The association of rods
and spirochetes is medically much less essential and isn’t described
on this introductory chapter.
Micro organism vary in measurement from about 0.2 to five μm (Determine 2–2).
The smallest micro organism (Mycoplasma) are about the identical measurement as
the biggest viruses (poxviruses) and are the smallest organisms
able to present exterior a number. The longest micro organism rods are
the dimensions of some yeasts and human crimson blood cells (7 μm).
STRUCTURE OF BACTERIA
The construction of a typical bacterium is illustrated in Determine 2–3,
and the essential options of every element are offered in
The cell wall is the outermost element frequent to all bac-
teria (besides Mycoplasma species, that are bounded by a cell
membrane, not a cell wall). Some micro organism have floor options
exterior to the cell wall, similar to capsule, flagella, and pili, which
are much less frequent elements and are mentioned subsequent.
The cell wall is situated exterior to the cytoplasmic mem-
brane and consists of peptidoglycan (see web page 6). The
peptidoglycan supplies structural help and maintains the
attribute form 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 form (e.g., Neisseria; A-4). B: Rods (bacilli): with sq.
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. twentieth ed.
Initially printed by Appleton & Lange. Copyright 1992, McGraw-Hill.)
Cell Partitions of Gram-Optimistic and Gram-Damaging
The construction, chemical composition, and thickness of the
cell wall differ in gram-positive and gram-negative micro organism
(Desk 2–2, Determine 2–4A, and “Gram Stain” field).
(1) The peptidoglycan layer is way thicker in gram-positive
than in gram-negative micro organism. Many gram-positive micro organism
even have fibers of teichoic acid that protrude exterior the pepti-
doglycan, whereas gram-negative micro organism don’t have teichoic
(2) In distinction, the gram-negative micro organism have a posh
outer layer consisting of lipopolysaccharide, lipoprotein, and
phospholipid. Mendacity between the outer-membrane layer and the
cytoplasmic membrane in gram-negative micro organism is the
periplasmic house, which is the location, in some species, of
enzymes referred to as β-lactamases that degrade penicillins and different
The cell wall has a number of different essential properties:
(1) In gram-negative micro organism, it incorporates endotoxin, a lipo-
polysaccharide (see pages 8 and 43).
(2) Its polysaccharides and proteins are antigens which might be use-
ful in laboratory identification.
(3) Its porin proteins play a job in facilitating the passage of
small, hydrophilic molecules into the cell. Porin proteins within the
outer membrane of gram-negative micro organism act as a channel to
enable the entry of important substances similar to sugars, amino
acids, nutritional vitamins, and metals in addition to many antimicrobial medicine
similar to penicillins.
Cell Partitions of Acid-Quick Micro organism
Mycobacteria (e.g., Mycobacterium tuberculosis) have an uncommon
cell wall, ensuing of their lack of ability to be Gram-stained (Determine
2–4B). These micro organism are stated to be acid-fast as a result of they
resist decolorization with acid–alcohol after being stained with
carbolfuchsin. This property is said to the excessive focus
of lipids, referred to as mycolic acids, within the cell wall of Mycobacteria.
Observe that Nocardia asteroides, a bacterium that may trigger
lung and mind infections in immunocompromised individu-
als, is weakly acid-fast. The that means of the time period “weakly” is
that if the acid-fast staining course of makes use of a weaker resolution of
hydrochloric acid to decolorize than that used within the stain for
Mycobacteria, then N. asteroides won’t decolorize. Nevertheless,
if the regular-strength hydrochloric acid is used, N. asteroides
In view of their significance, three elements of the cell
wall (i.e., peptidoglycan, lipopolysaccharide, and teichoic acid)
are mentioned intimately right here.
Peptidoglycan is a posh, interwoven community that surrounds
the whole cell and consists of a single covalently linked mac-
romolecule. It’s discovered solely in bacterial cell partitions. It supplies inflexible
help for the cell, is essential in sustaining the attribute
form of the cell, and permits the cell to face up to low osmotic
stress. A consultant section of the peptidoglycan layer is
proven in Determine 2–5. The time period peptidoglycan is derived from
the peptides and the sugars (glycan) that make up the molecule.
Synonyms for peptidoglycan are murein and mucopeptide.
Determine 2–5 illustrates the carbohydrate spine, which
consists of alternating N-acetylmuramic acid and N-
acetylglucosamine molecules. Connected to every of the muramic
acid molecules is a tetrapeptide consisting of each d- and
l-amino acids, the exact composition of which differs from
one bacterium to a different. Two of those amino acids are wor-
thy of particular point out: diaminopimelic acid, which is exclusive
to bacterial cell partitions, and d-alanine, which is concerned within the
cross-links between the tetrapeptides and within the motion of peni-
cillin. Observe that this tetrapeptide incorporates the uncommon d-isomers
of amino acids; most proteins comprise the l-isomer. The opposite
essential element on this community is the peptide cross-link
This staining process, developed in 1884 by the Danish
doctor Christian Gram, is a very powerful staining
process in microbiology. It separates most micro organism into
two teams: the gram-positive micro organism, which stain purple,
and the gram-negative micro organism, which stain crimson. The Gram
stain entails the next four-step process:
(1) The crystal violet dye stains all cells purple.
(2) The iodine resolution (a mordant) is added to kind a
crystal violet–iodine advanced; all cells proceed to look
(3) The natural solvent, similar to acetone or ethanol, extracts
the purple dye advanced from the lipid-rich, thin-walled,
gram-negative micro organism to a larger diploma than from the lipid-
poor, thick-walled, gram-positive micro organism. The gram-negative
organisms seem colorless; the gram-positive micro organism stay
(4) The crimson dye safranin stains the decolorized gram-
unfavourable cells crimson/pink; the gram-positive micro organism stay
The Gram stain is beneficial in two methods:
(1) Within the identification of many micro organism.
(2) In influencing the selection of antibiotic as a result of, in gen-
eral, gram-positive micro organism are extra inclined to penicillin
G than are gram-negative micro organism.
Nevertheless, not all micro organism could be seen within the Gram stain.
Desk 2–3 lists the medically essential micro organism that can’t
be seen and describes the explanation why. The choice micro-
scopic strategy to the Gram stain can be described.
Observe that it takes roughly 100,000 micro organism/mL to see
1 bacterium per microscopic area utilizing the oil immersion
(100×) lens. So the sensitivity of the Gram stain process is
low. This explains why a affected person’s blood is never stained
instantly however moderately is incubated in blood cultures over-
evening to permit the micro organism to multiply. One essential excep-
tion to that is meningococcemia during which very excessive
concentrations of Neisseria meningitidis can happen within the
between the 2 tetrapeptides. The cross-links differ amongst spe-
cies; in Staphylococcus aureus, for instance, 5 glycines hyperlink the
terminal d-alanine to the penultimate l-lysine.
As a result of peptidoglycan is current in micro organism however not in
human cells, it’s a good goal for antibacterial medicine. A number of
of those medicine, similar to penicillins, cephalosporins, and van-
comycin, inhibit the synthesis of peptidoglycan by inhibiting
the transpeptidase that makes the cross-links between the 2
adjoining tetrapeptides (see Chapter 10).
Lysozyme, an enzyme current in human tears, mucus, and
saliva, can cleave the peptidoglycan spine by breaking its
glycosyl bonds, thereby contributing to the pure resistance of
the host to microbial an infection. Lysozyme-treated micro organism might
swell and rupture on account of the entry of water into the cells,
which have a excessive inner osmotic stress. Nevertheless, if the
lysozyme-treated cells are in an answer with the identical osmotic
stress as that of the bacterial inside, they are going to survive as
spherical types, referred to as protoplasts, surrounded solely by a cyto-
The lipopolysaccharide (LPS) of the outer membrane of the cell
wall of gram-negative micro organism is endotoxin. It’s accountable for
most of the options of illness, similar to fever and shock (espe-
cially hypotension), attributable to these organisms (see web page 43). It
known as endotoxin as a result of it’s an integral a part of the cell wall,
in distinction to exotoxins, that are actively secreted from the
micro organism. The constellation of signs attributable to the endo-
toxin of 1 gram-negative bacterium is just like one other, however
the severity of the signs can differ enormously. In distinction,
FIGURE 2–6 Endotoxin (lipopolysaccharide [LPS]) construction.
The O-antigen polysaccharide is uncovered on the outside of the cell,
whereas the lipid A faces the inside. (Reproduced with permission from
Brooks GF et al. Medical Microbiology. nineteenth ed. Initially printed by Appleton &
Lange. Copyright 1991, McGraw-Hill.)
the signs attributable to exotoxins of various micro organism are usu-
ally fairly completely different.
The LPS consists of three distinct items (Determine 2–6):
(1) A phospholipid referred to as lipid A, which is accountable for
the poisonous results.
(2) A core polysaccharide of 5 sugars linked by way of keto-
deoxyoctulonate (KDO) to lipid A.
(3) An outer polysaccharide consisting of as much as 25 repeating
items of three to 5 sugars. This outer polymer is the impor-
tant somatic, or O, antigen of a number of gram-negative micro organism
that’s used to establish sure organisms within the scientific labora-
tory. Some micro organism, notably members of the genus Neisseria,
have an outer lipooligosaccharide (LOS) containing only a few
repeating items of sugars.
Teichoic acids are fibers situated within the outer layer of the
gram-positive cell wall and prolong from it. They’re composed
of polymers of both glycerol phosphate or ribitol phosphate.
Some polymers of glycerol teichoic acid penetrate the peptido-
glycan layer and are covalently linked to the lipid within the cyto-
plasmic membrane, during which case they’re referred to as lipoteichoic
acid; others anchor to the muramic acid of the peptidoglycan.
The medical significance of teichoic acids lies of their capability to
induce irritation and septic shock when attributable to sure
gram-positive micro organism; that’s, they activate the identical pathways
as does endotoxin (LPS) in gram-negative micro organism. Teichoic acids
additionally mediate the attachment of staphylococci to mucosal cells.
Gram-negative micro organism don’t have teichoic acids.
Simply contained in the peptidoglycan layer of the cell wall lies the cyto-
plasmic membrane, which consists of a phospholipid bilayer
related in microscopic look to that in eukaryotic cells.
They’re chemically related, however eukaryotic membranes comprise
sterols, whereas prokaryotes usually don’t. The one prokary-
otes which have sterols of their membranes are members of the
genus Mycoplasma. The membrane has 4 essential capabilities:
(1) energetic transport of molecules into the cell, (2) power genera-
tion by oxidative phosphorylation, (3) synthesis of precursors of
the cell wall, and (4) secretion of enzymes and toxins.
The cytoplasm has two distinct areas when seen within the electron
(1) An amorphous matrix that incorporates ribosomes, nutrient
granules, metabolites, and plasmids.
(2) An interior, nucleoid area composed of DNA.
Bacterial ribosomes are the location of protein synthesis as in
eukaryotic cells, however they differ from eukaryotic ribosomes
in measurement and chemical composition. Bacterial ribosomes are
70S in measurement, with 50S and 30S subunits, whereas eukaryotic
ribosomes are 80S in measurement, with 60S and 40S subunits. The dif-
ferences in each the ribosomal RNAs and proteins represent
the idea of the selective motion of a number of antibiotics that inhibit
bacterial, however not human, protein synthesis (see Chapter 10).
The cytoplasm incorporates a number of several types of granules that
function storage areas for vitamins and stain characteristically
with sure dyes. For instance, volutin is a reserve of excessive
power saved within the type of polymerized metaphosphate. It
seems as a “metachromatic” granule because it stains crimson with
methylene blue dye as a substitute of blue as one would anticipate. Meta-
chromatic granules are a attribute function of Corynebacte-
rium diphtheriae, the reason for diphtheria.
The nucleoid is the realm of the cytoplasm during which DNA is
situated. The DNA of most prokaryotes is a single, round mol-
ecule; nevertheless, there are essential exceptions. As an example,
the genome of Vibrio cholerae, the causative agent of cholera, is
composed of two round chromosomes. Borrelia burgdorferi,
the spirochete that causes Lyme illness, consists of a linear
chromosome and a number of round and linear plasmids (see
under). The scale of bacterial genomes varies broadly, with the
smallest genome containing simply over 130 genes and the biggest
containing roughly 11,600 genes. Against this, human
DNA has roughly 25,000 genes.
As a result of the bacterial nucleoid incorporates no nuclear mem-
brane, no nucleolus, no mitotic spindle, and no histones, there
is little resemblance to the eukaryotic nucleus. One main dif-
ference between bacterial DNA and eukaryotic DNA is that
bacterial DNA has no introns, whereas eukaryotic DNA does.
Plasmids are extrachromosomal, double-stranded, round
DNA molecules which might be able to replicating independently
of the bacterial chromosome. Though plasmids are often
extrachromosomal, they are often built-in into the bacterial
chromosome. Plasmids happen in each gram-positive and gram-
unfavourable micro organism, and several other several types of plasmids can
exist in a single cell:
(1) Transmissible plasmids could be transferred from cell to
cell by conjugation (see Chapter 4 for a dialogue of conjuga-
tion). They’re massive (molecular weight [MW] 40–100 million),
since they comprise a couple of dozen genes accountable for synthesis
of the intercourse pilus and for the enzymes required for switch. They
are often current in a couple of (1–3) copies per cell.
(2) Nontransmissible plasmids are small (MW 3–20 million)
since they don’t comprise the switch genes; they’re continuously
current in lots of (10–60) copies per cell.
Plasmids carry the genes for the next capabilities and
constructions of medical significance:
(1) Antibiotic resistance, which is mediated by a wide range of
enzymes, such because the β-lactamase of S. aureus, Escherichia coli,
and Klebsiella pneumoniae.
(2) Exotoxins, such because 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 micro organism
to epithelial cells.
(4) Resistance to heavy metals, similar to mercury, the energetic
element of some antiseptics (e.g., Merthiolate and Mercuro-
chrome), and silver, which is mediated by a reductase enzyme.
(5) Resistance to ultraviolet gentle, which is mediated by DNA
(6) Bacteriocins, that are poisonous proteins produced by
sure micro organism which might be deadly for different micro organism. Two frequent
mechanisms of motion of bacteriocins are (i) degradation of
bacterial cell membranes by producing pores within the membrane
and (ii) degradation of bacterial DNA by DNAse. Examples of
bacteriocins produced by medically essential micro organism are
colicins made by E. coli and pyocins made by Pseudomonas
aeruginosa. Micro organism that produce bacteriocins have a selective
benefit within the competitors for meals sources over those who
don’t. Nevertheless, the medical significance of bacteriocins is that
they could be helpful in treating infections attributable to antibiotic-
resistant micro organism.
Transposons are items of DNA that transfer readily from one
web site to a different both inside or between the DNAs of micro organism,
plasmids, and bacteriophages. Due to their uncommon capability
to maneuver, they’re nicknamed “leaping genes.” Some transpo-
sons transfer by replicating their DNA and inserting the brand new copy
into one other web site (replicative transposition), whereas others
are excised from the location with out replicating after which inserted
into the brand new web site (direct transposition). Transposons can code
for drug-resistant enzymes, toxins, or a wide range of metabolic
enzymes and may both trigger mutations within the gene into which
they insert or alter the expression of close by 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 Rules of Microbiology. New York, NY: McGraw-Hill; 2009.)
Transposons usually have 4 identifiable domains. On
every finish is a brief DNA sequence of inverted repeats, which
are concerned within the integration of the transposon into the recipi-
ent DNA. The second area is the gene for the transposase,
which is the enzyme that mediates the excision and integration
processes. The third area is the gene for the repressor that
regulates the synthesis of each the transposase and the protein
encoded by the fourth area, which, in lots of instances, is an
enzyme mediating antibiotic resistance (Determine 2–7). Observe that
for simplicity, the repressor gene isn’t proven in Determine 2–7.
Antibiotic resistance genes are transferred from one bacte-
rium to a different primarily by conjugation (see Chapter 4). This
switch is mediated primarily by plasmids, however some transpo-
sons, referred to as conjugative transposons, are able to transfer-
ring antibiotic resistance as properly.
In distinction to plasmids or bacterial viruses, transposons
usually are not able to impartial replication; they replicate as
a part of the DNA during which they’re built-in. A couple of
transposon could be situated within the DNA; for instance, a plasmid
can comprise a number of transposons carrying drug-resistant genes.
Insertion sequences are a kind of transposon which have fewer
bases (800–1500 base pairs), since they don’t code for his or her
personal integration enzymes. They will trigger mutations at their web site
of integration and could be present in a number of copies on the ends of
bigger transposon items.
Constructions Outdoors the Cell Wall
The capsule is a gelatinous layer protecting the whole bacterium.
It’s usually composed of polysaccharide. The sugar compo-
nents of the polysaccharide differ from one species of micro organism to
one other and continuously decide the serologic kind (serotype)
inside a species. For instance, there are 91 completely different serotypes
of Streptococcus pneumoniae, that are distinguished by the
antigenic variations of the sugars within the polysaccharide capsule.
The capsule is essential for 4 causes:
(1) It’s a determinant of virulence of many micro organism since
it limits the flexibility of phagocytes to engulf the micro organism. Nega-
tive fees on the capsular polysaccharide repel the negatively
charged cell membrane of the neutrophil and forestall it from
ingesting the micro organism. Variants of encapsulated micro organism which have
misplaced the flexibility to provide a capsule are often nonpathogenic.
(2) Particular identification of an organism could be made by utilizing
antiserum in opposition to the capsular polysaccharide. Within the presence of
the homologous antibody, the capsule will swell enormously. This swell-
ing phenomenon, which is used within the scientific laboratory to iden-
tify sure organisms, known as the Quellung response.
3) Capsular polysaccharides are used because the antigens in cer-
tain vaccines as a result of they’re able to eliciting protecting
antibodies. For instance, the purified capsular polysaccharides
of 23 varieties of S. pneumoniae are current within the present vaccine.
(4) The capsule might play a job within the adherence of micro organism
to human tissues, which is a vital preliminary step in inflicting
Flagella are lengthy, whiplike appendages that transfer the micro organism
towards vitamins and different attractants, a course of referred to as chemo-
taxis. The lengthy filament, which acts as a propeller, consists
of many subunits of a single protein, flagellin, organized in sev-
eral intertwined chains. The power for motion, the proton
driver, is offered by adenosine triphosphate (ATP),
derived from the passage of ions throughout the membrane.
Flagellated micro organism have a attribute quantity and loca-
tion of flagella: some micro organism have one, and others have many;
in some, the flagella are situated at one finish, and in others, they
are all around the outer floor. Solely sure micro organism have flagella.
Many rods do, however most cocci don’t and are subsequently nonmo-
tile. Spirochetes transfer by utilizing a flagellum-like construction referred to as
the axial filament, which wraps across the spiral-shaped cell to
produce an undulating movement.
Flagella are medically essential for 2 causes:
(1) Some species of motile micro organism (e.g., E. coli and Proteus
species) are frequent causes of urinary tract infections. Flagella
might play a job in pathogenesis by propelling the micro organism up the
urethra into the bladder.
(2) Some species of micro organism (e.g., Salmonella species) are
recognized within the scientific laboratory by way of particular anti-
our bodies in opposition to flagellar proteins.
Pili are hairlike filaments that reach from the cell floor. They
are shorter and straighter than flagella and are composed of
subunits of pilin, a protein organized in helical strands. They’re
discovered primarily on gram-negative organisms.
Pili have two essential roles:
(1) They mediate the attachment of micro organism to particular
receptors on the human cell floor, which is a vital step
within the initiation of an infection for some organisms. Mutants of
Neisseria gonorrhoeae that don’t kind pili are nonpathogens.
(2) A specialised form of pilus, the intercourse pilus, types the
attachment between the male (donor) and the feminine (recipient)
micro organism throughout conjugation (see Chapter 4).
FIGURE 2–8 Bacterial spores. The spore incorporates the whole DNA genome of the bacterium surrounded by a thick, resistant coat.
Glycocalyx (Slime Layer)
The glycocalyx is a polysaccharide coating that’s secreted by
many micro organism. It covers surfaces like a movie and permits the bac-
teria to stick firmly to numerous constructions (e.g., pores and skin, coronary heart
valves, prosthetic joints, and catheters). The glycocalyx is an
essential element of biofilms (see web page 36). The medical
significance of the glycocalyx is illustrated by the discovering that it
is the glycocalyx-producing strains of P. aeruginosa that trigger
respiratory tract infections in cystic fibrosis sufferers, and it
is the glycocalyx-producing strains of Staphylococcus epider-
midis and viridans streptococci that trigger endocarditis. The
glycocalyx additionally mediates adherence of sure micro organism to the
floor of enamel. This performs an essential function within the formation
These extremely resistant constructions are fashioned in response to
antagonistic circumstances by two genera of medically essential
gram-positive rods: the genus Bacillus, which incorporates the
agent of anthrax, and the genus Clostridium, which incorporates
the brokers of tetanus and botulism. Spore formation (sporula-
tion) happens when vitamins, similar to sources of carbon and
nitrogen, are depleted (Determine 2–8). The spore types inside
the cell and incorporates bacterial DNA, a small quantity of cyto-
plasm, cell membrane, peptidoglycan, little or no water, and
most significantly, a thick, keratin-like coat that’s accountable
for the exceptional resistance of the spore to warmth, dehydra-
tion, radiation, and chemical compounds. This resistance could also be medi-
ated by dipicolinic acid, a calcium ion chelator discovered solely
As soon as fashioned, the spore has no metabolic exercise and may
stay dormant for a few years. Upon publicity to water and
the suitable vitamins, particular enzymes degrade the coat,
water and vitamins enter, and germination right into a doubtlessly
pathogenic bacterial cell happens. Observe that this differentiation
course of isn’t a way of replica since one cell produces
one spore that germinates into one cell.
The medical significance of spores lies of their extraordinary
resistance to warmth and chemical compounds. On account of their resistance
to warmth, sterilization can’t be achieved by boiling. Steam heat-
ing underneath stress (autoclaving) at 121°C, for at the very least 15 min-
utes, is required to make sure the sterility of merchandise for medical
use. Spores are sometimes not seen in scientific specimens recovered
from sufferers contaminated by spore-forming organisms as a result of the
provide of vitamins is sufficient.
Desk 2–4 describes the medically essential options of bac-