The Intricate Structure and Composition of Bacterial Cells: Unveiling the Complicated Constructions That Underlie Microbial Life

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

Desk 2–1.

Cell Wall

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

Micro organism

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

acids.

(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

β-lactam medicine.

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

will decolorize.

In view of their significance, three elements of the cell

wall (i.e., peptidoglycan, lipopolysaccharide, and teichoic acid)

are mentioned intimately right here.

Peptidoglycan

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

 

GRAM STAIN

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

purple.

(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

purple.

(4) The crimson dye safranin stains the decolorized gram-

unfavourable cells crimson/pink; the gram-positive micro organism stay

purple.

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

blood.

 

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-

plasmic membrane.

Lipopolysaccharide

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 Acid

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.

Cytoplasmic Membrane

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.

Cytoplasm

The cytoplasm has two distinct areas when seen within the electron

microscope:

(1) An amorphous matrix that incorporates ribosomes, nutrient

granules, metabolites, and plasmids.

(2) An interior, nucleoid area composed of DNA.

Ribosomes

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).

Granules

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.

Nucleoid

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

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

restore enzymes.

(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

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

Capsule

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

an infection.

Flagella

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 (Fimbriae)

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

of plaque.

Bacterial Spores

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

in spores.

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-

terial spores.