THE STRUCTURE OF

BACTERIAL CELL
BACTERIAL STRUCTURES

Essential components Nonessential components

cell wall capsule
cytoplasmic membrane pilus or fimbria
ribosome flagellum
nucleoid spore
mesosome plasmid
periplasm granule
glycocalyx
STRUCTURE OF BACTERIAL CELL
I. NUCLEUS
•A cell's content of genetic material ( DNA) is known as its genome. The
basic molecular structures of prokaryotic and eukaryotic DNA are
indistinguishable. Both are composed of two helical chains of purine and
pyrimidine nucleotides associated with one another by hydrogen bonding
to form a double helix.

•The prokaryotic genome normally consists of a single circular

molecule.

• Further-more, the prokaryotic DNA molecule is found within the

cytoplasm of the cell in a discrete area known as the nucleoid

•Because the absence of a nuclear membrane in prokaryotes, the

physical separation of the mRNA synthesis (transcription) and
translation does not occur in prokaryotes, the two processes often
occur simultaneously and are said to be coupled.
•Some bacteria contain additional DNA molecules beyond the
genomic DNA. These addidonal molecules are referred to as
PLASMIDS - double stranded and normally circular - significantly
smaller in size than the chromosomal DNA. Plasmids are most
commonly found in gram-negative bacteria, and although not usually
essential for cellular survival.
•Many plasmids confer resistance to one or more antibiotics.
Because they can be spread from cell to cell, they contribute to
proliferation of multidrug-resistant bacterial strains, making
treatment of infections caused by these organisms very difficult.

• Bacteria, in contrast, with eukaryotes reproduce by a process

known as binary fission:
• the bacterial cell grows to a genetically predetermined length at
which point a cross-wall is formed -and two new cells are produced.
•coupled with this process is the replication of the chromosome and
any plasmid DNA, so that the daughter cell contains a duplicate of
the parental genome.
II. CELLULAR MEMBRANE

•all cells, both prokaryotic and eukaryotic, are surrounded by the cellular
or cytoplasmic membrane.

•It is composed primarily of phospholipids arranged in two layers. It

functions as a barrier to the leakage of substances out of the cell or
entry of unwanted materials. Embedded in this lipid bilayer are
multiple proteins, which facilitate transport of substances across the
membrane and also carry out various metabolic reactions.

•in contrast with eukaryotic cells which contain multiple internal

organelle within which different cellular processes occur, prokaryotic
cells do not contain any membrane-bound organelles, but here is the
centre of respiratory enzimes-the most important of these is aerobic
respiration by which metabolic energy is produced
Cellular
membrane
 III. CYTOPLASM

• ribosomes (70 S)

• granules (storage areas for nutrients)

• nucleoid (area in cytoplasm where DNA located)
• plasmids (extrachromosomal, ds, circular DNA)
– transmissible
– nontransmissible
Eukaryotic ribosomes vs prokaryotic ribosomes

•Protein synthesis in both bacteria and eukaryodc cells takes

place on ribosomes, which are a complex of protein and
ribonucleic acid (rRNA).
•Eukaryotic ribosomes are somewhat larger and heavier than
their prokaryotic counterparts. The former is referred to as
80S, whereas the latter is termed 70S based on this size
difference and resultant differences in migration in a
centrifugal field.
•This difference in ribosome structure is the mechanism by
which certain antibiotics selectively inhibit protein
synthesis by prokaryotic but not eukaryotic ribosomes.
•Bacteria, do not possess mitochondria, but here are
localized some enzymes to carry out these same functions.
 Cell Envelope

• Cell membrane + cell wall (+ plus outer membrane)

• Cell wall
 peptidoglycan
 attached structures
Gram positive
Gram negative
AAR bacteria
 Gram stain (+ or -) correlates with structure
IV. CELL WALL
IV.1 PEPTIDOGLICAN
•one of these structures unique to bacteria and upon which classification
is based is the peptidoglycan of prokaryotic cell walls. With the
exception of the Archaeobacteria (which contain a pseudoglycan or
pseudomurein related to peptidoglycan) and mycoplasmas (which have
no cell walls at all) the cytoplasmic membranes of all prokaryotes are
surrounded by a rigid peptidoglycan layer.

• the peptidoglycan provides rigidity and it also determines the shape

of the particular bacterial cell!!!

•Chemically: a regular heteropolymer of alternating molecules of N-

acetylglucoseamine (GlcNAc) and N-acetylmuramic acid (MurNAc).

•The individual chains are cross-linked via short peptides consisting

of L-alanine, D-glutamic acid, meso-diaminopimelic acid (DAP) or
lysine, and D-alanine.
•This cross-linking of the long individual chains forms a
strong, rigid network providing an integral component of
bacterial cell walls.

•The β-lactam antibiotics (penicillins and cephalosporins)

inhibit peptidoglycan synthesis. Thus the interruption of the
integrity of the peptidoglycan usually results in death of the
bacterial cell.

•Biologically, peptidoglycan interferes with phagocytosis, is

mitogenic (stimulates mitosis of lymphocytes), and has
pyrogenic activity (induces fever).

• The peptidoglycan of some streptococcal species contributes

to the establishment of the chronic inflammatory response
in a rheumatoid arthritis and rheumatic fever model system.
 Peptidoglycan
• single macromolecule glycan backbone
muramic acid
• highly cross-linked glucosamine
peptide side chain
• surrounds cell peptide cross-bridge
D-and L- amino acids
• provides rigidity diaminopimelic acid

Muramic acid, D-amino acids, diaminopimelic acid

– not synthesized by mammals
 Cell wall

- peptidoglican
 Oxidative phosphorylation occurs at cell membrane
(since there are no mitochondria).

Cell Wall

Cytoplasm Cell membrane

The cell wall is outside of the cell membrane

– rigid, protecting cell from osmotic lysis.
IV.2.1 Gram-positive Cell Wall
•both gram-positive and gram-negative bacteria contain
peptidoglycan in their cell walls (multiple layers are often
cross-linked in three dimensions, providing a very strong,
rigid cell wall); the two types of cell walls are very different.
•First, the amount of peptidoglycan is quite different in that
the peptidoglycan of gram-positive cell walls is usually
30 to 200 molecules thick, providing 40% to 80% of the
total cell wall dry weight. In contrast, the peptidoglycan in
gram-negative cell walls is usually only one molecule
(layer) thick.
•Gram-positive cell walls also contain so called teichoic
acids, which are common surface antigens and function
as structures that attach to other bacteria, as well as to
specific receptors on mammalian cell surfaces. These
teichoic acids are important factors in virulence.
The Cell Envelope

Gram Positive Gram Negative

IV.II.2 Gram-negative Cell Wall
• are much more complex, both structurally and chemically.
•structurally, the gram-negative cell wall contains two layers external to
the cytoplasmic membrane.
• immediately external to the CM is a thin peptidoglycan layer,
which accounts for only 5% to 10% of the gram-negative cell wall
by weight. It is unusual for this layer to contain interpeptide bridges;
thus it is much less rigid than the gram-positive peptidoglycan.
•external to the peptidoglycan is the outer membrane
•the area between the external surface of the CM and the
internal face of the outer membrane is referred to as the
periplasmic space. This space is containing:
 hydrolytic enzymes - breakdown of large macromolecules into smaller
products that the cell uses for metabolism (proteases, phosphatases, lipases,
nucleases, carbohydrate-degrading).
many of the lytic virulence factors (collagenases, hyaluronidases, and
proteases) - in the case of pathogenic gram-negative species
binding proteins, (PBPs)which bind specific molecules (penicillin) for
Outer Membrane

external to the peptidoglican

it is unique to gram-negative prokaryotes.
functions by regulating the passage of molecules
greater than 700 daltons (d) into the cell.
This membrane is a typical lipid bilayer but differs from
any other biological membrane in its lipid composition and
number of proteins.
The inner lipid membrane contains phospholipids
that are normally found in bacterial membranes.
The outer one is composed by a so called
lipopolysaccharide (LPS). Because of its toxic and
other biological properties this molecule was originally
named the endotoxin. This is the only location where
LPS molecules are found.
Importantly, many of the proteins of the outer membrane
traverse the entire lipid bilayer and are trans-membrane
proteins.
A group of these proteins are known as porins because
they form pores (channals) through the membrane that
allow the diffusion of hydrophilic molecules through the
membrane.
The outer membrane also contains integral proteins,
which, maintain the integrity of the outer membrane, as
well as peripheral proteins or surface proteins, some of
which are receptor molecules for bacteriophages.

 The association between the outer membrane and the

peptidoglycan is maintained by a lipoprotein, which is an
intrinsic protein of the outer membrane and is linked to the
peptidoglycan.
Outer Membrane

Gram negative bacteria

 major permeability barrier
 space between inner membranes
periplasmic space
store degradative enzymes
 Gram positive bacteria
 no periplasmic space
enzymes secreted
extracellular digestion
Lipopolysaccharide (LPS)
The LPS molecule is composed of three main regions:
(1) the hydrophobic lipid A
-consists of a glucosamine disaccharide whose hydroxyl groups are
esterified with β-hydroxy fatty acids, the only β-hydroxy acids found in
prokaryotes.
(2) the core polysaccharide - links the lipid A region to
the 0-antigen polysaccharide and
(3) the hydrophilic O-antigen or O-specific
polysaccharide- consists of repeating units of a tetrasaccharide or
pentasaccharide subunit. The specific composition of these repeating
units varies, producing the 0-somatic antigen on which serotyping of
many different gram-negative species is based (there are hundreds of
different serotypes of Salmonella based on the 0-antigen composition.
-All chemical forms of LPS that have very little or no O-antigen are
called "rough" LPS molecules (R), as opposed "smooth" molecules(S).
•the presence of the O-antigen is important in resistance to
certain antibiotics and may be important for survival in
the host.
•LPS is a heat stable toxin (even stable to autoclaving), which is
liberated from gram-negative bacteria upon lysis and cell death. This
endotoxic activity is associated with the lipid A component and
produces a broad spectrum of pathophysiological effects.
•In lesser amounts, LPS functions as an activator of many
inflammatory mediators including activation of the complement
cascade by the alternate pathway, and activation of tumor necrosis
factor, interleukin 1, and prostaglandins.
•When present in sufficient quantities in the blood, LPS will cause
death within 1 or 2 hours because of irreversible shock and
cardiovascular collapse. In addition, it is pyrogenic, causes platelet
aggregation, increases resistance to certain antibiotics, causes
resistance to phagocytosis, and plays a role in bone resorption.
•LPS of GNB is among the most active components of prokaryotes!!!
Structure of Lipopolysaccharide
GRAM NEGATIVE
CELL ENVELOPE
Outer Membrane
(Major permeability barrier) Lipopolysaccharide
Porin

Braun lipoprotein

degradative enzyme

Inner (cytoplasmic) membrane

Cytoplasm
GRAM POSITIVE CELL ENVELOPE
secrete exoenzymes and perform extracellular digestion

Lipoteichoic acid Peptidoglycan-teichoic acid

Cytoplasmic membrane

Cytoplasm
Gram negative Gram positive
 GRAM POSITIVE
Lipoteichoic acid Peptidoglycan-teichoic acid

Cytoplasmic membrane

Cytoplasm

GRAM NEGATIVE Porin

Lipopolysaccharide

Outer Membrane Braun lipoprotein

Inner (cytoplasmic) membrane

Cytoplasm
 IV.II.3 Acid fast -AAR -and related
(mycobacteria, nocardia)

• Ziehl Nielsen stain (AAR)

• peptidoglycan
– mycolic acid-polysaccharide

• Outer membrane
– complex lipids (mycolic acid)
SPECIALIZED STRUCTURES OUTSIDE THE CELL WALL

 CAPSULE
 FLAGELLA
 PILI (FMBRIAE)
 GLYCOCALYX (SLIME LAYER)
Capsules and Slimes
-pathogenic and commensal bacteria are capable of producing amounts
of extracellular slime or capsular material.
•When this material is closely associated with the cell surface, it is
referred to as a capsule.
•In cases when it is loosely adherent and nonuniform in density or
thickness, the material is referred to as a slime layer.
-Except for the case of Bacillus anthracis, which produces a polypeptide
capsule, all other host-associated bacteria studied to date produce
polysaccharidc capsules or slimes. These capsules and slimes are
extremely hydrated;
-Capsules and slimes are known as accessory structures because they are
not required for cell growth in vitro. Interestingly most bacterial species
produce capsules or slimes when first cultured from a host. Upon
successive transfers, many of the isolates will no longer produce these
materials (possibly because the presence of the capsule on the cell
surface offers no selective advantage in normal in vitro situations).
•The primary function of most capsules and slime layers is to
serve as -antiphagocytic structures. Importantly, in mixed
infections, the presence of a capsule or a slime layer on one
bacterial species may protect neighboring unrelated bacteria
from phagocytosis.
-may also protect the cell from other hazards of the
environment including the presence of antibiotics (strains of P.
aeruginosa). Thus higher concentrations of antibiotics are
often required for treatment.
-In some cases bacterial capsules also function as
adhesins and provide the specific adherence interactions
between the bacterial cell and the host tissues or between the
bacterial cell and other bacterial cells (Str. mutans- the
etiological agent of dental caries). Because this tight adherence
to the tooth surface is required for caries formation, the
capsules are considered virulence factors.
 Capsules and slime layers

 outside cell envelope

 well defined: capsule
 not defined: slime layer or
glycocalyx
 usually polysaccharide
 often lost on in vitro culture
 protective in vivo
SURFACE APPENDAGES -FLAGELLA

•are helically coiled - organelles of motility that extend outward

from the cell surface.
• embedded in cell membrane
• flagellin (protein) subunits
• bacterial species may have one or several flagella on their
surface. The arrangement and number of flagella on the bacterial
surface is characteristic of a particular species, flagellar
arrangement is often used as a means of taxonomic
classification.
•until recently, no relationship between motility and virulence
was known.
• it is now apparent that motility and thus the presence of
functioning flagella is a virulence factor for some bacteria.
Pili (fimbriae)

Fimbriae (Latin for "fringe"), or the original term pili, are hairlike
projections on the bacterial cell surface. Pili were originally described as
protein projections on gram-negative cell surfaces.
Fimbriae can be distinguished from flagella morphologically because
they are smaller in diameter 3 to 8 nm vs 15 to 20 nm and usually not
coiled in structure.
 because gram-positive bacteria may also have hairlike projections and
these may also contain carbohydrates, the term fimbriae is now used to
denote any nonflagellar hairlike projection.
The term pili is presently reserved for the appendages of gram-negative
bacterial species involved in the formation of conjugal pairs (sexual
conjugation)for the transfer of DNA between bacterial cells via
conjugation.
There are generally several hundred fimbriae arranged peritrichously
(uniformly) over the entire surface of the bacterial cell. They may be as
long as 15 to 20 μm, or many times the length of the cell.
The primary function of fimbriae is to mediate adherence (adhesion
to host epithelium) of the bacterial cell to other bacteria, to mammalian
cells, or to hard and soft surfaces. Fimbriae are very specific in their
attachment to other surfaces because they interact with only certain
sugars.
they are often considered virulence factors.
The presence of fimbriae is a characteristic most common among
pathogenic bacteria of the mucosal surfaces.
As such, urinary tract pathogens such as Escherichia coli typically
are fimbriated. The presence of specific fimbriae is a requirement for
E. coli to colonize and infect the urinary tract.
 One of the strongest cases for the direct relationship of fimbriation
and virulence is Neisseria gonorrhoeae. Virulent strains of N.
gonorrhoeae are fimbriated and are able to adhere to genital tract
mucosal surfaces.
SPORES

 highly resistant to heating

 highly resitant to many disinfectants
 produced only by Bacillus and Clostridium
 nutrition deprivation
 survive for many years (soil)
 no metabolic activity
 contain dipicolinic acid calcium chelator unique
in the bilogical world
 The endospore

exosporium
coat

outer
membrane

cortex Inner
membrane
Spore position