Bacterial Cell Structure

Microorganisms have a tremendous impact on all life and chemical make up of our planet

Chemical elements cycling Photosynthesis 5×1030 microbial cells 90% of biomass 90% of the cells in our body

Prokaryotes Eukaryotes

Prokaryotes:

Eubacteria

Archaebacteria

Bacterial Cell Structure

• Nucleoid• Cytoplasmic Structures• Cell Envelope• Capsule & Glycocalyx• Flagella• Pili (Fimbriae)• Endospore

The nucleoid

No true nucleus

Nucleoid

absence of mitotic apparatus

Binary fission

single circular DNA molecule

Haploid

Exceptions:

Planktomycetes (aquatic bacteria) have two nuclear membranes

A few bacteria have 2, 3 or 4 dissimilar chromosomes

Vibrio cholerae and Brucella melitensis have 2 dissimilar chromosomes

Some prokaryotes have a linear chromosome Borrelia burgdorferi and several Streptomyces spp.

Cytoplasmic Structures

reserve materials store in the form of insoluble granules

Inclusion bodies

Bounded by a thin nonunit membrane consisting of lipids

One of the most common inclusion bodies consists of poly β-hydroxybutyric acid (PBH)

when the source of nitrogen, sulfate, or phosphorous is limited, or when the pH is low

And there is excess carbon

starch and glycogen ( polymers of glucose )

sulfur granules

polyphosphate granules (reserves of inorganic phosphate) volutin granules or metachromatic

granules (Corynebacteria)

protein bounded vesicles:

carboxysomes (CO2 fixation enzyme)

magnetosome (magnetite [Fe3O4] ) magnetotaxis

gas vesicles (buoyancy)

Ribosome 70S

No Endoplasmic Reticulum

The cell envelope

The Cell membrane The cell wall Capsule or S-layer

The Cell Membrane

A. Structure

“unit membrane”

composed of phospholipids and proteins

sterols are absent (except: mycoplasmas)

mesosomes (invaginations of CM)

2types : septal mesosome (cross wall formation) lateral mesosome (electron transport enzyme)

bacterial chromosome is attached to a septal mesosome

B- Function

1) selective permeability2) electron transport and oxidative phosphorylation3) excretion of hydrolytic exoenzymes4) biosynthetic functions5) chemotactic systems and other sensory transduction

systems

1- selective permeability

hydrophobic barrier

transport systems (nutrient, waste products)

require energy in some forms

transport systems:

a) Passive transport b) Active transport c) Group translocation d) Special transport processes

a) Passive transport

the only form that does not require energy simple diffusion neither speed nor selectivity dissolved oxygen, carbon dioxide, water

Facilitated diffusion selective channel proteins glycerol

b) Active transport

I) Ion coupled transportII) ABC transport

I) Ion coupled transport

Move a molecule across the CM at the expense of proton motive force (pmf)

transport protein

3 basic types: uniport (independent of any coupled ion)

symport (simultaneous transport of a solute [negative charge or neutral] and a positively charged ion, [H+] in the same direction )

antiport (simultaneous transport of two like-charged compounds in the opposite directions: H+, Na+ )

II) ABC transport ATP-binding cassette transport

binding proteins (BP)

Then BPs transfer the bound substrate to a membrane-bound protein complex

ATP hydrolyzed and the energy is used to open the membrane pore

Allow the unidirectional movement of the substrate into the cell

c) Group translocation (vectorial metabolism)

Change in structure certain sugars (eg, glucose and mannose) phosphorylated during the transport

"Phosphotransferase system“

Coupled transport with metabolism

d. Special transport processes

Iron (Fe) is an essential nutrient no free Fe in internal compartments of

animals Transferrin and lactoferrin

Siderophores

Some pathogenic bacteria Specific receptors Bind host transferrin and lactoferrin Fe is removed

2) electron transport and oxidative phosphorylation cytochromes and other enzymes functional analog of the mitochondrial

membrane (theory: mitochondria have evolved from

symbiotic bacteria)

3) excretion of hydrolytic exoenzymes

some nutrients are polymers hydrolytic enzymes degrade these polymers In Gram positive bacteria: external medium In Gram negative bacteria: periplasmic space

4) biosynthetic functions

enzymes for the cell wall biosynthesis enzymes for phospholipid synthesis some proteins of the DNA replicating

complex (septal mesosome)

carrier lipids

5) chemotactic systems and other sensory transduction systems

attractants and repellents bind to specific receptors in CM

Regulate the behavior in the environment

The Cell Wall

Outer of the CM

internal osmotic pressure 5-20 atm burst the cell Peptidoglycan (PG) (mucopeptide, murein)

Hans Christian Gram

Based on the composition of the cell wall bacteria are classified as:

Gram positive Gram negative

Cell wall functions

osmotic protection

nonselectively permeable

plays an essential role in cell division

serving as a primer for its own biosynthesis

antigenic properties

Bacterial shapes

A- The Peptidoglycan Layer complex polymer consisting of three parts: 1) a backbone (alternating N-acetylglucosamine(NAG) and N-acetylmuramic acid(NAM)) 2) tetrapeptide side chains attached to NAM

3) cross-bridges

For example in staphylococci NAM-NAG-NAM-NAG-NAM... L- Alanine D- Glutamine L- Lysine D- Alanine-

[Glycine]5

the backbone is the same in all bacteria

the tetrapeptide side chains and cross-bridges vary from species to species

the tetrapeptide side chains of all species:

a) most have L-alanine at position 1

b) D-glutamate at position 2

c) D-alanine at position 4

d) position 3 is the most variable one in G+ L-lysin , in G- mostly Diaminopimelic acid

Diaminopimelic acid( DAP) is the immediate precursor of Lysine

DAP is a unique element of prokaryotic cell walls

cross bridges varies among different species

for example in Staphylococcus, pentaglycin

in many gram negative a direct peptide linkage between the DAP of one side chain and

the alanine of a second side chain

In gram-positive bacteria, there are as many as 40 sheets of PG

In gram-negative bacteria there are only one or two sheets

B- Special components of Gram-Positive Cell Walls

1- Teichoic acids

2- polysaccharide molecules

Teichoic acids

Polymers containing ribitol or glycerol residues

there are 2 types of teichoic acids a) wall teichoic acid b) membrane teichoic acid (lipoteichoic acid) linked to membrane glycolipid

the teichoic acid constitute major surface antigens of G+

Streptococcus pneumoniae forssman antigen

Streptococcus pyogenes , attachment of S. pyogenes to animal cells

C- Special components of Gram-Negative Cell Walls

1- Outer membrane (OM)3- Lipoproteins4. The periplasmic space

1- Outer membrane (OM)

Chemically distinct bilayered structure the inner leaflet resembles in

composition that of cell membrane while the phospholipids of the outer

leaflet are replaced by Lipopolysaccharide (LPS) molecules

OM exclude hydrophilic molecules, because of its lipid nature

OM exclude hydrophobic molecules (unusual in biologic membranes)

protect the cells from the bile salts (enterics)

special protein channels (porins) Permit passive diffusion of hydrophilic compounds

G- bacteria are relatively high antibiotic resistance

Because large Antibiotic molecules penetrate the OM relatively slowly

major proteins of the OM (omp) receptors for several bacteriophages diffusion of nutrients

OmpA protein anchoring of the OM to PG sex pillus receptor (conjugation)

Minor proteins transport of specific molecules (vit B12 and iron siderophore complexes)

phospholipases and proteases

zone of adhesion (Bayer junction) between CM and OM

Lipopolysaccharide (LPS) lipid A polysaccharide

Lipid A consists of:

phosphorylated glucosamine disaccharide

fatty acids

Polysaccharide is made up of:

a core

a terminal series of repeat units

P

The polysaccharide core, is similar in all gram-negative species

But, Each species contains a unique repeat unit

LPS is extremely toxic to animals,

Endotoxin

because it is firmly bound to the cell surface and is released only when the cells are lysed.

When LPS is split into lipid A and polysaccharide,

all of the toxicity is associated with the Lipid A

The polysaccharide is a major surface antigen

0 antigen

Antigenic specificity is conferred by the terminal repeat units

Over 1000 antigenic types in Salmonella

2- Lipoproteins cross link the outer membrane and

peptidoglycan layers

its function is to stabilize the OM and anchor it to the PG layer

3. The periplasmic space

The space between the inner and outer membranes, called the periplasmic space,

Contains: the peptidoglycan layer and a gel-like solution of proteins.

The periplasmic proteins include:

Binding proteins for specific substrates (eg, amino acids, sugars, vitamins, and ions)

hydrolytic enzymes that break down non-transportable substrates into transportable ones

detoxifying enzymes that inactivate certain antibiotics (eg, β-lactamase).

the Gram positive cell envelope

cell membrane thick Peptidoglycan (PG) Teichoic Acid capsule or S layer

the Gram negative cell envelope

C.M (inner membrane) single sheet of PG outer membrane periplasmic space capsule or S layer

ENZYMES THAT ATTACK CELL WALLS

Lysozyme hydrolyze the linkage between NAG-NAM

found in animal secretions (tears, saliva, nasal secretions) as well as in egg white

autolysins

Bacteria themselves possess a number of, hydrolytic enzymes that attack peptidoglycan,

including glycosidases, amidases, and peptidases.

cell growth

division

autolysis after death

protozoa and the phagocytic cells

Have enzymes that degrade bacterial cell walls

L- form

Gram-positive bacteria:

In osmotically protected media

Lysozyme

L- form

protoplast

Gram-Negative bacteria:

In osmotically protected media,

The outer membrane prevents access of lysozyme

ethylenediaminetetraacetic acid (EDTA)

cells treated with EDTA+lysozyme

L- form

spheroplast

Some L forms can revert to the normal form

The factor that determines their capacity to revert is the presence of residual peptidoglycan

Some bacteria produce L- form in hosts:

Spontaneously Antibiotics

The spontaneous or antibiotic-induced formation of L forms in the host may produce chronic infections

they present special problems in chemotherapy.

relapses of the infection.

Capsule & Glycocalyx

extracellular polymers

Made of polysaccharide

known exception: Bacillus anthracis and Bacillus licheniformis

capsules are made of poly-D-glutamic acid

Glycocalyx more inclusive term

capsule Condensed Well defined layer Closely surrounding the cell Exclude particles, such as India ink

slime layer Loosly associated with the cell Does not exclude particles

The capsule contributes to the invasiveness of pathogenic bacteria

protection from phagocytosis

Until anticapsular antibody is formed

The glycocalyx plays a role in the adherence of bacteria to surfaces

Resistance to desiccation (bind water)

S-layer Many bacteria (G+, G- ) Have a surface layer composed of Protein

or glycoprotein molecules

As the outermost component of the cell envelope

Generally composed of a single molecular species

Function: Protection against: Wall-degrading enzymes

Invasion by: -predatory bacteria Bdellovibrio bacteriovorus -Bacteriophages

Host Cell adhesion

Flagella Thread-like appendages composed entirely

of protein

Motility

protein subunits called flagellin.

H antigens

Basal body

Hook

Filament

Flagellum has a Counterclockwise rotation

Rotation is driven by proton motive force

chemotaxis

chemical attractants (such as a sugar or an amino acid) or repellants bind to specific receptors located in the cell membrane

The mechanism by which a change in cell behavior is brought about in response to a change in the environment is called sensory transduction

sensory transduction chemotaxis aerotaxis (movement toward the optimal

oxygen concentration), phototaxis (movement of photosynthetic

bacteria toward the light), electron acceptor taxis (movement of

respiratory bacteria toward alternative electron acceptors, such as nitrate and fumarate).

4 types of arrangement are known:

Monotrichous (single polar flagellum)

Amphitrichous ( flagella at both poles)

Lophotrichous (multiple polar flagella)

Peritrichous (flagella distributed over the entire cell)

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Pili (Fimbriae) In many gram-negative, in some Gram positive bacteria surface appendages called pili (L “hairs”) or fimbriae (L “Fringes”)

They are shorter and finer than flagella

and do not rotate (twitching motility)

composed of structural protein subunits termed pilins

Two classes :

ordinary pili for adherence “colonization antigens”

sex pili used in bacterial conjugation

Like capsule , pili inhibit the phagocytic ability of leukocytes

Endospore endospores form in several bacterial

genera

The two most common are gram-positive rods:

Bacillus

Clostridium

The other bacteria known to form endospores are:

the gram-positive coccus sporosarcina

possibly the rickettsial agent of Q fever, Coxiella burnetii

Vegetative cells

Mother cell: spore forming cell

The spore is a: resting cell, highly resistant to desiccation, heat, and

chemical agents

A. SPORULATION

begins when nutritional conditions become unfavorable,

depletion of the nitrogen or carbon source (or both) being the most significant factor

Sporulation involves:

the production of many new structures, enzymes, and metabolites

the disappearance of many vegetative cell components.

B- Properties of Endospores

1. Core The core is the spore protoplast.

It contains: -a complete chromosome, -the protein synthesizing apparatus, -an energy generating system based on glycolysis.

some unique enzymes are formed (eg, dipicolinic acid synthetase)

The energy for germination is stored as 3PGA rather than as ATP

The heat resistance of spores is due to:

their dehydrated state

the presence of calcium dipicolinate

result in the stabilization of the spore enzymes

2. Spore wall

It contains normal peptidoglycan

3. Cortex The cortex is the thickest layer of the spore

envelope. It contains an unusual type of peptidoglycan

Cortex peptidoglycan is extremely sensitive to lysozyme,

and its autolysis plays a role in spore germination.

4. Coat The coat is composed of a keratin-like

protein

The impermeability of this layer confers on spores their relative resistance to antibacterial chemical agents.

5. Exosporium

The exosporium is a lipoprotein membrane containing some carbohydrate.

C. GERMINATION

The germination process occurs in three stages:

activation, initiation, outgrowth.

1. Activation

Some agents can overcome spore dormancy:

heat, abrasion, acidity, compounds containing free sulfhydryl groups

(SH)

2. Initiation

Once activated, a spore will initiate germination if the environmental conditions are favorable.

Spores have receptors that recognize different effectors as signaling a rich medium

Such as: L-alanine adenosine

Binding of the effector activates an autolysin that rapidly degrades the cortex peptidoglycan.

Water is taken up,

calcium dipicolinate is released,

3. Outgrowth

Degradation of the cortex and outer layers results in the emergence of a new vegetative cell

A period of active biosynthesis follows;

this period, which terminates in cell division, is called outgrowth.