Bacteria pdf - غير معروف

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

Bacteriology

Cell Structure

Learning Outcomes:

• Review molecules and structures of the bacterial cell.

• Describe the roles played by each of the major components of bacterial cell.

• Discuss briefly the features and the structures of the cytoplasmic membrane

and cell well of bacterial cell.

3-Cytoplasmic membrane



The bacterial cell membrane is a typical "unit membrane" composed of

phospholipid bilayer, similar in microscope to that in eukaryotic cells, and

upward of 200 different kinds of proteins.



The prokaryotic and eukaryotic cell membranes are chemically similar, but

eukaryotic membranes contain sterols, whereas prokaryotes generally do

not. The only prokaryotes that have sterols in their membrane are members

of the genus Mycoplasma.



Functions of cytoplasmic membrane:

1. Selective permeability and transport of solutes

The cytoplasmic membrane forms a hydrophobic barrier impermeable to

most hydrophilic molecules. However, several mechanisms (transport

systems) exist that enable the cell to transport nutrients into and waste

products out of the cell. These transport systems work against a

concentration gradient to increase the concentration of nutrients inside the

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

cell, a function that requires energy in some form. There are three general

transport mechanisms involved in membrane transport:

A. Passive transport:

This mechanism relies on diffusion, uses no

energy, and operates only when the solute is at higher concentration

outside than inside the cell. Simple diffusion accounts for the entry of

very few nutrients including dissolved oxygen, carbon dioxide, and

water itself. Simple diffusion provides neither speed nor selectivity.

Facilitated diffusion also uses no energy so the solute never achieves

an internal concentration greater than what exists outside the cell.

However, facilitated diffusion is selective. Channel proteins form

selective channels that facilitate the passage of specific molecules.

Facilitated diffusion is common in eukaryotic microorganisms (eg,

yeast), but is rare in prokaryotes.

B. Active transport: Many nutrients are concentrated more than a

thousand fold as a result of active transport.

C. Group translocation: Is a process in which an organic molecule such

as glucose or mannose is transported into the cell while being

chemically modified.

2. Electron transport & oxidative phosphorylation: The cytochromes

and other enzymes and components of the respiratory chain are located

in the cell membrane. The bacterial cell membrane is thus a functional

analog of the mitochondrial membrane in eukaryotic.

3. Excretion of hydrolytic exoenzymes & pathogenicity proteins:

All

organisms that rely on macromolecular organic polymers as a source of

nutrients (eg, proteins, polysaccharides, lipids) excrete hydrolytic

enzymes that degrade the polymers to subunits small enough to

penetrate the cell membrane.

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

4. Biosynthetic functions: The cell membrane is the site of the carrier

lipids and enzymes used in cell wall synthesis.

5. Chemotactic systems: Attractants and repellents bind to specific

receptors in the bacterial membrane e.g., flagella.

4- Cell wall

 Most prokaryotes have a rigid external cell wall that contains peptidoglycan,

a polymer of amino acids and sugars. Eukaryotes on the other hand, do not

contain peptidoglycan.

 The cell wall is the outermost component common to all bacteria (except

Mycoplasma species, which bounded by a cell membrane, not a cell wall).

 The cell wall is located external to the cytoplasmic membrane.

 Cell wall functions:

1- Osmotic protection.

2- Plays an essential role in cell division as well as serving as a primer

for its own biosynthesis.

3- Various layers of the wall are the sites of major antigenic

determinants of the cell surface, and one component—the

lipopolysaccharide of gram-negative cell walls—is responsible for the

nonspecific endotoxin activity of gram-negative bacteria.

4- Not selectively permeable.

5- Bacteria are classified to G+ve or G-ve according to their response to

the differential stains method called Gram’s stains. The difference

reside on the cell wall differences.

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

Peptidoglycan

• It is a complex polymer consists of a backbone that composed of alternating

N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM).

• A set of identical tetrapeptide side chains attached to N-acetylmuramic acid.

• A set of identical peptide cross-bridges (Figure 1).

• 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 vancomycin, inhibit the synthesis of

peptidoglycan by inhibiting the transpeptidase that makes the cross-links

between the two adjacent tetrapeptides.

• Scientists describe two basic types of bacterial cell walls: gram-positive and

gram-negative.

Figure  1:  Peptidoglycan  structure.  A:  Peptidoglycan  is  composed  of  a  glycan
chain  (NAM&NAG),  a  tetrapeptide  chain,  and  a  cross-link  (peptide  interbridge).
B:  Three-dimensional  structure.  (Levinson  W.  Review  of  Medical  Microbiology
and Immunology.12

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

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.

 Gram-positive
 The basal structure of cell wall is the peptidoglycan layer.

 Relatively thick layer of peptidoglycan.

 Contains fibers of teichoic acids.

 There are 2 types of teichoic acid:

a. Wall teichoic acid that linked to peptidoglycans.

b. Membrane teichoic acid that linked to membrane glycolipid.

 Because the latter are intimately associated with lipids, they have been

called lipoteichoic acids (LTA). (Figure 2)

 The medical importance of teichoic acids lies in their ability to induce 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 staph to mucosal cells. Gram

positive bacteria do not have teichoic acids.

 Retains crystal violet dye in Gram staining procedure; appear purple.

 Gram-negative

 Have only a thin layer of peptidoglycan

 Have a complex outer layer consisting of Lipopolysaccharide (LPS),

lipoprotein, and phospholipid.

 Lipoprotein: that cross link the outer membrane and peptidoglycan layer and

stabilize them together.

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

 Outer membrane: it’s a bilayer structure: and inner leaflet resembles in

composition the cell membrane. While the phospholipids of the outer leaflet

are replaced by lipopolysaccharides molecules. Thus this bilayer differs

 from the bilayer of cell membrane. The outer membrane protects the cell

from hydrophobic as well as hydrophilic molecules. The outer membrane

has special channels called porins that permit the passive diffusion of low

molecular weight hydrophilic compounds as sugars and amino acid also

large antibiotic molecules penetrate the outer membrane slowly that makes

G-ve bacteria much resistant to antibiotics.

 Periplasmic Space: Located between outer membrane and cell membrane

Contains water, nutrients, and substances secreted by the cell, such as

digestive enzymes. The periplasmic proteins include binding proteins for

amino acids, vitamins, sugars hydrolytic enzymes and detoxifying enzymes

that inactivate certain antibiotics (Figure 2)

 Following Gram staining procedure, cells appear pink

Figure 2: Cell wall of gram-positive and gram-negative bacteria (Levinson W.
Review of Medical Microbiology and Immunology.12

McGraw-Hill.)

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

Lipopolysaccharide (LPS)

• The LPS of the outer membrane of the cell wall is endotoxin.

• It is responsible for many of the features of disease, such as fever and shock

caused by these organisms.

• Composed of three distinct units: (Figure 3)

1. A phospholipid called lipid A, which is responsible for the toxic effects.

2. A core polysaccharide of five sugars linked to lipid A.

3. An outer polysaccharide consisting of up to 25 repeating units of three to

five sugars. This outer polymer is the important somatic, or O, antigen of

several gram-negative bacteria that used to identify certain organisms in the

clinical laboratory.

Figure  3:  Endotoxin  (LPS)  structure.  The  O-antigen  polysaccharide  is
exposed  on  the  exterior  of  the  cell,  whereas  the  lipid  A  faces  the  interior.
(Levinson  W.  Review  of  Medical  Microbiology  and  Immunology.12

ed.

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

Structures outside the Cell Wall

1- Capsule

• A gelatinous layer covering the entire bacteria.

• Composed of polysaccharide, except anthrax bacillus, which contain

polymerized D-glutamic acid.

• The capsule is important because:

1- It is a determinant of virulence of many bacteria since it limits the ability

of phagocytes to engulf the bacteria.

2- Capsular polysaccharide is used as the antigens in certain vaccines

because they are capable of eliciting protective antibodies.

3- Specific identification of an organism can be made by using antiserum

against the capsular polysaccharide.

4- The capsule may play a role in the adherence of bacteria to human

tissues, which is an important initial step in causing infection.

2- Flagella

• Thread-like appendages composed of protein.

• Three types of arrangement are know:

Monotrichous : single polar Flagellum

Lophotrichous : multiple polar Flagellum

Peritrichous : Flagella are distributed over the entire cell.

• Flagellum proteins called flagellin that aggregate to form a helical structure.

They are highly antigenic (H antigens) and some of immune responses to

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

infection are directed against these proteins. Flagella are structure

composed of a hook and body.

• Flagella play a role in the pathogenesis.

• Same species of bacteria (e.g., Salmonella) are identified in the clinical Lab.

By the use of specific antibodies against flagellar protein.

• 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 in to the bladder.

3- Pili (Fimbriae)

 Hair like filaments that extend from the cell surface.

 Shorter and straighter than flagella.

 Composed of subunits of pilin, a protein arranged in helical strand.

 Found mainly on gram-negative organism.

 Ordinary pili mediate the attachment of bacteria to specific receptors on cell

surface, which is a necessary step in the initiation of infection for some
organisms. Mutant of Neisseria gonorrhoeae that do not form pili are
nonpathogenic.

 Sex pili responsible for the attachment of donor and recipient cells in

bacteria conjugation.

 Pilli are present in motile and non-motile strains of many species of

bacteria.

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

Glycocalyx

 Polysaccharide (slime layer) secreted by certain bacteria.

 It attached bacteria firmly to the surface of human cells and to the surface of

catheters, prosthetic joints, and heart valves.

 The medical importance of the Glycocalyx is illustrated by the findings that

it is the Glycocalyx-producing strains of Pseudomonas aeruginosa, which

cause respiratory tract infections in cystic fibrosis patient.

Endospore

• Members of several bacteria genera are capable of forming endospores.

Commonly the G+ve rods.

• Under conditions of nutritional depletion each cell forms a single internal

spore

that is liberated when the mother cell under goes autolysis.

• Highly resistant to heat and chemicals when returned to favourable

nutritional conditions and activated, the spore germinates to produce a
single vegetative cell.

• Sporulation accurse in culture that have terminated growth as a result of

depletion of carbon and N sources.

The spore consists of:

1- The core: spore protoplasm.

2- Cell envelope of spore composed of :

3- Spore wall.

4- Cortex.

5- Coat: keratin like protein.

Lec.2

Dr. Inas Khalifa Al-Sharquie

2012-2013

Summary

• The cytoplasmic membrane of bacteria consist of a phospholipid bilayer

(without sterols) located just inside the peptidoglycan.

• It regulates the transport of nutrients into the cells and the secretion of toxins

out of the cells.

• Cell wall is peptidoglycan Polymer (amino acids + sugars). Sugars; NAG &

NAM.

• G+ vs. G- bacteria.

• G+ Thicker cell wall, Teichoic Acids

• G- Endotoxin – LPS

• Between the inner cell membrane and the outer membrane of G-ve bacteria

lies the periplasmic space

• The cell wall of mycobacterium has more lipid than either G+ or G- ve.

• Capsule is anti-phagocytic.

• Pili are filaments of protein that extended from the bacterial surface and

mediate attachment of bacteria to the surface of human cells.

• The Glycocalyx is a polysaccharide (slime layer) secreted by certain

bacteria.

• Spores are medically important because. They are highly heat resistant and

are not killed by many disinfectants.

Main References:

1. Jawetz, Melnick and Adelberg’s Medical Microbiology (Brooks,

Butel,Morse), 2010.

2. Levinson W. Review of Medical Microbiology and Immunology.12

ed.