Bacterial Capsules

@Firm, gelatinous layer covering cell wall.

@Microcapsule detected by E.M. & serology

@Composed of water, saccharides & peptides

@Slime layer is common in capsulated bacteria

growing on solid media (K. pneumoniae).

@Carbohydrates stimulate capsules @Failure of production is due to

starvation & , accumulation of enzymes.

Capsule Demonstration

@Negative staining by India ink.

@Serology of enterobacteria shows K-antigen

( capsule )masking somatic antigen )cell wall(.

@E.M. with difficulty due to shrinkage of microcapsule on drying.

@Capsule-swelling reaction is conducted by

adding a specific antiserum to organism.

Functions of Capsules

@Protect bacteria against: bacteriophages ,

complement, antibiotics, phagocytes , lysozymes, colicines.

@Determine virulence of organism: Bacillus ,

Streptococcus, Yersinia, Haemophilus.

@Non-capsulated mutants of capsulated bacteria are found to be non-virulent.

Bacterial Cell Wall•Functions:

@Gives the bacterial cell its shape & rigidity.

@Protects cell against high osmotic pressure

and environmental imbalance

@Serves as antigenic determinant to cell.

@Contributes to virulence of organism.

@Structure of gram positive cell wall is different from that of gram negative.

Cell Wall of Gram Positive Bacteria

@ Peptidoglycan found on surface & made of 16 square units & each unit is made of 4 subunits

@Peptidoglycan contains murein which forms

the rigid elements of the wall.

@Peptidoglycan is found throughout the wall

forming a complex matrix with teichoic acids.

Peptidoglycan Structure:

@Contains specific sugars as per each species:

# Rhamnose: Found in streptococci. # Arabinose: Found in mycobacteria.

@It is resistant to complement and sensitive

to lysozyme and lytic enzymes.

@It is composed of N-acetyl glucosamine &

N-acetylmuramic acid molecules linked together alternately in a chain by

glycosidic bonds.

@The carboxyl group of N-acetylmuramic acid

is linked to a tetrapeptide chain containing

L-alanine, D-isoglutamamide, L-lysine, and

D-alanine.

@These tetrapeptides are joined together by

covalent bond to make a chain.

@N-acetylmuramic acid molecules each carrying tetrapeptide side chains cross-

linked together by peptide bridges.

Biosynthesis of Peptidoglycan :

#In the cytoplasm a nucleotide is formed from UDP & five aminoacids: L-alanine, L-lysine ,

D-isoglutamamide, D-alanine and D-alanine.

#The nucleotide is transferred from cytoplasm to cell membrane where it loses UDP and forms a complex with a lipid called bactoprenol.NAG & NAM are added to the nucleotide-bactoprenol complex .

#This complex is transferred to cell wall where bactoprenol is liberated & the peptide subunit is cross-linked by covalent bonds, and terminal D-alanine is liberated.

Teichoic Acids

@There are two types of teichoic acids: *Glycerol-teichoic acid: in cell

membrane *Ribitol teichoic acid: in cell wall.

@Cell wall teichoic acid is composed of a polymer of ribitol phosphate and N-

acetyl - glucosamine. This polymer is joined to

glycine molecule by ester bonds

@Teichoic acid is joined to the peptidoglycan in

the cell wall by covalent bonds.

Functions of Teichoic Acids

@Bacterial antigenic determinant, used for

serological identification of bacterial genera.

@Regulate action of reactions needed for cell

growth and division.

@Binding of cell cations, e.g. Mg++

Cell Wall of Gram Negative Bacteria

* Peptidoglycan inner layer:10% of cell wt

*Outer membrane layer containing :

1 .Lipopolysaccharide that consists of:@12 sugars forming somatic O antigens .

@Lipid A which acts as an endotoxin .

2. Lipoprotein links outer membrane layer with peptidoglycan inner layer by a covalent bond

3. Porins: acting as pores for passing small hydrophilic molecules

4. Proteins 5. phospholipids

@The periplasmic space is found between the

peptidoglycan layer and the cell membrane.

@G-ve: No teichoic acids or lipoteichoic acids.

@G-ve: Have a convoluted surface structure.

@Lipopolysaccharide is responsible for:

*Somatic (O) antigen

* Endotoxic activity, bacteraemia, shock.

* Leukopenia, hyperglycaemia

* Impaired blood circulation