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