Bacterial Capsule

Capsule

• Gelatinous secretion of bacteria, forming a thick coat around the cell wall
• Dependent on favorable environmental conditions, such as high sugar concentration, blood serum, or growth in a living host
• Can be composed of complex polysaccharide, polypeptide, or hyaluronic acid
• Examples: pneumococci, Klebsiella, Bacillus anthracis, and Streptococcus pyogenes
• Thinner capsules are called microcapsules, e.g. Meningococci, Streptococcus pyogenes, and Haemophilus influenzae

Functions of Capsule

• Protects against deleterious agents, such as lytic enzymes
• Contributes to virulence of pathogenic bacteria by inhibiting phagocytosis
• Capsular antigen is a hapten in nature, specific for bacteria

Cell Wall

• A rigid, chemically complex structure between the cell membrane and capsule/slime layer
• Main component is peptidoglycan, responsible for shape and strength
• Composed of a disaccharide with two sugar derivatives: N-acetylglucosamine and N-acetylmuramic acid
• Joined together by short peptide chains

Functions of Cell Wall

• Protects internal structure (supporting layer)
• Gives shape to the cell and provides rigidity
• Plays a role in bacterial division and offers resistance to environmental stress
• Protects bacteria from changes in osmotic pressure

Classification of Bacterial Cells

• Based on structural differences between Gram-positive and Gram-negative cell walls
• Gram-positive bacteria have simpler chemical structures compared to Gram-negative bacteria