Bacterial capsules — identifying the typical chemical composition In pathogenic bacteriology, “capsules” and very thin “microcapsules” are surface layers that contribute to immune evasion. What is the predominant chemical composition of these bacterial capsules?

Introduction / Context:
Bacterial capsules are hydrated, gelatinous layers external to the cell wall that help resist phagocytosis, desiccation, and complement-mediated killing. They are major virulence factors and important vaccine targets (for example, pneumococcal polysaccharide vaccines). Knowing their chemistry guides staining methods and immunology.

Given Data / Assumptions:

• Most classic capsules are polysaccharides composed of repeating oligo- or polysaccharide units.
• Some exceptions exist (for example, Bacillus anthracis has a polypeptide capsule of poly-D-glutamic acid).
• “Microcapsule” refers to a thin or loosely adherent polysaccharide layer that may be harder to visualize.

Concept / Approach:
Although rare proteinaceous exceptions occur, the overwhelming majority of bacterial capsules are polysaccharide based. This explains why capsule detection often relies on negative staining (for example, India ink), Quellung reaction (capsular swelling with specific antisera), and why purified polysaccharides can be used as antigens in conjugate vaccines. Lipids do not form capsules; peptidoglycan is part of the rigid cell wall, not the capsule.

Step-by-Step Solution:
Recall that “capsule” generally means sugar polymers external to the cell wall.Consider notable exception (poly-D-glutamate of B. anthracis) but recognize it is not the rule.Eliminate options proposing lipids or peptidoglycan as capsule material.Select polysaccharides as the predominant composition.

Verification / Alternative check:
Laboratory manuals describe capsular serotyping based on polysaccharide antigenic variation (for example, K antigens in Enterobacteriaceae, pneumococcal serotypes), confirming sugar composition.

Why Other Options Are Wrong:

• A: S-layers are distinct protein lattices, not capsules.
• C: Lipids organize into membranes, not hydrated capsules.
• D: Not equally common; polysaccharides dominate.
• E: Peptidoglycan forms the rigid sacculus, not the capsule.

Common Pitfalls:
Assuming all extracellular layers are the same. Differentiate capsules (polysaccharide), S-layers (protein), and slime layers (loosely adherent exopolysaccharide).

Final Answer:
Polysaccharides (for example, repeating sugar polymers)