Which bacterial structure is described as a sticky layer that helps evade the immune system and adhere to substances?

• A. Capsule
• B. Plasmids
• C. Pili
• D. Endospore

Answer: (A)

A capsule is a sticky layer that surrounds some bacteria, made mostly of polysaccharides (sometimes proteins). This capsule helps bacteria cling to surfaces and tissues, which supports adherence and colonization. More importantly, it acts as a shield that can make immune cells less able to recognize and engulf the bacteria, helping the organism evade phagocytosis. The capsule also contributes to biofilm formation, increasing persistence in the host.

In contrast, plasmids are small DNA circles that carry genes and aren’t a surface layer; pili are hair-like structures used for attachment or genetic exchange but are not the broad sticky coating; endospores are dormant, highly resistant forms used for survival, not for adhesion. Thus, the sticky, adherence-enhancing and immune-evading role points to the capsule.