Which externalstructure protects bacteria fromphagocytosis? Select one: a. Slime layer b. Cell membrane c. Capsule d. Fimbriae
The correct answer and explanation is:
Correct Answer: c. Capsule
Explanation:
The capsule is an external structure of some bacteria that plays a crucial role in protecting the organism from phagocytosis, which is the process by which immune cells (like macrophages and neutrophils) engulf and destroy invading microbes.
Capsules are typically made of polysaccharides (though some are polypeptide-based), and they form a thick, gelatinous outer layer surrounding the cell wall. This structure serves several key protective and pathogenic functions:
- Anti-Phagocytic Shield: The capsule can prevent immune cells from recognizing and binding to the bacterium. It masks surface antigens that immune cells would normally detect, and also makes the surface slippery, making it physically harder for phagocytes to engulf the bacteria.
- Immune Evasion: By covering the bacterial surface, the capsule reduces the effectiveness of opsonization—the process where antibodies and complement proteins coat a pathogen to enhance recognition by phagocytes.
- Increased Virulence: Bacteria with capsules, such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis, are more likely to cause serious infections because of their resistance to the body’s natural immune defenses.
Let’s contrast the other options:
- a. Slime layer: Similar to a capsule but loosely attached and less structured. It provides some protection and helps in adherence, but is not as effective against phagocytosis.
- b. Cell membrane: This is the innermost boundary of the bacterial cell, important for transport and metabolism, but it is not involved in protection from phagocytosis.
- d. Fimbriae: Hair-like appendages used for attachment to surfaces or host cells, not for evading phagocytosis.
Conclusion:
The capsule is the bacterial structure most directly responsible for protecting against phagocytosis, making it a key virulence factor in many pathogenic bacteria.