Describe the Type III secretion system.

Describe the Type III secretion system.

• A. A capsule that resists phagocytosis
• B. Protein appendage common in pathogenic Gram-negative bacteria that senses eukaryotic cells and injects effector proteins into host cells
• C. A toxin that disrupts cell membranes
• D. A flagellar motor used for motility

Answer: (B)

The main idea is that this system is a syringe‑like delivery apparatus used by many pathogenic Gram-negative bacteria to inject proteins directly into host cells. It spans the bacterial membranes, forming a basal structure and a needle that protrudes toward the host, with a translocation pore in the host membrane. When the bacteria contact a eukaryotic cell, the secretion system activates and transfers its effector proteins through the needle into the host cell cytosol, where these effectors hijack signaling pathways, rearrange the cytoskeleton, or dampen immune responses to help the infection. This description matches a secretion system that injects bacterial proteins into host cells, rather than being a capsule that blocks phagocytosis, a toxin that lyses membranes, or a flagellum used for movement.

The main idea is that this system is a syringe‑like delivery apparatus used by many pathogenic Gram-negative bacteria to inject proteins directly into host cells. It spans the bacterial membranes, forming a basal structure and a needle that protrudes toward the host, with a translocation pore in the host membrane. When the bacteria contact a eukaryotic cell, the secretion system activates and transfers its effector proteins through the needle into the host cell cytosol, where these effectors hijack signaling pathways, rearrange the cytoskeleton, or dampen immune responses to help the infection. This description matches a secretion system that injects bacterial proteins into host cells, rather than being a capsule that blocks phagocytosis, a toxin that lyses membranes, or a flagellum used for movement.