​Bacteria employ various mechanisms to invade or enter host cells, which can be either phagocytic or non-phagocytic in nature. Given below are mechanisms commonly used for bacterial entry into non-phagocytic cells.

Statements:
(A) Some bacteria express a protein called invasin that is recognized by host-cell β1 integrins.
(B) Actin polymerization along with assembly of the clathrin coat results in the internalization of bacteria by the zipper mechanism.
(C) Some bacteria, including Salmonella enterica, use the trigger mechanism to inject a set of effector molecules into the cytosol through the type III secretion system.
(D) Some bacteria attach to host cell surface receptors inducing local elevation of Ca²⁺ in the cytosol, leading to the fusion of lysosomes with bacteria-containing plasma membrane vesicles.

Which one of the following options represents the combination of correct mechanisms for invading non-phagocytic cells?

Correct option is A

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Explanation:

(A) Invasin and β1 Integrins → Zipper Mechanism

• Certain bacteria (e.g., Yersinia spp.) produce invasins, which bind β1 integrins on host cells.
• This induces cytoskeletal rearrangement leading to bacterial internalization.
• This process follows a "zipper" mechanism, where bacteria closely interact with the host cell membrane.

(B) Actin Polymerization and Clathrin Assembly → Zipper Mechanism

• Bacterial uptake via the zipper mechanism requires actin polymerization at the site of bacterial attachment.
• Example: Listeria monocytogenes and Yersinia spp. use this pathway.
• Clathrin coating is not typically involved in bacterial entry but is essential for endocytosis, which bacteria exploit.

(C) Type III Secretion System (T3SS) → Trigger Mechanism

• Salmonella enterica, Shigella, and Pseudomonas use the trigger mechanism for invasion.
• The T3SS injects effector proteins into host cells, inducing membrane ruffling and cytoskeletal changes.
• This results in bacterial engulfment and internalization.

(D) Ca²⁺ Elevation and Lysosomal Fusion → Not a Typical Entry Mechanism

• Ca²⁺ elevation plays a role in intracellular trafficking and phagosome maturation.
• However, fusion with lysosomes is NOT a mechanism for bacterial invasion into non-phagocytic cells.
• Instead, bacteria avoid lysosomal fusion to prevent degradation.

Information Booster:

1. Zipper Mechanism (Used by Yersinia & Listeria):
• Involves tight receptor-ligand interactions.
• Uses β1 integrins & cadherins to mediate actin polymerization.
2. Trigger Mechanism (Used by Salmonella & Shigella):
• Bacteria inject effectors via T3SS, causing membrane ruffling.
• Leads to macropinocytosis-like uptake of bacteria.
3. Actin Cytoskeleton Remodeling:
• Required for both zipper & trigger mechanisms.
• Pathogens manipulate actin polymerization to enter cells.
4. Role of Calcium (Ca²⁺) in Infection:
• Intracellular Ca²⁺ regulates vesicular trafficking, but it is not a direct invasion strategy.
• Some bacteria manipulate Ca²⁺ signaling to disrupt host immune responses.