What virulence trait enables Listeria monocytogenes to cross intestinal and placental barriers?

• A. Capsule formation
• B. Exotoxin A production
• C. Internalin-mediated invasion
• D. Beta-hemolysin production

Answer: (C)

Listeria monocytogenes crosses intestinal and placental barriers by actively invading non-phagocytic cells through internalin proteins. The surface proteins InlA and InlB bind receptors on host cells—InlA targets E-cadherin on intestinal epithelial cells, triggering uptake, while InlB interacts with Met to broaden invasion to other cell types, including placental trophoblasts. Once inside, the bacterium uses listeriolysin O to escape the phagosome and ActA-driven actin-based motility to spread from cell to cell, enabling dissemination to the bloodstream and placenta without exposed extracellular traversal. This invasion capability directly explains how it breaches mucosal and placental barriers.

Capsule formation isn’t the primary driver of barrier crossing, exotoxin A is not a virulence factor for Listeria, and while listeriolysin O (a hemolysin) helps escape the phagosome, the specific trait that enables crossing those barriers is the internalin-mediated invasion.