Which statement best describes Listeria crossing barriers?

• A. It forms spores to survive in harsh environments
• B. It seeds the bloodstream...
• C. It uses flagellin to invade from gut lumen
• D. It invades through internalin-mediated invasion and can disseminate to placenta causing fetal infection

Answer: (D)

Listeria crosses barriers by using internalins to invade host cells and then spread from cell to cell, allowing dissemination through the body and into the placenta where fetal infection can occur. It is a non-spore-forming, facultative intracellular bacterium; spore formation is not part of its strategy. The invasion at body temperature relies on internalins (InlA and InlB) binding to host receptors like E-cadherin and Met, not on flagellin-mediated invasion. Once inside cells, Listeria escapes the phagosome and uses actin-based motility to move between cells, facilitating spread to the placenta and fetal tissues. That combination—internalin-mediated invasion with placental dissemination—best describes how Listeria crosses barriers.