Which of the following describes Listeria monocytogenes?

• A. It is an obligate anaerobe
• B. It is not pathogenic to humans
• C. It is an intracellular pathogen
• D. It is a virus

Answer: (C)

Listeria monocytogenes is a bacterium that can live inside host cells. It invades cells, escapes the phagosome, and multiplies in the cytoplasm, then uses actin-based movement to push from one cell to another. This intracellular lifestyle is why it’s described as an intracellular pathogen. It can grow with or without oxygen, so it is not an obligate anaerobe. It is indeed pathogenic to humans, causing listeriosis, especially in pregnant women, newborns, the elderly, and those with weakened immune systems. It is not a virus.