Final answer:
Listeriolysin O (LLO) produced by Listeria monocytogenes is active at acidic pH levels, which enables the bacterium to escape from the phagosome into the host's cytoplasm, thereby evading the host's immune response and facilitating intracellular survival.
Step-by-step explanation:
The bacterium Listeria monocytogenes, which can cause listeriosis, possesses several virulence factors that enable it to invade host cells and evade the immune system. Key to its pathogenicity is a protein called listeriolysin O (LLO), which is regulated by the transcriptional factor PrfA.
LLO is critical for the bacterium's escape from the host cell's phagosome into the cytoplasm. This protein is active at an acidic pH, which is typically encountered within the phagosome. Once L. monocytogenes is ingested by a host cell through phagocytosis, it becomes enclosed within a phagosome. Before the phagosome can fuse with a lysosome to create a phagolysosome, where the bacterium would be destroyed, LLO degrades the phagosome membrane, allowing the bacterium to escape into the host's cytoplasm. As a result, the bacteria can survive, multiply, and continue the infection process within the host cell.