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About LLO / Listeriolysin:

Listeriolysin O (LLO) is a hemolysin produced by Listeria monocytogenes, the pathogen that causes listeriosis. The toxin may be considered a virulence factor because it is essential for L. monocytogenes virulence.
Listeriolysin O is a cytolytic, thiol-activated, cholesterol-dependent cytolysin. It is activated by reducing agents and inhibited by oxidizing agents. On the other hand, LLO differs from other thiol-activated toxins in that its cytolytic activity is maximized at pH 5.5.

LLO Mechanism
LLO is selectively activated within the acidic phagosomes (average pH 5.9) of cells with phagocytosed L. monocytogenes by maximizing activity at a pH of 5.5. The bacterium escapes into the cytosol after LLO lyses the phagosome, where it can grow intracellularly. The toxin's activity in the more basic cytosol decreases after release from the phagosome. As a result, LLO allows L. monocytogenes to escape from phagosomes into the cytosol without damaging the infected cell's plasma membrane. This allows bacteria to live intracellularly, safe from extracellular immune system factors like the complement system and antibodies.
Listeriolysin O is encoded by the hly gene, which is part of the LIPI-1 pathogenicity island. The protein encoded by the prfA gene activates transcription of hly and other L. monocytogenes virulence factors within LIPI-1. The PrfA thermoregulator UTR element regulates prfA translation so that it is maximal at 37 °C and nearly silent at 30 °C. Because 37 °C is within the normal body temperature range, PrfA protein and listeriolysin O and other virulence factors regulated by PrfA are only produced when L. monocytogenes are present in a host.

Listeriolysin Function
During the early stages of infection, before the entry of L. monocytogenes into the host cell, LLO also causes dephosphorylation of histone H3 and deacetylation of histone H4.