Differential function of Listeria monocytogenes listeriolysin O and phospholipases C in vacuolar dissolution following cell-to-cell spread

We investigated the role of listeriolysin O (LLO) and the bacterial phospholipases PI-PLC and PC-PLC in cell-to-cell spread of Listeria monocytogenes. We showed that LLO is essential for cell-to-cell spread in primary murine macrophages. Electron micrographs revealed that in the absence of continued LLO expression, bacteria remain trapped in secondary spreading vacuoles having either a double or single membrane. In bacteria lacking PI-PLC and PC-PLC, cessation of LLO expression after initiation of infection resulted in a significant increase in the proportion of bacteria trapped in double-membrane compartments. We propose that the bacterial phospholipases are involved in the dissolution of the inner membrane of the spreading vacuole, yet are not sufficient for disruption of the outer membrane. As a consequence, we identified LLO as a key factor in the disruption of the outer membrane. This model is consistent with the observation that LLO is dispensable for cell-to-cell spread from human macrophages into a cell type in which LLO is not required for vacuolar escape. These data suggest that during human infection, spreading of L. monocytogenes to distant organs is likely to occur even in the absence of LLO expression, and that the bacterial phospholipases may be sufficient to mediate continued cell-to-cell spread.