Hemolysin supports survival but not entry of the intracellular bacterium Listeria monocytogenes

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Abstract
The gram-positive bacterium Listeria monocytogenes is a facultative intracellular pathogen. The only known property of L. monocytogenes which has been shown to be involved in virulence is a hemolysin, listeriolysin (J. L. Gaillard, P. Berche, and P. Sansonetti, Infect. Immun. 52:50-55, 1986; S. Kathariou, P. Metz, H. Hof, and W. Goebel, J. Bacteriol. 169:1291-1297, 1987). Using our previously obtained transposon Tn916-induced hemolysin-negative mutants of L. monocytogenes Sv1/2a (Mackaness strain), we demonstrated that the loss of hemolysin reduced significantly the rate of survival of the bacteria in mouse peritoneal macrophages but did not reduce their uptake. It was further shown that virulent L. monocytogenes strains could invade the mouse embryo fibroblast 3T6 cell line, i.e., mammalian cells which are nonprofessional phagocytes. This uptake was inhibited by cytochalasin B and hence seems to be accomplished by parasite-induced endocytosis. Hemolysin was not essential for this step. Strains of other Listeria species could not efficiently penetrate the 3T6 cells.