CONVERSION OF AN EXTRACELLULAR CYTOLYSIN INTO A PHAGOSOME-SPECIFIC LYSIN WHICH SUPPORTS THE GROWTH OF AN INTRACELLULAR PATHOGEN

Listeria monocytogenes is a facultative intracellular pathogen which s ecretes a pore-forming cytolysin, listeriolysin O (LLO), necessary for intracellular growth. Clostridium perfringens is an extracellular pat hogen which secretes a related cytolysin, perfringolysin O (PFO). When PFO is secreted by intracellular L. monocytogenes, it is toxic to the infected host cell. PFO-mediated toxicity renders the infected host c ell permeable to gentamicin and leads to the death of the intracellula r bacteria, In this study, we selected for L. monocytogenes mutants in which PFO supported the intracellular growth of L. monocytogenes. Six independent mutants were isolated, each containing a single amino aci d change within the PFO protein. Three classes of PFO mutations were i dentified, all capable of mediating lysis of the vacuole but without a toxic effect upon the infected host cell. The first class had a sever e defect in haemolytic activity. The second class had a change in the pH optimum of PFO. The third class had nearly wild-type levels of haem olytic activity, but had a decrease in protein half-life in the host-c ell cytosol. Acquisition of single amino acid changes in PFO were suff icient to convert an extracellular cytolysin into a vacuole-specific l ysin which mediated growth of L. monocytogenes in cultured cells.