Listeria monocytogenes secretes two distinct phospholipases C, a phosphatidylinositol-specific phospholipase C (PI-PLC) and a broad-range phospholipase C (PC-PLC). In this study, single in-frame deletion mutants with mutations in each PLC and a double mutant lacking both PLCs were characterized with regard to virulence in mice, escape from a primary vacuole, and cell-to-cell spread in cell culture. The mutant lacking PI-PLC, previously shown to be twofold less virulent than the wild type in mice, had a minor defect in escape from a primary vacuole but was not notably affected in cell-to-cell spread. The mutant lacking PC-PLC was 20-fold less virulent in mice and was defective in cell-to-cell spread but had no measurable defect in escape from a primary vacuole. The mutant lacking both PLCs was 500-fold less virulent in mice and was severely diminished in its ability to escape from the primary vacuole and to spread cell to cell. Cellular levels of diacylglycerol and ceramide, products of PLC activity, accumulated beginning 3 to 4 h after infection of cells with wild-type bacteria. The bacterial PLCs were partially responsible for this activity, since cells infected with the mutant lacking both PLCs had a reduced increase in diacylglycerol and no increase in ceramide. Elevation of diacylglycerol in the absence of bacterial PLCs indicated that host cell phospholipase(s) was activated during infection. The results of this study were consistent with the two bacterial PLCs having overlapping functions throughout the course of intracellular infection. Furthermore, the PC-PLC, and possibly PI-PLC, appeared to be enzymatically active intracellularly.

Listeriolysin 0 (LLO) is a pore-forming cytolysin that enables Listeria monocytogenes to escape from a host cell vacuole. The structural gene for the related cytolysin perfringolysin 0 (pfo) was cloned downstream from the promoter for hly, the gene encoding LLO, both on a plasmid and on the L. monocytogenes chromosome. Both strains secreted active PFO, although regulation was not identical to that of LLO. The chromosomal PFO-expressing strain was characterized for intracellular growth and cell-to-cell spread. It escaped from a host cell vacuole with 64% efficiency compared with the wild type as determined by immunofluorescent staining of bacteria for F-actin, a marker for entry into the cytoplasm. In addition, it replicated intracellularly with a doubling time similar to that of the wild type for 5 h, after which growth was aborted because of a cytotoxic effect on the host cell and influx of extracellular gentamicin. The chromosomal PFO strain was able to plaque in mouse L2 fibroblasts, but it did so at 20%o efficiency compared with the wild type and the plaques were significantly smaller. Both strains expressing PFO were completely avirulent in mice. These results indicate that PFO can mediate escape from a host cell vacuole but cannot complement an hly deletion strain for virulence.

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Elderly men and women with HIV have lower bone mass than HIV negative controls. Decreased body mass index was the most important risk factor associated with decreased BMD. Bone demineralization was observed among HIV-infected subjects receiving either tenofovir or a protease inhibitor.

Listeria monocytogenes is a facultative intracellular pathogen which secretes a pore-forming cytolysin, listeriolysin O (LLO), necessary for intracellular growth. Clostridium perfringens is an extracellular pathogen 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 cell permeable to gentamicin and leads to the death of the intracellular 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 acid change within the PFO protein. Three classes of PFO mutations were identified, all capable of mediating lysis of the vacuole but without a toxic effect upon the infected host cell. The first class had a severe 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 haemolytic activity, but had a decrease in protein half-life in the host-cell cytosol. Acquisition of single amino acid changes in PFO were sufficient to convert an extracellular cytolysin into a vacuole-specific lysin which mediated growth of L. monocytogenes in cultured cells.