RAPID AND COMPLETE FUSION OF MACROPHAGE LYSOSOMES WITH PHAGOSOMES CONTAINING SALMONELLA-TYPHIMURIUM

The virulence of Salmonella typhimurium for mice results, in part, fro m its ability to survive after phagocytosis by macrophages. Although i t is generally agreed that intracellular bacteria persist in membrane- bound phagosomes, there remains some question as to whether these phag osomes fuse with macrophage lysosomes. This report describes the matur ation of phagosomes containing S. typhimurium inside mouse bone marrow -derived macrophages. Macrophages were infected briefly and incubated for various intervals; then they were examined by fluorescence microsc opy for colocalization of bacteria with lysosomal markers. These marke rs included LAMP-1, cathepsin L, and fluorescent proteins or dextrans preloaded into lysosomes by endocytosis. By all measures, phagosomes c ontaining S. typhimurium merged completely with the lysosomal compartm ent within 20 min of phagocytosis. The rate of phagosome-lysosome fusi on was similar to the rate for phagocytosed latex beads. Phagolysosome s remained accessible to fluid-phase probes and contained lysosomal ma rkers for many hours. Moreover, a large percentage of the wild-type ba cteria that were viable 20 min after infection survived longer incubat ions inside macrophages, indicating that the survivors were not a mino r subpopulation that avoided phagosome-lysosome fusion. Therefore, we conclude that S. typhimurium survives within the lysosomal compartment s of macrophages.