The unique trafficking pattern of Salmonella typhimurium-containing phagosomes in murine macrophages is independent of the mechanism of bacterial entry

Although it has been known for some time that Salmonella typhimurium is able to survive and even replicate in the normally bactericidal environment of the macrophage phagosome, the mechanisms by which this organism accomplishes this feat remain obscure. In this study, a murine macrophage cell line and confocal immunofluorescence microscopy were used to more thoroughly define the specific nature of phagosomes containing latex beads or wild-type S. typhimurium (viable or heat-killed organisms). Live S. typhimurium organisms were observed to reside in phagosomes that diverge from the degradative pathway of the macrophage. These compartments contain lysosomal glycoproteins and lysosomal acid phosphatase, endocytic markers delivered to vacuoles by mannose 6-phosphate receptor-independent mechanisms, but are devoid of the mannose 6-phosphate receptor and cathepsin L. In contrast, phagosomes containing latex beads or heat-killed organisms appeared to be processed along the degradative pathway of the host cell; these compartments colocalized not only with lysosomal glycoproteins and lysosomal acid phosphatases but also with mannose 6-phosphate receptors and cathepsin L. The uniqueness of the phagosome containing viable S. typhimurium was confirmed by the observation that these compartments, in comparison to phagosomes containing latex beads, do not readily interact with incoming endocytic traffic. Finally, we show that an isogenic, noninvasive mutant of S. typhimurium, BJ66, ends up in an intracellular compartment identical to the wild-type S. typhimurium-containing phagosome. Thus, modifications of the Salmonella-containing compartment occur independently of the mechanism of bacterial entry.