Unusual intracellular trafficking of Salmonella typhimurium in human melanoma cells

Salmonella spp. are enterobacteria capable of invading and replicating in both professional and non-professional phagocytes. Here, we investigate the fate of S. typhimurium in human melanoma MelJuSo cells. The bacterium entered MelJuSo cells by a trigger mechanism and resided within a unique organelle, the Salmonella-containing vacuole (SCV). The SCV acquired early endosomal markers transiently and then underwent a series of membrane modifications. In HeLa cells, vacuole maturation is characterized by the simultaneous acquisition of the lysosomal membrane glycoproteins (Lgps) Lamp1, CD63 and vacuolar (v)-ATPase; in MelJuSo cells, however, acquisition of CD63 and v-ATPase preceded that of Lamp1. A very striking event in MelJuSo cells was the arrest of bacterial septation starting from 8 h after infection. Bacteria nevertheless continued to elongate, remained morphologically intact and viable and were eventually exocytosed. This original feature was observed in several skin-related cells including melanocytes, suggesting that it may provide the basis for an efficient host defence mechanism against Salmonella infection.