Drosophila S2 cells: an alternative infection model for Listeria monocytogenes.

Listeria monocytogenes is a Gram-positive facultative intracellular bacterial pathogen that infects humans and animals. Its pathogenic strategy involves the expression of virulence proteins that mediate intracytosolic growth and cell-to-cell spread. A key virulence protein is the cholesterol-dependent cytolysin, listeriolysin O (LLO), which is largely responsible for mediating escape from the phagosome into the host cytosol. To study further the host processes exploited during L. monocytogenes infection, we sought to develop Drosophila S2 cells as a model for infection. Here, we show that S2 cells share a number of properties with mammalian cell culture models of infection. As with mouse macrophages, LLO was required for phagosomal escape from S2 cells. Furthermore, vacuolar escape was dependent on their acidification via the ATPase proton pumps, as bafilomycin A1 treatment sharply decreased escape. However, unlike in mouse macrophages, LLO mutants replicated in the phagosome of S2 cells. Drosophila cells are cholesterol auxotrophs, and exogenous cholesterol increased the infection rate of L. monocytogenes (LLO independent) and also augmented the efficiency of vacuolar escape (LLO dependent). With available genetic tools such as RNA interference, S2 cells could become an important model in the study of host-pathogen interactions.