Recognition of bacteria in the cytosol of mammalian cells by the ubiquitin system

Recent studies have suggested the existence of innate host surveillance systems for the detection of bacteria in the cytosol of mammalian cells [1-4]. The molecular details of how bacteria are recognized in the cytosol, however, remain unclear. Here we examined the fate of Salmonella typhimurium, a gram-negative bacterial pathogen that can infect a variety of hosts [5], in the cytosol of mammalian cells. These bacteria typically occupy a membrane bound compartment, the Salmonella-containing vacuole (SCV), in host cells. We show that some wild-type bacteria escape invasion vacuoles and are released into the cytosol. Subsequently, polyubiquitinated proteins accumulate on the bacterial surface, a response that was witnessed in several cell types. In macrophages but not epithelial cells, the proteasome was observed to undergo a dramatic subcellular relocalization and become associated with the surface of bacteria in the cytosol. Proteasome inhibition promoted replication of S. typhimurium in the cytosol of both cell types, in part through destabilization of the SCV. Surprisingly, the cytosol-adapted pathogen Listeria monocytogrenes avoided recognition by the ubiquitin system by using actin-based motility. Our findings indicate that the ubiquitin system plays a major role in the recognition of bacterial pathogens in the cytosol of mammalian cells.