pH-dependent secretion of SseB, a product of the SPI-2 type III secretion system of Salmonella typhimurium

The type III secretion system of Salmonella pathogenicity island 2 (SPI-2) is required for bacterial replication inside macrophages. SseB has; been co nsidered a putative target of the secretion system on the basis of its simi larity with EspA, a protein secreted by the type III secretion system of en teropathogenic Escherichia coil (EPEC). EspA forms a filamentous structure on the bacterial cell surface and is involved in translocation of proteins into the eukaryotic cytosol. In this paper, we show that SseB is a secreted protein that associates with the surface of the bacterial cell and might, therefore, also be required for delivery of SPI-2 effector proteins to the eukaryotic cell cytosol. SseB begins to accumulate inside the bacterial cel l when the culture enters early stationary phase. However, SseB is only sec reted if the bacteria are grown at low pH or if the pH is shifted after gro wth from 7.0 to below pH 5.0. The secretion occurs within minutes of acidif ication and is totally dependent on a functional SPI-2 type III secretion s ystem. As the pH of the Salmonella-containing vacuole inside host cells has been shown to acidify to between pH4.0 and 5.0, and as SPI-2 gene expressi on occurs inside host cells, low pH might be a physiological stimulus for S PI-2-mediated secretion in vivo.