SRA

Flagella-driven motility of Salmonella enterica serovar Typhimurium facilitates host colonization. However, the large extracellular flagellum is also a prime target for the immune system. As consequence, expression of flagella is bistable within a population of Salmonella, resulting in flagellated and non-flagellated subpopulations. This allows the bacteria to maximize fitness in hostile environments. The degenerate EAL-domain protein RflP (formerly YdiV) is responsible for the bistable expression of flagella by directing the flagellar master regulatory complex FlhD4C2 to proteolytic degradation. The environmental cues controlling expression of rflP and thus the bistable flagellar biosynthesis remain ambiguous. Here, we demonstrate that RflP responds to cell envelope stress and alterations of outer membrane integrity. Lipopolysaccharide (LPS) truncation mutants of Salmonella Typhimurium exhibited increasing motility defects due to downregulation of flagellar gene expression. Transposon mutagenesis and genetic profiling revealed that s24 (RpoE) and Rcs phosphorelay-dependent cell envelope stress response systems sense modifications of lipopolysaccharide, low pH activity of the complement system. This subsequently results in activation of RflP expression and degradation of FlhD4C2 via ClpXP. We speculate that diverse hostile environments inside the host might result in cell envelope damage and would thus trigger the repression of resource-costly and immunogenic flagella biosynthesis via activation of the cell envelope stress response. Overall design: S. Typhimurium planktonic cultures representing wildtype, ?rfaL, ?rfaG, ?rfaD