FliC is a natural antigen recognized by the innate and adaptive immune systems during Salmonella infection in mice and humans; however, the regulatory mechanisms governing its expression in vivo are incompletely understood. Here, we use flow cytometry to quantify fliC gene expression in single bacteria. In vitro, fliC transcription was not uniformly positive; a viable fliC-negative subpopulation was also identified. Intracellular Salmonella repressed transcription of fliC and its positive regulator fliA, but constitutively transcribed the master regulator flhD; fliC repression required ClpXP protease, known to degrade FlhD. In orally infected mice, fliC transcription was anatomically restricted: Salmonella transcribed fliC in the Peyer's Patches (PP) but not in the mesenteric lymph nodes and spleen. The intracellularly transcribed pagC promoter was upregulated by Salmonella in all tissues, defining the infected PP as a unique environment that initiates expression of intracellularly induced genes and yet permits transcription of fliC. Because a single bacterium can escape the GI tract to colonize deeper tissues, heterogeneous gene expression may have important implications for Salmonella pathogenesis: FliC-positive bacteria in the PP could stimulate inflammation and facilitate the priming of FliC-specific immune responses, while FliC-negative bacteria escape host detection in the gut and spread to systemic sites of replication.
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