Abstract
ABSTRACTFimbriae play an important role in adhesion and are therefore essential for the interaction of bacteria with the environments they encounter. Most of them are expressed in vivo but not in vitro, thus making difficult the full characterization of these fimbriae. Here, we characterized the silencing of plasmid-encoded fimbriae (Pef) expression, encoded by the pef operon, in the worldwide pathogen Salmonella Typhimurium. We demonstrated that the nucleoid-associated proteins H-NS and Hha, and their respective paralogs StpA and YdgT, negatively regulate at pH 5.1 and pH 7.1 the transcription of the pef operon. Two promoters, PpefB and PpefA, direct the transcription of this operon. All the nucleoid-associated proteins silence the PpefB promoter and H-NS also targets the PpefA promoter. While Hha and YdgT are mainly considered as acting primarily through H-NS to modulate gene transcription, our results strongly suggest that Hha and YdgT silence pef transcription at acidic pH either by interacting with StpA or independently of H-NS and StpA. We also confirmed the previously described post-transcriptional repression of Pef fimbriae by CsrA titration via the fim mRNA and CsrB and CsrC sRNA. Finally, among all these regulators, H-NS clearly appeared as the major repressor of Pef expression. These results open new avenues of research to better characterize the regulation of these bacterial adhesive proteins and to clarify their role in the virulence of pathogens.
Highlights
Fimbriae are filamentous appendages present on the surface of bacteria which play an important role in the virulence of pathogens
In vitro, the 5ʹUTR of the fimAICDHF transcript cooperates with the CsrB and CsrC sRNA, which are positively regulated by SirA, to antagonize CsrA activity leading to an absence of plasmid-encoded fimbriae (Pef) expression in a wild-type strain
In Salmonella, several genes/proteins have been implicated in the repression of fimbrial expression but the mechanisms of regulation remain unknown [21,46] except in the case of the posttranscriptional control of Pef fimbriae expression via CsrA protein [23]
Summary
Fimbriae are filamentous appendages present on the surface of bacteria which play an important role in the virulence of pathogens They mediate adhesion to target host cells and to many surfaces encountered by bacteria in their host and in the environment [1,2,3]. Typhimurium is only able to assemble Type 1 fimbriae encoded by the fim operon in standard laboratory growth conditions [11,12,13]. These data are in agreement with a tight regulation of fimbrial expression characterized by a strong in vitro repression that can be relieved by the environmental conditions encountered by the bacteria. Little is known about the physiological environments allowing the expression of fimbriae and the regulatory mechanisms involved
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