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Enterohemorrhagic Escherichia coli serotype O157:H7 (O157) is a critical, foodborne, human intestinal pathogen that causes severe acute hemorrhagic diarrhea, abdominal cramping, and even death. Small RNAs (sRNAs) are noncoding regulatory molecules that sense environmental changes and trigger various virulence-related signaling pathways; however, few such sRNAs have been identified in O157. Here, we report a novel sRNA, EsrF that senses high ammonium concentrations in the colon and enhances O157 pathogenicity by promoting bacterial motility and adhesion to host cells. Specifically, EsrF was found to directly interact with the 5' untranslated regions of the flagellar biosynthetic gene, flhB , mRNA and increase its abundance, thereby upregulating expression of essential flagellar genes, including flhD , flhC , fliA , and fliC , leading to elevated O157 motility and virulence. Meanwhile, an infant rabbit model of O157 infection showed that deletion of esrF and flhB significantly attenuates O157 pathogenicity. Furthermore, NtrC-the response regulator of the NtrC/B two-component system-was found to exert direct, negative regulation of esrF expression. Meanwhile, high ammonium concentrations in the colon release the inhibitory effect of NtrC on esrF , thereby enhancing its expression and subsequently promoting bacterial colonization in the host colon. Our work reveals a novel, sRNA-centered, virulence-related signaling pathway in O157 that senses high ammonium concentrations. These findings provide novel insights for future research on O157 pathogenesis and targeted treatment strategies. IMPORTANCE The process by which bacteria sense environmental cues to regulate their virulence is complex. Several studies have focused on regulating the expression of the locus of enterocyte effacement pathogenicity island in the typical gut pathogenic bacterium, O157. However, few investigations have addressed the regulation of other virulence factors in response to intestinal signals. In this study, we report our discovery of a novel O157 sRNA, EsrF, and demonstrate that it contributed to bacterial motility and virulence in vitro and in vivo through the regulation of bacterial flagellar synthesis. Furthermore, we show that high ammonium concentrations in the colon induced esrF expression to promote bacterial virulence by releasing the repression of esrF by NtrC. This study highlights the importance of sRNA in regulating the motility and pathogenicity of O157.

A Novel Small RNA Promotes Motility and Virulence of Enterohemorrhagic Escherichia coli O157:H7 in Response to Ammonium