The two-component system CpxAR controls biofilm formation by directly regulating the T3SS needle tip protein EseB in .

The type III secretion system (T3SS) translocon protein EseB (needle tip protein) forms filamentous appendages on the surface of to facilitate autoaggregation and biofilm formation. By contrast, another T3SS translocon protein EseC inhibits biofilm formation by sequestering EseC's chaperone EseE, which also functions as a positive regulator of the operon, in which EseB is encoded. The two-component system (TCS) EsrAB and the regulator EsrC tightly and positively regulate the T3SS in . The TCS CpxAR provides an adaptive response to external environmental changes. In this study, we have shown that disruption of the histidine kinase CpxA (sensor) instead of CpxR (response regulator) significantly reduces biofilm formation in . CpxR is negatively regulated by CpxA, and significant amounts of CpxR accumulate in in the absence of CpxA. CpxR, together with EsrB and EsrC, directly binds the promoter of the operon to promote CpxR transcription and expression. The elevated phosphorylated CpxR (CpxR-) binds to the promoter of the operon to repress transcription and expression, while EseE, EsrB, and EsrC bind directly to the same promoter to promote EseB transcription and expression. is an enteric pathogen that senses microbiota-derived indole in the gut lumen. EseB filament-mediated biofilm formation in is inversely proportional to exogenous indole. Together, CpxR inhibits while EsrB, EsrC, and EseE stimulate transcription and expression of the operon, thereby coordinately controlling EseB filament-mediated biofilm formation in in response to environmental stimuli.IMPORTANCE is primarily an enteric pathogen of fish and can form a biofilm to resist the lethal effects of host or antimicrobial agents. The assembly of filamentous appendages on the bacterial surface, mediated by the type III secretion system (T3SS) needle tip protein EseB, promotes bacterial-bacterial interactions and biofilm formation when is cultured in Dulbecco's modified Eagle's medium (DMEM). In this study, we have shown that the histidine kinase CpxA regulates biofilm formation in by negatively regulating its response regulator CpxR. Binding to the promoter of the operon, CpxR negatively regulates, whereas EsrB, EsrC, and EseE positively regulate the operon, of which EseB is encoded, coordinately regulating biofilm formation in .