Signaling input from the polar flagellum uncouples quorum sensing from cell-density regulation in Vibrio species

Quorum sensing (QS) is traditionally recognized as a signaling mechanism that monitors cell density to coordinate bacterial group behaviors. Although the architecture of many QS pathways and their dependence on cell density are well established, the impact from other physiological cues on QS response is largely unclear. We demonstrate here that flagellar integrity profoundly influences QS responses across multiple Vibrio species. Systematic disruption of flagellar components in Vibrio cholerae revealed that loss of the hook cap protein FlgD induces QS sRNAs Qrr1-4 production independent of cell density and the canonical QS regulator LuxO. This bypass mechanism also overrides normal cell density-dependent regulation of several QS-controlled phenotypes. Notably, {Delta}flgD mutation restores intestinal colonization of {Delta}luxO mutants in an infant mouse model, indicating that flagellar defects reshape V. cholerae QS during infection. This bypass mechanism is evolutionarily conserved as {Delta}flgD mutation induced LuxO-independent qrr expression in multiple Vibrio species. The FlrBC two-component system, essential for flagellar gene expression, is required for the activation of qrr expression in {Delta}flgD mutants. FlgD specifically interacts with the FlrB histidine kinase but not the FlrC response regulator, and a hyperactive FlrC variant is sufficient to drive qrr expression even in cells with FlgD. These findings suggest that not only FlrBC senses cytoplasmic FlgD levels to monitor flagellar completeness to direct flagellar gene expression, this signaling system also functions as a link to bypass the canonical cell-density control of QS by integrating cellular structural information to coordinate group behaviors.