RESEARCH ARTICLE Quadruple Quorum-Sensing Inputs Control Vibrio cholerae Virulence and Maintain System Robustness

Bacteria use quorum sensing (QS) for cell-cell communication to carry out group behaviors. This intercellular signaling process relies on cell density-dependent production and detec-tion of chemical signals called autoinducers (AIs). Vibrio cholerae, the causative agent of cholera, detects two AIs, CAI-1 and AI-2, with two histidine kinases, CqsS and LuxQ, re-spectively, to control biofilm formation and virulence factor production. At low cell density, these two signal receptors function in parallel to activate the key regulator LuxO, which is essential for virulence of this pathogen. At high cell density, binding of AIs to their respective receptors leads to deactivation of LuxO and repression of virulence factor production. How-ever, mutants lacking CqsS and LuxQ maintain a normal LuxO activation level and remain virulent, suggesting that LuxO is activated by additional, unidentified signaling pathways. Here we show that two other histidine kinases, CqsR (formerly known as VC1831) and VpsS, act upstream in the central QS circuit of V. cholerae to activate LuxO. V. cholerae strains expressing any one of these four receptors are QS proficient and capable of coloniz-ing animal hosts. In contrast, mutants lacking all four receptors are phenotypically identica