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 detection 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, respectively, 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. However, 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 colonizing animal hosts. In contrast, mutants lacking all four receptors are phenotypically identical to LuxO-defective mutants. Importantly, these four functionally redundant receptors act together to prevent premature induction of a QS response caused by signal perturbations. We suggest that the V. cholerae QS circuit is composed of quadruple sensory inputs and has evolved to be refractory to sporadic AI level perturbations.

Photorhabdus temperata is a symbiont of the entomopathogenic nematode Heterorhabditis bacteriophora and an insect pathogen. This bacterium produces a wide variety of virulence factors and hemolytic activity. The goal of this study was to identify hemolysin-defective mutants and test their virulence. A genetic approach was used to identify mutants with altered hemolytic activity by screening a library of 10 000 P. temperata transposon mutants. Three classes of mutants were identified: (i) defective (no hemolytic activity), (ii) delayed (delayed initiation of hemolytic activity), and (iii) early (early initiation of hemolytic activity). The transposon insertion sites for these mutants were identified and used to investigate other physiological properties, including insect pathogenesis and motility. The hemolysin-defective mutants, P10A-C11, P10A-H12, and P79-B5, had inserts in genes involved in RNA turnover (RNase II and 5'-pentaphospho-5'-adenosine pyrophosphohydrolase) and showed reduced virulence and production of extracellular factors. These data support the role of RNA turnover in insect pathogenesis and other physiological functions.