Distinct sensory pathways in Vibrio cholerae El Tor and classical biotypes modulate cyclic dimeric GMP levels to control biofilm formation

Quorum sensing (QS), or cell-cell communication in bacteria, is achieved through the production and subsequent response to the accumulation of extracellular signal molecules called autoinducers (AIs). To identify AI-regulated target genes in Vibrio cholerae El Tor (V. cholerae_El), the strain responsible for the current cholera pandemic, luciferase expression was assayed in an AI^- strain carrying a random lux transcriptional reporter library in the presence and absence of exogenously added AIs. Twenty-three genes were identified and shown to require the QS transcription factor, HapR, for their regulation. Several of the QS-dependent target genes, annotated as encoding hypothetical proteins, in fact encode HD-GYP proteins, phosphodiesterases that degrade the intracellular second messenger cyclic dimeric GMP (c-di-GMP), which is important for controlling biofilm formation. Indeed, overexpression of a representative QS-activated HD-GYP protein in V. cholerae_El reduced the intracellular concentration of c-di-GMP, which in turn decreased exopolysaccharide production and biofilm formation. The V. cholerae classical biotype (V. cholerae_Cl), which caused previous cholera pandemics and is HapR^-, controls c-di-GMP levels and biofilm formation by the VieA signaling pathway. We show that the VieA pathway is dispensable for biofilm formation in V. cholerae_El but that restoring HapR in V. cholerae_Cl reestablishes QS-dependent repression of exopolysaccharide production. Thus, different pandemic strains of V. cholerae modulate c-di-GMP levels and control biofilm formation in response to distinct sensory pathways.