Combinatorial control of Pseudomonas aeruginosa biofilm development by quorum-sensing and nutrient-sensing regulators

The human pathogen Pseudomonas aeruginosa, a leading cause of hospital-acquired infections, inhabits and forms sessile antibiotic-resistant communities called biofilms in a wide range of biotic and abiotic environments. In this study, we examined how two global sensory signaling pathways - the RhlR quorum-sensing system and the CbrA/CbrB nutritional adaptation system - intersect to control biofilm development. Previous work has shown that individually these two systems repress biofilm formation. Here, we used biofilm analyses, RNA-seq, and reporter assays to explore the combined effect of information flow through RhlR and CbrA on biofilm development. We find that the delta rhlR delta cbrA double mutant exhibits a biofilm morphology and an associated transcriptional response distinct from wildtype and the parent delta rhlR and delta cbrA mutants indicating codominance of each signaling pathway. The delta rhlR delta cbrA mutant rapidly gains suppressor mutations that map to the carbon catabolite repression protein Crc. The combined absence of RhlR and CbrA leads to drastic reduction in the abundance of the Crc antagonist small RNA CrcZ. Thus, CrcZ acts as the molecular convergence point for quorum- and nutrient-sensing cues. Furthermore, in the absence of antagonism by CrcZ, Crc promotes the expression of biofilm matrix components - Pel exopolysaccharide, and CupB and CupC fimbriae. Therefore, this study uncovers a regulatory link between nutritional adaption and quorum sensing with potential implications for anti-biofilm targeting strategies.