PhoPQ is an upstream regulator of quorum sensing in small colony variant subpopulations of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic human pathogen that is hospital-endemic, forming biofilms on medical equipment and causing thousands of hospital-acquired infections each year. The success of P. aeruginosa as an opportunistic pathogen is linked to its phenotypic and genotypic adaptability. Relatedly, P. aeruginosa forms a small colony subpopulation in response to stresses like oxygen limitation and antibiotic exposure. Additionally, P. aeruginosa coordinates population-level decisions using a mechanism of cell-cell communication called quorum sensing. P. aeruginosa uses these signaling pathways to control virulence factor production and biofilm formation in the host. We show that certain quorum-sensing mutations promote phenotypic variation; specifically, deletion of lasR and autoinducer modulating mutations in rhlI enhanced small colony formation in a time course-dependent manner. Using transcriptome analyses of isogenic small and large colony variants, we show that small colony formation is driven in part by the PhoPQ two-component signal transduction system in quorum-sensing mutant backgrounds. Specifically, our data show that unphosphorylated PhoP represses rhlR gene expression, and that subsequent de-repression of quorum sensing contributes to the production of virulence factors and the small colony phenotype. In total, these findings provide insight on how mutations evolved by clinical strains might serve as a bet-hedging strategy to promote the formation of a small colony phenotype and alter quorum-sensing signaling within a subpopulation of a community.