Investigating the mechanism of surface-induced virulence in Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa is one of the most intensely studied bacterial pathogens and is a leading cause of health-care associated infections in the United States. An intriguing aspect of P. aeruginosa is its ability increase its virulence in response to surface attachment, which is an early step in the colonization of multiple host environments. However, the downstream toxin(s) that mediate surface-induced virulence in P. aeruginosa and the signaling pathways that lead to their activation are unknown. Here, I demonstrate that the alkyl-quinolone (AQ) secondary metabolites are rapidly induced upon bacterial surface association, and directly target host cells to promote cytotoxicity. Using several reporters for AQ production, I identify the signal transduction pathways downstream of the two known regulators of surface-dependent virulence, the putative mechanosensor, PilY1, and the quorum sensing receptor, LasR. PilY1 regulates surface-induced AQ production by repressing the AlgR-AlgZ two-component system. AlgR repression upregulates RhlR, which can induce the AQ biosynthesis operon under specific conditions, implicating a complex integration of the PilY1 and LasR activity at the pqsABCDE operon in a surface-dependent manner. I further demonstrate that packaging of AQs in secreted outer-membrane vesicles (OMVs) increases their cytotoxicity to host cells but not their ability to stimulate downstream quorum-sensing pathways in bacteria. OMVs lacking AQs are significantly less cytotoxic, suggesting these molecules play a major role in OMV cytotoxicity, in addition to their previously characterized role in OMV biogenesis. These findings collectively suggest that the induction of AQs following surface association may be an important strategy used by P. aeruginosa to promote host colonization and infection.