Evolution of the <i>Pseudomonas aeruginosa</i> quorum-sensing hierarchy

Abstract

The bacterial pathogen <i>Pseudomonas aeruginosa</i> activates expression of many virulence genes in a cell density-dependent manner by using an intricate quorum-sensing (QS) network. QS in <i>P. aeruginosa</i> involves two acyl-homoserine-lactone circuits, LasI-LasR and RhlI-RhlR. LasI-LasR is required to activate many genes including those coding for RhlI-RhlR. <i>P. aeruginosa</i> causes chronic infections in the lungs of people with cystic fibrosis (CF). In these infections, LasR mutants are common, but <i>rhlR-rhlI</i> expression has escaped LasR regulation in many CF isolates. To better understand the evolutionary trajectory of <i>P. aeruginosa</i> QS in chronic infections, we grew LasR mutants of the well-studied <i>P. aeruginosa</i> strain, PAO1, in conditions that recapitulate an environment where QS signal synthesis by other bacteria might still occur. When QS is required for growth, addition of the RhlI product butyryl-homoserine lactone (C4-HSL), or bacteria that produce C4-HSL, to LasR mutants results in the rapid emergence of a population with a LasR-independent RhlI-RhlR QS system. These evolved populations exhibit subsequent growth without added C4-HSL. The variants that emerge have mutations in <i>mexT</i>, which codes for a transcription factor that controls expression of multiple genes. LasR-MexT mutants have a competitive advantage over both the parent LasR mutant and a LasR-MexT-RhlR mutant. Our findings suggest a plausible evolutionary trajectory for QS in <i>P. aeruginosa</i> CF infections where LasR mutants arise during infection, but because these mutants are surrounded by C4-HSL-producing <i>P. aeruginosa,</i> variants rewired to have a LasR-independent RhlIR system quickly emerge.