Evolution of the Pseudomonas aeruginosa quorum-sensing hierarchy

Abstract

The bacterial pathogenPseudomonas aeruginosaactivates expression of many virulence genes in a cell density-dependent manner by using an intricate quorum-sensing (QS) network. QS inP. aeruginosainvolves two acyl-homoserine-lactone circuits, LasI-LasR and RhlI-RhlR. LasI-LasR is required to activate many genes including those coding for RhlI-RhlR.P. aeruginosacauses chronic infections in the lungs of people with cystic fibrosis (CF). In these infections, LasR mutants are common, butrhlR-rhlIexpression has escaped LasR regulation in many CF isolates. To better understand the evolutionary trajectory ofP. aeruginosaQS in chronic infections, we grew LasR mutants of the well-studiedP. aeruginosastrain, 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 inmexT, 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 inP. aeruginosaCF infections where LasR mutants arise during infection, but because these mutants are surrounded by C4-HSL–producingP. aeruginosa,variants rewired to have a LasR-independent RhlIR system quickly emerge.

Allelic polymorphism shapes community function in evolving Pseudomonas aeruginosa populations

“…One of best-known signatures of P. aeruginosa adaptation to the CF lung environment, is mutation in the QS regulator LasR. Several studies on P. aeruginosa strains isolated from CF sputum samples, found significant genomic changes in lasR, including truncation, deletion, frame shifts and SNPs, with many resulting in a loss of function (30,39,40,49,69). Although there can be a high percentage of lasR-deficient isolates collected from patients, studies have also shown differential frequencies of functional and intact lasR alleles within patients (13,70).…”

“…One of the most commonly described adaptations of P. aeruginosa to the CF lung is the loss of the las QS system, predominantly through point mutations, frameshifts and deletions in the lasR gene (17,(38)(39)(40). We used our evolved populations to specifically focus on the impact of lasR mutation frequency on QS phenotypes.…”

Combinatorial quorum sensing in Pseudomonas aeruginosa allows for novel cheating strategies

“…Importantly, the regulators lasR, rhlR, and pqsR were all-intact and identical when compared to the ancestor strain and the PAO1 reference library. We did not observe any mutations in the mex family of genes which have previously been linked to similar QS phenotypes [23,24]. It therefore remains unknown what mechanism (or a combination of mechanisms) impact on combinatorial signaling, however, the lack of mutation in known QS-regulated genes, suggests novel ways to circumvent the known QS system in P. aeruginosa, which allows strains to evolve to bypass the restrictions levied by QSprivate goods metabolism.…”

“…A simple mechanistic explanation for the lack of response to C4-HSL would be a defect in the rhl QS system; however, when we whole genome sequenced NCRi, we found no changes in the rhl genes or a range of known QS genes (including pqs genes) that could easily explain our observed phenotype. We also did not observe any mutations in the mex family of genes which have recently been linked to similar QS phenotypes [23,24]. To determine whether indirect effects on the rhl system were responsible for the NCRi phenotype, we added the rhlR gene on a constitutive promoter into NCRi.…”

Uncovering the dynamic mechanisms of thePseudomonas aeruginosaquorum sensing and virulence networks using Boolean modelling

“…However, other mutations can arise in the lasR mutants to turn the cheaters back into co-operators. Such a compensatory mechanism can involve the up-regulation of rhll [34][27], so that it has been suggested to target RhlR for treatment [27], [35]. In our model, when the Las system repressor rsaL was ON and there was the minimum Las Al to activate LasREG only for a limited number of steps before it was overtaken by rsaL, Rhl REG was able to reach sustained activation (Fig 2A).…”