The end of the reign of a "master regulator''? A defect in function of the LasR quorum sensing regulator is a common feature of Pseudomonas aeruginosa isolates

Abstract

Pseudomonas aeruginosa, a bacterium causing infections in immunocompromised individuals, regulates several of its virulence functions using three interlinked quorum sensing (QS) systems (las, rhl, and pqs). Despite its presumed importance in regulating virulence, dysfunction of the las system regulator LasR occurs frequently in strains isolated from various environments, including clinical infections. This newfound abundance of LasR-defective strains calls into question existing hypotheses regarding their selection. Indeed, current assumptions concerning factors driving the emergence of LasR-deficient isolates and the role of LasR in the QS hierarchy must be reconsidered. Here, we propose that LasR is not the primary master regulator of QS in all P. aeruginosa genetic backgrounds, even though it remains ecologically significant. We also revisit and complement current knowledge on the ecology of LasR-dependent QS in P. aeruginosa, discuss the hypotheses explaining the putative adaptive benefits of selecting against LasR function, and consider the implications of this renewed understanding.

Keywords: Pseudomonas aeruginosa; adaptive mutations; ecology; opportunistic infections; quorum sensing; virulence regulation.

Fig 1

Clustering analysis performed on a panel of 92 strains of P. aeruginosa isolated from acute and chronic infections. Clustering analysis is based on selected variables from a previous study. Briefly, HAQ concentrations and pyocyanin production were measured in King’s A medium at two different time points (4). Strains PA14 (Functional LasR; top) and PA14 lasR::Gm (LasR-defective; bottom, “lasR”) were included in the analysis as references. Three robust clusters were identified. One of these clusters comprises LasR-defective strains, whereas another includes strains with a functional LasR protein. The third cluster comprises strains that produce negligible levels of HAQs. Further analyses are required to confirm the functionality of LasR in this subset of isolates, as described before (4). Raw data used to generate this analysis are presented in Table S2. Statistical analyses were made as previously described using R software (4, 44). Robustness (*) represents the proportion of clustering runs in which a pair of isolates appeared together in some clusters, given that they were clustered together in at least one run, averaged over all such pairs (***: 80%–89%).

Fig 2

Clustering analysis performed on a subset of LasR-defective isolates of Pseudomonas aeruginosa shown in Fig. 1. Clustering analysis was based on chosen variables from a previous study. Briefly, HAQ concentrations, pyocyanin production, and activity of a rhlA-gfp reporter were measured in King’s A medium at two different time points using the same methods as previously described (4, 82). RhlR Active Independently of LasR (RAIL) strain E90 was included in the analysis as a reference. Raw data used to generate this analysis are presented in Table S2. Statistical analyses were made as previously described using R software (4, 44). Robustness (*) represents the proportion of clustering runs in which a pair of isolates appeared together in some clusters, given that they were clustered together in at least one run, averaged over all such pairs (****: >90%).