Swarming motility growth favours the emergence of a subpopulation ofPseudomonas aeruginosaquorum-sensing mutants

Pseudomonas aeruginosaexploits several types of motility behaviours to colonize diverse environments. One of these is swarming motility, a coordinated group movement on a semi-solid surface. This bacterium needs to express a functional flagellum and produce rhamnolipids to display this type of social motility. A Delta hptBmutant, a gene part of the Gac/Rsm signalling pathway, produces rhamnolipids and expresses a functional flagellum but has an important swarming defect. Experimental-directed evolution was performed on this mutant under swarming conditions to obtain compensatory mutations and thus identify genes responsible for its deficient swarming phenotype. Unexpectedly, a gain-of-function subpopulation emerged from this evolution with mutations inlasR, which codes for a key quorum-sensing transcriptional regulator. Furthermore, we found thatlasR(-)mutants even emerge at high frequencies in the wild-type strain when using the same experimental evolution strategy. The resulting evolved population, largely composed of LasR-defective mutants, is fitter than the original strain in swarming motility. We also established thatlasR(-)mutants have a growth advantage under swarming conditions when compared with wild-type. Our results demonstrate that a social phenotype, that is, swarming motility, favours the emergence of mutants deficient in a quorum-sensing regulatory pathway to the benefit of the whole population.