Efflux pump mutations in Pseudomonas aeruginosa cause low-level clinical resistance and high-level tolerance to antibiotics in patients

Pseudomonas aeruginosa is one of the microorganisms with high-risk regarding antimicrobial resistance, since it has an overwhelming capacity to acquire antibiotic resistance, mainly by mutations during persistent lung infections. Mutations in mexZ, encoding the local negative regulator of genes encoding the MexXY efflux pump, are very frequently acquired at early stages of P. aeruginosa infections, while they are rarely selected for in vitro. Although traditionally related to resistance to the first-line drug tobramycin, caused by the overproduction of the aminoglycosides MexXY efflux pump, mutations in mexZ are actually associated with low levels of aminoglycosides resistance when determined in the clinical microbiology laboratory. This very moderate but frequent phenotype suggests that these mutations may shape the infection process, beyond causing conventional resistance. Here we investigated the colonization strategy of a mexZ mutant, compared to a wild-type strain, in a human airway infection model. We observed that the mexZ mutant tends to accumulate inside the epithelial cell layer. This behaviour allows bacteria to colonise the epithelium while being more protected against diverse antibiotics. The altered colonization phenotype was caused by the overexpression of lecA, a Quorum Sensing regulated gene encoding a lectin involved in P. aeruginosa tissue invasiveness. lecA upregulation was underlied by the competition for the shared porin, OprM, between the overproduced MexXY and the MexAB efflux pump, responsible for extruding Quorum Sensing molecules. These findings suggest that standardised antimicrobial susceptibility determined in the clinic may be misleading because antibiotic resistance often depends on the infection environment.