Tobramycin adaptation alters the antibiotic susceptibility of Pseudomonas aeruginosa quorum sensing-null mutants

The opportunistic bacterium Pseudomonas aeruginosa uses the LasR-I quorum sensing system to increase resistance to the aminoglycoside antibiotic tobramycin. Paradoxically, lasR-null mutants are commonly isolated from chronic human infections treated with tobramycin, suggesting there may be a mechanism allowing the lasR-null mutants to persist under tobramycin selection. We hypothesized that the effects of inactivating lasR on tobramycin resistance might be dependent on the presence or absence of other gene mutations in that strain, a phenomenon known as epistasis. To test this hypothesis, we inactivated lasR in several highly tobramycin-resistant isolates from long-term evolution experiments. We show that the effects of {Delta}lasR on tobramycin resistance are strain dependent, which is due to a single mutation in the fusA1 gene encoding the translation elongation factor EF-G1A (G61A nucleotide substitution). The fusA1 G61A mutation confers a strong selective advantage to {Delta}lasR mutants under tobramycin treatment. The effects of fusA1 G61A on {Delta}lasR-dependent tobramycin resistance are dependent on the MexXY efflux pump and the MexXY regulator ArmZ. The fusA1 mutation also modulates {Delta}lasR mutant resistance to two other antibiotics, ciprofloxacin and ceftazidime. Our results provide a possible explanation for the emergence of lasR-null mutants in clinical isolates and illustrate the importance of epistatic gene interactions in the evolution of quorum sensing.