Tobramycin adaptation alters the antibiotic susceptibility ofPseudomonas aeruginosaquorum sensing-null mutants

Abstract

ABSTRACTThe opportunistic bacteriumPseudomonas aeruginosauses 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 thelasR-null mutants to persist under tobramycin selection. We hypothesized that the effects of inactivatinglasRon 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 inactivatedlasRin several highly tobramycin-resistant isolates from long-term evolution experiments. We show that the effects of ΔlasRon tobramycin resistance are strain dependent, which is due to a single mutation in thefusA1gene encoding the translation elongation factor EF-G1A (G61A nucleotide substitution). ThefusA1G61A mutation confers a strong selective advantage to ΔlasRmutants under tobramycin treatment. The effects offusA1G61A on ΔlasR-dependent tobramycin resistance are dependent on the MexXY efflux pump and the MexXY regulator ArmZ. ThefusA1mutation also modulates ΔlasRmutant resistance to two other antibiotics, ciprofloxacin and ceftazidime. Our results provide a possible explanation for the emergence oflasR-null mutants in clinical isolates and illustrate the importance of epistatic gene interactions in the evolution of quorum sensing.