An Amino Acid Substitution in Elongation Factor EF-G1A Alters the Antibiotic Susceptibility of Pseudomonas Aeruginosa LasR-Null Mutants

The opportunistic bacterium Pseudomonas aeruginosa uses the LasR-I quorum-sensing system to increase resistance to the aminoglycoside antibiotic tobramycin. Paradoxically, -null mutants are commonly isolated from chronic human infections treated with tobramycin, suggesting there may be a mechanism that permits the emergence of -null mutants under tobramycin selection. We hypothesized that some other genetic mutations that emerge in these isolates might modulate the effects of -null mutations on antibiotic resistance. To test this hypothesis, we inactivated in several highly tobramycin-resistant isolates from long-term evolution experiments. In some of these isolates, inactivating further increased resistance, compared with decreasing resistance of the wild-type ancestor. These strain-dependent effects were due to a G61A nucleotide polymorphism in the gene encoding amino acid substitution A21T in the translation elongation factor EF-G1A. The EF-G1A mutational effects required the MexXY efflux pump and the MexXY regulator ArmZ. The mutation also modulated Δ mutant resistance to two other antibiotics, ciprofloxacin and ceftazidime. Our results identify a gene mutation that can reverse the direction of the antibiotic selection of mutants, a phenomenon known as sign epistasis, and provide a possible explanation for the emergence of -null mutants in clinical isolates. One of the most common mutations in Pseudomonas aeruginosa clinical isolates is in the quorum sensing gene. In laboratory strains, disruption decreases resistance to the clinical antibiotic tobramycin. To understand how mutations emerge in tobramycin-treated patients, we mutated in highly tobramycin-resistant laboratory strains and determined the effects on resistance. Disrupting enhanced the resistance of some strains. These strains had a single amino acid substitution in the translation factor EF-G1A. The EF-G1A mutation reversed the selective effects of tobramycin on mutants. These results illustrate how adaptive mutations can lead to the emergence of new traits in a population and are relevant to understanding how genetic diversity contributes to the progression of disease during chronic infections.