The Pseudomonas aeruginosa LasR-I quorum-sensing system regulates secreted proteases that can be exploited by cheaters, such as quorum sensing receptor-defective (lasR) mutants. lasR mutants emerge in populations growing on casein as a sole source of carbon and energy. These mutants are exploitative cheaters because they avoid the substantial cost of engaging in quorum sensing. Previous studies showed that quorum sensing increases resistance to some antibiotics, such as tobramycin. Here, we show that tobramycin suppressed the emergence of lasR mutants in casein-passaged populations. Several mutations accumulated in those populations indicating evidence of antibiotic adaptation. We found that mutations in one gene, ptsP, increased antibiotic resistance and also pleiotropically increased production of a quorum sensing-controlled phenazine, pyocyanin. When passaged on casein, ptsP mutants suppressed cheaters in a manner that was tobramycin independent. We found the mechanism of cheater suppression in ptsP mutants relied on pyocyanin, which acts as a policing toxin by selectively blocking growth of cheaters. Thus, tobramycin suppresses lasR mutants through two mechanisms: first, through direct effects on cheaters and second, by selecting mutations in ptsP that suppressed cheating in a tobramycin-independent manner. This work demonstrates how adaptive mutations can alter the dynamics of cooperator-cheater relationships, which might be important for populations adapting to antibiotics during interspecies competition or infections. IMPORTANCE The opportunistic pathogen Pseudomonas aeruginosa is a model for understanding quorum sensing, a type of cell-cell signaling important for cooperation. Quorum sensing controls production of cooperative goods, such as exoenzymes, which are vulnerable to cheating by quorum sensing-defective mutants. Because uncontrolled cheating can ultimately cause a population to collapse, much focus has been on understanding how P. aeruginosa can control cheaters. We show that an antibiotic, tobramycin, can suppress cheaters in cooperating P. aeruginosa populations. Tobramycin suppresses cheaters directly because the cheaters are more susceptible to tobramycin than cooperators. Tobramycin also selects for mutations in a gene, ptsP, that suppresses cheaters independent of tobramycin through pleiotropic regulation of a policing toxin, pyocyanin. This work supports the idea that adaptation to antibiotics can have unexpected effects on the evolution of quorum sensing and has implications for understanding how cooperation evolves in dynamic bacterial communities.

The Pseudomonas aeruginosa LasR-I quorum sensing (QS) system regulates secreted proteases that can be exploited by cheaters, such as QS receptor-defective (lasR) mutants. lasR mutants emerge in populations growing on casein as a sole source of carbon and energy and increase in the population because they do not incur the substantial cost of engaging in QS. QS also increases resistance to some antibiotics, such as tobramycin. Here, we show that tobramycin suppresses the emergence of lasR mutants in casein-passaged populations. We also identify several mutations that accumulate in those populations indicating evidence of antibiotic adaptation. Mutations in one gene, ptsP, increase activity of the LasR-I system and production of a QS-controlled phenazine, pyocyanin. We find that mutations in ptsP lead to suppression of cheaters independent of tobramycin. Cheater suppression relies on pyocyanin, which acts as a policing toxin by targeting cheaters. These results show that tobramycin suppresses lasR mutants through two mechanisms: first, by directly acting on tobramycin-susceptible cheaters and second, by selecting mutations in ptsP that lead to pyocyanin-dependent policing. This work demonstrates how adaptive mutations can alter the dynamics of cooperator-cheater relationships, which might be important for populations adapting to antibiotics during infections.

The opportunistic bacterium Pseudomonas aeruginosa uses the LasR-I quorum sensing system to increase resistance to the aminioglycoside 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 δlasR on tobramycin resistance are strain dependent. The effects can be attributed to a point mutation in the gene encoding the translation elongation factor fusA1 (G61A nucleotide substitution), which confers a strong selective advantage to lasR-null PA14 under tobramycin selection. This fusA1 G61A mutation results in increased activity of the MexXY efflux pump and expression of the mexXY regulator ArmZ. The fusA1 mutation can also modulate δlasR mutant resistance to two other antibiotics, ciprofloxacin and ceftazidime. Our results demonstrate the importance of epistatic gene interactions on antibiotic susceptibility of lasR-null mutants. These results support of the idea that gene interactions might play a significant role in the evolution of quorum sensing in P. aeruginosa.

The bacterium Pseudomonas aeruginosa causes serious disease in immunocompromised patients and is a model for studying quorum sensing, a cell‐cell signaling system that becomes activated at a certain population or “quorum.” The P. aeruginosa master quorum sensing regulator is LasR, which drives transcription of dozens of genes including those coding for virulence and antibiotic resistance. Paradoxically, lasR‐null mutants are common in infections of tobramycin‐treated patients, presenting a challenge to ongoing efforts to develop anti‐LasR therapeutics. To understand how these mutants might arise, we performed a laboratory evolution experiment. We grew P. aeruginosa populations with sublethal tobramycin. Every day, we transferred the population to fresh medium and every three days we increased the tobramycin concentration. Variants from these populations became highly tobramycin resistant. Surprisingly, inactivating LasR in some of the variants further increased resistance, which was the opposite effect as that observed in the ancestral strain where LasR inactivation decreased tobramycin resistance. This phenomenon is known as sign epistasis. The evolved variants with the altered LasR phenotype all encoded a point mutation in the translation elongation factor fusA1. fusA1frequently incurs mutations as a mechanism of adaptation to tobramycin in antibiotic‐treated patient infections. We showed that the fusA1 mutation reversed the effect of LasR on antibiotic resistance, and thus is responsible for sign epistatic effects on LasR. Our results suggest that antibiotic selection might drive the accumulation of mutations that reverse the effect of LasR mutants on tobramycin resistance. These results could possibly explain how lasR mutants emerge in infected patients and have implications for developing novel therapeutics to treat antibiotic‐resistant infections.

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