Tobramycin adaptation enhances policing of social cheaters in Pseudomonas aeruginosa
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.