Quorum-sensing regulation of antibiotic resistance and antibiotic-dependent effects on social cheating in the bacterial pathogen Pseudomonas aeruginosa

Abstract

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. The LasR-I system is essential for virulence and increases resistance to aminoglycoside antibiotics. However, lasR is a mutation hotspot in clinical P. aeruginosa isolates. Here, we explore how adaptation to aminoglycosides tobramycin and gentamicin influences the evolutionary trajectory of quorum sensing. We show that both antibiotics suppressed the emergence of lasR mutants in casein-passaged populations. Several mutations accumulated in those populations indicating evidence of antibiotic adaptation. We found that inactivation of the nitrogen phosphotransferase gene, ptsP, suppressed cheaters through the policing toxin pyocyanin. Cheater suppression through ptsP-independent mechanisms were also evident in gentamicin-adapted variants. Although ptsP inactivation suppresses cheating, ΔptsP populations are prone to collapse. Population collapse was also observed in ΔygdP and ΔmdpA. The mechanistic pathway behind the population collapse remains unknown. We also show that a point mutation in the elongation factor, fusA1 G61A (FusA1A21T) reverses the role of LasR in tobramycin resistance. This mutation increases the antibiotic resistance of the lasR mutants through an ArmZ-dependent upregulation of the MexXY efflux pump. Inactivation of lasR has an additive effect on ribosome stalling which induces ArmZ expression. This illustrates how losing LasR function may be advantageous in antibiotic-treated environments and may provide a potential explanation for the high frequency of lasR mutants in clinical samples. Overall, this work provides insights on how antibiotic selection alters quorum sensing and quorum sensing-regulated functions. These pleiotropic effects of antibiotics on quorum sensing might be important for populations adapting to antibiotics during interspecies competition or infections.