Data from: The SOS response increases bacterial fitness, but not evolvability, under a sub-lethal dose of ciprofloxacin

Exposure to stressors, including antibiotics, induces the expression of bacterial response pathways that could increase fitness over the short term, by increasing physiological tolerance, and over the long term, by increasing the ability of bacterial populations to genetically adapt to stress as a result of stress-induced mutagenesis. Here, we directly measure the contribution of stress responses to bacterial fitness over both ecological and evolutionary timescales using the antibiotic-induced expression of the mutagenic SOS response pathway as a model system. Using short-term competition experiments between P. aeruginosa and an isogenic LexA mutant that is incapable of inducing the SOS response, we demonstrate that the SOS pathway increases competitive fitness in the presence of ciprofloxacin. Continued exposure to ciprofloxacin results in the rapid evolution of increased fitness and antibiotic resistance, but we find no evidence that SOS-induced mutagenesis accelerates the rate of adaptation to ciprofloxacin during a 200 generation selection experiment.Our findings suggest that the primary evolutionary benefit of the SOS response is to increase bacterial competitive ability, and that stress-induced mutagenesis is an unwanted side-effect, and not a selected attribute, of this pathway.

暴露于抗生素等胁迫因子时，细菌会诱导应答通路表达：此类通路可通过提升生理耐受性在短期内提升细菌适合度，亦可通过胁迫诱导诱变（stress-induced mutagenesis）使细菌种群获得遗传适应胁迫的能力，从而在长期内提升适合度。本研究以抗生素诱导的致突变性SOS应答通路（SOS response pathway）为模型系统，直接测定了细菌应答通路在生态与进化时间尺度上对细菌适合度的贡献。我们通过铜绿假单胞菌（P. aeruginosa）与无法诱导SOS应答的同基因LexA突变体（LexA mutant）之间的短期竞争实验，证实了环丙沙星（ciprofloxacin）存在的条件下，SOS通路可提升竞争适合度。持续暴露于环丙沙星会导致适合度与抗生素抗性快速进化，但在为期200代的选择实验中，我们未发现任何证据表明SOS诱导的诱变可加速细菌对环丙沙星的适应速率。本研究结果显示，SOS应答的主要进化益处在于提升细菌的竞争能力，而胁迫诱导诱变仅是该通路的意外副产物，并非该通路被选择保留的性状。