Citation
Torres-Barceló, Clara; Kojadinovic, Mila; Moxon, Richard; MacLean, Roderick Craig (2015), Data from: The SOS response increases bacterial fitness, but not evolvability, under a sublethal dose of antibiotic, Dryad, Dataset, https://doi.org/10.5061/dryad.rv766
Exposure to antibiotics induces the expression of mutagenic bacterial stress–response pathways, but the evolutionary benefits of these responses remain unclear. One possibility is that stress–response pathways provide a short-term advantage by protecting bacteria against the toxic effects of antibiotics. Second, it is possible that stress-induced mutagenesis provides a long-term advantage by accelerating the evolution of resistance. Here, we directly measure the contribution of the Pseudomonas aeruginosa SOS pathway to bacterial fitness and evolvability in the presence of sublethal doses of ciprofloxacin. Using short-term competition experiments, 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. Intriguingly, we find that the expression of the SOS pathway decreases during adaptation to ciprofloxacin, and this helps to explain why this pathway does not increase long-term evolvability. Furthermore, we argue that the SOS pathway fails to accelerate adaptation to ciprofloxacin because the modest increase in the mutation rate associated with SOS mutagenesis is offset by a decrease in the effective strength of selection for increased resistance at a population level. 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.
SOS expression cip doses
Impacts of ciprofloxacin on SOS expression. Mean relative expression (+/- s.e.m; n≥3) of a bioluminescent SOS reporter construct across a gradient of ciprofloxacin (ug/L) in a WTpLex:Lux reporter strain and a WT control lacking the reporter construct
Cell density cip doses
Impacts of ciprofloxacin on bacterial population density. Mean (+/- sem; n≥3) density of viable cells in cultures of the WT strain that were grown overnight across a gradient of ciprofloxacin (ug/L).
Fitness wt-lex cip-no ratios
Short-term fitness effects of the SOS response. Competitive fitness of the WT strain relative to the LexA mutant, as measured using overnight competition experiments in the presence of ciprofloxacin or not. Values are the average of 20 independent replicates starting from different WT:LexA frequencies and error bars are SEM.
5-days competition lex-wt
Short-term fitness effects of the SOS response. Frequency of the WT strain in competition with the LexA strain across multiple cycles of competition, in the presence of ciprofloxacin or not. After 16 generations, the WT strain had achieved such a high frequency that it could no longer be accurately measured.
Mutation rate wt-lex with cip
Mutation rate of WT and LexA in the absence or presence of ciprofloxacin, as calculated using a fluctuation test that measures the rate of mutation to rifampicin resistance. Estimation of mutation rate values (and 95 % confidence intervals) was done using MSS maximum likelihood method from eight independent replicates.
Fitness lex-wt time-evolvability
Fitness changes of WT and LexA populations that evolved in the presence and absence of ciprofloxacin. Fitness was measured relative to the ancestral WT strain in 10 replication populations for each treatment.
MIC wt-lex time
MIC scores (ciprofloxacin concentration in ug/L) in WT and LexA populations that were allowed to evolve for 200 generations in the presence and absence of ciprofloxacin. We also present the date from the ancestor (T0) populations.
SOS expression cip-no time
Relative expression of a bioluminescent SOS reporter construct in populations of WTpLex:Lux that were allowed to evolve in culture medium lacking ciprofloxacin or containing ciprofloxacin. We measured the expression of populations under the conditions in which they evolved.