DOI: 10.5061/dryad.rv766
title: Data from: The SOS response increases bacterial fitness, but not evolvability, under a sublethal dose of antibiotic

description: 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.
description: SOS expression cip
 dosesImpacts 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 constructCell density cip
 dosesImpacts 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
 ratiosShort-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-wtShort-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 cipMutation 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-evolvabilityFitness 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
 timeMIC 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
 timeRelative 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.