Subinhibitory concentrations of ciprofloxacin induce SOS response and mutations of antibiotic resistance in bacteria

In this work, Salmonella typhimurium strains (Sal94) containing plasmid-borne fusions of Vibrio fischeri lux to the recA promoter was used to test the SOS response induced by sub-minimum inhibitory concentration (sub-MIC) of ciprofloxacin. The SOS response of Sal94 strain was rapidly increased during 20 min when supplemented with sub-MIC ciprofloxacin, but came down thereafter. The induction level of 1/2 MIC was higher than that of 1/4 MIC, showing some dose-dependency. Both mutation frequencies (MF) of anti-ciprofloxacin resistance and transformation frequency of pMD18-T Vector ( lacZ ori Amp r ) caused by the treatment of 1/4 and 1/2 MIC ciprofloxacin in Escherichia coli AB1157 strain (wild-type recA , SOS inducible) were markedly increased, but there were no or only slight changes in Escherichia coli IC400 strain ( recA mutant, deficient in SOS response). These results, combining the positive relationship between the induction factor and the MF of anti-ciprofloxacin induced by sub-MIC ciprofloxacin in AB1157 strain, indicated that SOS response played an important role in the acquirement of antibiotic resistance resulting from sub-MIC ciprofloxacin treatment.