Effect of recA inactivation on mutagenesis of Escherichia coli exposed to sublethal concentrations of antimicrobials

Low concentrations of some antibiotics have been reported to stimulate mutagenesis and recombination, which may facilitate bacterial adaptation to different types of stress, including antibiotic pressure. However, the mutagenic effect of most of the currently used antibiotics remains untested. Furthermore, it is known that in many bacteria, including Escherichia coli, stimulation of mutagenesis is mediated by the SOS response. Thus, blockage or attenuation of this response through the inhibition of RecA has been proposed as a possible therapeutic adjuvant in combined therapy to reduce the ability to generate antibiotic-resistant mutants. The aim of this work was to study the capacity of sublethal concentrations of antimicrobials of different families with different molecular targets to increase the mutant frequency of E. coli, and the effect that inactivation of recA would have on antibiotic-mediated mutagenesis. We tested the mutagenicity of the following antimicrobials: ampicillin; ceftazidime; imipenem; fosfomycin; ciprofloxacin; trimethoprim; sulfamethoxazole; trimethoprim/sulfamethoxazole; colistin; tetracycline; gentamicin; rifampicin; and chloramphenicol. Eight out of the 13 antimicrobials tested stimulate E. coli mutagenesis (slightly in most cases), with trimethoprim, alone or in combination with sulfamethoxazole, producing the highest effect. Inactivation of recA abolishes the mutagenic effect and also produces increased susceptibility to some of the tested antimicrobials. The fact that inactivation of recA reduces mutagenicity and/or increases the activity of a large number of antimicrobials supports the hypothesis that RecA inhibition might have favourable effects on antibiotic therapy.