Mutators among CTX-M β-lactamase-producing Escherichia coli and risk for the emergence of fosfomycin resistance

Objectives: Fosfomycin is a possible oral treatment for lower urinary tract infections caused by Escherichia coli with CTX-M extended-spectrum beta-lactamases but is vulnerable to mutational resistance. Hypermutability among natural E. coli populations might facilitate the emergence of resistance to fosfomycin. We therefore examined the prevalence of mutators amongst urinary isolates of E. coli producing CTX-M beta-lactamases. Methods: Urinary E. coli isolates with CTX-M beta-lactamases (n = 220) were screened for resistance to both rifampicin and fosfomycin, as well as a mutator phenotype, by rifampicin and fosfomycin disc assays. Mutation frequencies for 10 isolates, identified as mutators by the initial disc screen, were determined in triplicate on agar with rifampicin or fosfomycin at 4x MIC and with fosfomycin or nitrofurantoin at 256 mg/L. Results: The disc screen identified 10 likely mutators and quantitative tests indicated that 9 of these had mutation frequencies of 8.0 x 10(-6)-1.5 x 10(-4) for fosfomycin and 0.1-2.3 x 10(-6) for rifampicin. These mutators were diverse in terms of PFGE type and 4 of the 10 were confirmed as strong mutators with rifampicin and fosfomycin. Only the strongest mutator isolate and hypermutable MutS(-) control strain consistently gave single-step mutants resistant to 256 mg/L fosfomycin. No nitrofurantoin-resistant mutants were selected from any isolate, although they could be selected from the hypermutable MutS(-) control strain. Conclusions: Mutator phenotypes were found among E. coli expressing CTX-M beta-lactamases and were independent of strain type. These had an increased propensity to fosfomycin resistance.