We looked for the presence of gyrA mutations in seven fluoroquinolone-resistant French clinical isolates of Campylobacter jejuni and Campylobacter coli . Three of the five isolates of C. jejuni and the two isolates of C. coli had high-level resistance to nalidixic acid (MICs 128-256 μ g/ml) and ciprofloxacin (MICs 32 μ g/ml). A gyrA mutation was found in all these isolates leading to the following substitutions: Thr86-Ile in four cases and Asp90-Tyr for one C. coli strain. One isolate had high-level resistance to nalidixic acid (MIC 64 μ g/ml) but low-level resistance to ciprofloxacin (MIC 2 μ g/ml) and also carried a gyrA mutation leading to a Thr86-Ala substitution. The last isolate of C. jejuni studied displayed an atypical resistance phenotype: It was resistant to high levels of ciprofloxacin (MIC 64 μ g/ml) but remained fully susceptible to nalidixic acid (MIC 2 μ g/ml). This phenotype was not explained by the presence of peculiar mutations in gyrA or gyrB . It carried a gyrA mutation leading to a Thr86-Ile substitution and was devoid of gyrB mutation. Despite numerous attempts with various degenerate oligonucleotide primers deduced from conserved regions of known parC genes, we were unable to amplify a corresponding sequence in C. jejuni or C. coli . First-step and second-step in vitro mutants, derived from reference strain C. coli ATCC 33559 with ciprofloxacin or moxifloxacin as selecting agents, were found to carry one and two mutations in gyrA , respectively. In contrast with the results obtained with clinical isolates, a variety of gyrA mutations were obtained in vitro .

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