Mutations in gyrA and parC genes in nalidixic acid-resistant Escherichia coli strains from food products, humans and animals.

Mutations in quinolone targets were analysed in 80 unrelated nalidixic acid-resistant (NALR) Escherichia coli strains whose nalidixic acid and ciprofloxacin MICs ranged from 32 to >256 mg/L and 0.03-64 mg/L, respectively. These strains were isolated from food products (23) and faecal samples from humans (15) and healthy animals (42). Thirteen nalidixic acid-susceptible (NALS) E. coli strains were also analysed. Mutations in gyrA and parC genes were studied by PCR and sequencing. No amino acid changes were detected in GyrA or ParC proteins of the 13 NALS strains. A single change in the GyrA protein was detected in all 61 NALR strains with ciprofloxacin MICs Leu (54), Ser-83-->Ala (one), Ser-83-->Val (one), Asp-87-->Asn (two), Asp-87-->Tyr (two) and Asp-87-->Gly (one). A double change in GyrA was found in 18 of 19 NALR strains with ciprofloxacin MICs >/= 4 mg/L. Amino acid substitutions were Ser-83-->Leu, with an additional change [Asp-87-->Asn (15), Asp-87-->Tyr (two) or Asp-87-->His (one)]. The remaining strain (ciprofloxacin MIC 4 mg/L) showed a single Ser-83-->Leu substitution. In respect of the ParC protein, a single change at Ser-80 or Glu-84 was found in 25 of 42 strains, with ciprofloxacin MICs ranging from 0.5 to 32 mg/L. A double substitution (Ser-80-->Ile and Glu-84-->Gly) was found in one strain (ciprofloxacin MIC 64 mg/L). No amino acid changes were detected in the GyrB protein of 18 NALR strains.