Two distinct mutations in gyrA lead to ciprofloxacin and nalidixic acid resistance in Campylobacter coli and Campylobacter jejuni isolated from chickens and beef cattle

Aims: The aim of this study was to identify point mutations in the gyrA quinolone resistance determining region (QRDR) of Campylobacter coli (n = 27) and Campylobacter jejuni (n = 26) that confer nalidixic acid (NAL) resistance without conferring resistance to ciprofloxacin (CIP). Methods and Results: Point mutations in the QRDR of gyrA from C. coli and C. jejuni isolates were identified by direct sequencing. All isolates (n = 14) with minimum inhibitory concentrations (MICs) >= 4 mu g ml(-1) for CIP and >= 32 mu g ml(-1) for NAL possessed a missense mutation leading to substitution of Ile for Thr at codon 86. Three isolates with a missense mutation leading to a Thr86Ala substitution had MICs < 4 mu g ml(-1) for CIP and >= 32 mu g ml(-1) for NAL. Conclusions: These data confirm previous findings that Thr86Ile mutations confer resistance to both CIP and NAL. However, resistance to NAL alone was conferred by a single Thr86Ala mutation. Significance and Impact of the Study: Resistance to NAL alone arises independently from CIP resistance. In addition, the role of other previously described point mutations in quinolone resistance is discussed.