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

Abstract

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). 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) >or=4 microg ml(-1) for CIP and >or=32 microg 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 mug ml(-1) for CIP and >or=32 microg ml(-1) for NAL. 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. Resistance to NAL alone arises independently from CIP resistance. In addition, the role of other previously described point mutations in quinolone resistance is discussed.