Mutations in the gyrA and parC genes of quinolone-resistant Shigella flexneri clinical isolates

Objective To identify the mutations of DNA gyrase gyrA and Topoisomerase Ⅳ parC genes in quinolone-resistant Shigella flexneri clinical isolates and evaluate the relevance of amino acid changes in GyrA and ParC to quinolone resistance. Methods Based on the antimicrobial susceptibility testing, 47 S.flexneri clinical isolates with different quinolone susceptibility were selected and the fragments including the quinolone resistance-determining region (QRDR) in gyrA and parC were PCR amplified and sequenced. SAS (V 8.2) software was used to analyze the correlation between quinolone resistance and changes in GyrA and ParC. Results Sense mutation(s) in gyrA and parC were commonly observed in all of 44 quinolone-resistant isolates, whereas no sense mutation was found in the 3 quinolone-susceptible ones. The most frequent mutation is at codon 83(TCG→TTG) of gyrA, which was observed in 43 quinolone-resistant isolates. The mixed model analysis indicated that the alterations in amino acid 83 of GyrA and amino acid 80 of ParC are close related to nalidixic acid resistance (P0.01, R~2=0.999) and alterations in amino acid 83,87 of GyrA are related to ciprofloxacin resistance (P0.05, R~2=0.872). Conclusions The substitution Leu to Ser83 in GyrA apparently showed to be the pivotal mutation resulting in quinolone resistance in S.flexneri clinical isolates tested here. This single mutation could lead to resistance to nalidixic acid, while to obtain resistance to ciprofloxacin, the presence of additional mutation(s) in GyrA and/or ParC or other resistance mechanisms are required.