Novel Combination of Mutations in the DNA Gyrase and Topoisomerase IV Genes in Laboratory-Grown Fluoroquinolone-Resistant Shigella flexneri Mutants

A ciprofloxacin-susceptible strain (wild type [WT]) ofShigella flexneri (MIC, 0.015 μg/ml), isolated from a stool sample in 1996 was passaged (about 1010 cells) onto ciprofloxacin-containing (0.5 to 32 μg/ml) Isosensitest medium (Oxoid) to obtain single colonies after 36-h incubation. Two resistant mutants (A and B) were thus obtained. Table1 shows MICs of nine antimicrobial agents, determined by the agar dilution method (14).Table 1.Mutations in the gyrA and parC QRDR in the WT and mutants A and B and MICs of nine antimicrobial agentsStrainMIC (μg/ml)agyrA codon 87parCCIPNALNORGENTOBACRTETCHLIMPCodon 79Codon 84WT0.01510.120.50.2516128640.25GAC (Asp)GAT (Asp)GAA (Glu)A16>1,024320.51325125120.25TAC (Tyr)GCT (Ala)GAA (Glu)B64>1,024640.51645125120.12TAC (Tyr)GCT (Ala)GCA (Ala)aCIP, ciprofloxacin; NAL, nalidixic acid; NOR, norfloxacin; GEN, gentamicin; TOB, tobramycin; ACR, acriflavin; TET, tetracycline; CHL, chloramphenicol; IMP, impenem.