Fluoroquinolone Resistance Mechanisms in Shigella Isolates in Shanghai, China, Between 2010 and 2015

Objectives: This study aimed to investigate the antimicrobial susceptibility of Shigella isolated in Shanghai, China and to determine the genetic basis of its resistance to fluoroquinolones. Materials and Methods: A total of 402 strains of Shigella , including 145 Shigella flexneri and 257 Shigella sonnei isolates, were analyzed. The Kirby–Bauer disk diffusion method was used to determine the susceptibility of the strains to 13 antimicrobials. Minimum inhibitory concentration of ciprofloxacin was determined by E-test. Mutations within the quinolone resistance-determining regions (QRDRs) of gyrA and parC and in the plasmid-mediated quinolone resistance (PMQR) genes, including qnrA, qnrB, qnrS, and aac (6′)-Ib-cr , were detected by polymerase chain reaction. All the products were then sequenced. Results: Most of the Shigella isolates were found to be resistant to nalidixic acid (96.4%), streptomycin (96.4%), ampicillin (86.2%), tetracycline (79.8%), and sulfamethoxazole/trimethoprim (80.6%). S. flexneri isolates showed a significantly higher resistance to cefepime (33.6%), ciprofloxacin (54.2%), norfloxacin (34.1%), and levofloxacin (12.1%) compared with that observed for the S . sonnei strains (χ 2 analysis, p