Objectives: The aim of this study was to characterize fluoroquinolone-resistant Shigella and determine whether the qnr and aac(6')-Ib-cr genes could contribute to sporadic shigellosis at the clinic in the Hangzhou area of China.
Methods: A total of 202 strains of Shigella (79 Shigella sonnei and 123 Shigella flexneri ) isolated from sporadic cases of shigellosis from 1998 to 2007 were analysed for their antimicrobial susceptibility. The gyrA, gyrB, parC, parE, qnr and aac(6')-Ib-cr genes and the profiles and incompatibility of plasmids were characterized. Chromosomal DNA fingerprinting was determined by XbaI-based digestion and PFGE.
Results: All strains of S. sonnei were susceptible to fluoroquinolones (ciprofloxacin and levofloxacin) while 15 out of 123 strains of S. flexneri were resistant. All of the 15 resistant strains displayed common mutations in the gyrA and parC genes and formed eight distinct groups with unique molecular characteristics. Notably, 10 isolates showed mutations at codon 87 of gyrA, and the other 5 were qnrS-positive. Two strains were positive for the aac(6')-Ib-cr gene. Importantly, this is the first report of qnrS- and aac(6')-Ib-cr-positive Shigella in China, the qnrS-positive S. flexneri serotypes 1a, 2a and 4c and the aac(6')-Ib-cr-positive S. flexneri serotypes 2a and 4c worldwide.
Conclusions: The common mutations at position 83 of gyrA and position 80 of parC were crucial for resistance to nalidixic acid in S. flexneri. The mutation at position 87 of gyrA or the presence of the qnrS gene is necessary for high-level resistance to fluoroquinolones in Shigella isolates from China.