Study On The Resistance-related Genes Of Shigella With Decreased Susceptibility Fluoroquinolones

Objectives:To investigate the prevalence of Nalidixic Acid-Resistant Shigella isolates with decreased susceptibility to fiuoroquinolones and explore its relationship with drug target genes gyrA and parC mutations as well as plasmid-mediated quinolones resistance genes.Methods:A total of153strains of Shigella isolated from2008to2010were analysed for their antimicrobial susceptibility. Mutations within the quinolone resistance determining regions of gyrA and parC and plasmid-mediated resistance determinants(qnr, aac(6’)-Ib-cr) were screened. PFGE was used to investigate the clonality of Nalidixic Acid-Resistant Shigella isolates. Conjugation was used to clarify the resistance genes located on plasmid.Results:Resistance to nalidixic acid, trimethoprim-sulfamethoxazole, ampicillin and tetracycline was high (100,92.8,93.2and93.9%, respectively). All of131strains of Shigella Sonnei was susceptible to norfloxacin, ciprofloxacin, levofloxacin, but94%of them showed reduced susceptibilities to fluoroquinolones. All of Nalidixic Acid-Resistant Shigella sonnei strains displaying a single mutation at codon83(TCG�'TTG) in the gyrA genes resulted in the replacement of serine by leucine, but no mutations in the parC genes and plasmid-mediated quinolones resistance genes qnr and aac(6’)-Ib-cr were detected. The PFGE analysis of all131strains of Shigella Sonnei revealed four major patterns.22strains of Shigella flexneri except one were resistant to nalidixic acid and showed reduced susceptibilities to fluoroquinolones.Among them6isolates were resistant to fluoroquinolones. Mutation analysis showed that ciprofloxacin-resistant flexneri Shigella have more than two sites mutations within the quinolone resistance determining regions of gyrA and parC. Each carry the PMQR genes qnr and aac(6’)-Ib-cr gene located on plasmids. The PFGE analysis of22strains of Shigella flexneri revealed two major patterns.Conclusions:The nalidixic acid-resistant Shigella sonnei strains showing reduced susceptibility to fluoroquinolones were common in the clinical strains, mainly due to a single mutation at codon83(TCG�'TTG) in the gyrA genes. Fluoroquinolones-resistant isolates are emerging in the nalidixic acid-resistant Shigella flexneri strains, mainly due to mutiple sites mutation in the gyrA and parC genes. Emergence of PMQR gene made it possible that the fluoroquinolone-resistant Shigella stains are Widely disseminated.