Plasmid-mediated quinolone resistance in Shigella sonnei and Shigella flexneri isolated from pediatric diarrheal

Fluoroquinolones are the most commonly antibiotic in treatment of Shigella infections worldwide. The presence of plasmids carrying quinolone-resistance genes is one of the most important mechanisms for resistance to these drugs. Therefore, the aim of this study was to investigate the presence of quinolone resistance plasmid genes (qnr) in Shigella sonnei and Shigella flexneri isolated from pediatric diarrhea.
In a 10-month study from the beginning of June to the end of March 2012, 91 isolates of Shigella were obtained from 358 non-repetitive diarrhea samples. After identification and confirmation of Shigella isolates, an antibiotic susceptibility test was performed based on the CLSI instruction. The molecular PCR test was performed to amplify qnrA, qnrB, qnrC, qnrD and qnrS genes.
69.2% and 30.7% of isolates were Shigella sonnei and Shigella flexneri in the Slide agglutination test, respectively. The results of the gel diffusion test showed that all Shigella flexneri were susceptible to levofloxacin. Most of the Shigella sonnei were resistant to Streptomycin. The frequency of qnrA, qnrB and qnrS genes were 26.4%, 74.7% and 46.2%, respectively. All strains were negative for qnrC and qnrD genes.
The qnrB gene is the most common PMQR gene in our treatment center; and it's related to plasmid source, the ability to transmit and high ability to spread to other isolates. It is suggested that, due to the continuous change of antibiotic susceptibility pattern, periodic bacterial sensitivity performed among different populations and appropriate experimental treatment should be selected.