Plasmid-Mediated Sulfamethoxazole Resistance Encoded by the sul2 Gene in the Multidrug-Resistant Shigella flexneri 2a Isolated from Patients with Acute Diarrhea in Dhaka, Bangladesh

In this study, mechanisms of plasmid-mediated sulfamethoxazole resistances in the clinical strains of multi-drug resistant (MDR) Shigella flexneri 2a were elucidated for the first time in Bangladesh. From 2006 to 2011, a total of 200 S. flexneri 2a strains were randomly selected from the stock of the Enteric and Food Microbiology Laboratory of icddr,b. Antimicrobial susceptibility of the strains showed 73%, 98%, 93%, 58%, 98%, 64% and 4% resistance to trimethoprim-sulfamethoxazole, nalidixic acid, ampicillin, erythromycin, tetracycline, ciprofloxacin and ceftriaxone respectively. Plasmid profiling revealed heterogeneous patterns and interestingly, all the trimethoprim-sulfamethoxazole resistant (SXT(R)) strains yielded a distinct 4.3 MDa plasmid compared to that of the trimethoprim-sulfamethoxazole susceptible (SXT(S)) strains. Curing of this 4.3 MDa plasmid resulted in the susceptibility to sulfamethoxazole alone suggesting the involvement of this plasmid in the resistance of sulfamethoxazole. Moreover, PCR analysis showed the presence of sul2 gene in SXT(R) strains which is absent in SXT(S) strains as well as in the 4.3 MDa plasmid-cured derivatives, confirming the involvement of sul2 in the resistance of sulfamethoxazole. Furthermore, pulsed-field gel electrophoresis (PFGE) analysis revealed that both the SXT(R) and SXT(S) strains were clonal. This study will significantly contributes to the knowledge on acquired drug resistance of the mostly prevalent S. flexneri 2a and further warrants continuous monitoring of the prevalence and correlation of this resistance determinants amongst the clinical isolates of Shigella and other enteric pathogens around the world to provide effective clinical management of the disease.