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Abstract
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 (SXTR) strains yielded a distinct 4.3 MDa plasmid compared to that of the trimethoprim-sulfamethoxazole susceptible (SXTS) 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 SXTR strains which is absent in SXTS 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 SXTR and SXTS 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.
Highlights
Shigellosis is becoming an increasing public health problem due to the emergence of multiple antimicrobial resistances, leading to high rate of global morbidity and mortality especially in the endemic areas like Bangladesh
In order to clarify the association of small plasmids in the drug resistance of Shigella, 200 S. flexneri 2a clinical strains were subjected to antimicrobial susceptibility testing and found that 73%, 98%, 93%, 58%, 98%, 64% and 4% resistance to SXT, NA, AMP, E, TET, CIP and CRO respectively (Data not shown)
All the SXTR (n = 146) strains harbored 4.3 MDa plasmid which is absent in all the SXTS strains (Table 1) and among the eight different plasmid patterns, pattern P1 (140, 4.3, 2.7 and 2.1 MDa) and P6 (140, 2.7, 2.1 MDa) were predominant in SXTR (n = 146) (91.2%) and SXTS (n = 54) (84%) strains
Summary
Shigellosis is becoming an increasing public health problem due to the emergence of multiple antimicrobial resistances, leading to high rate of global morbidity and mortality especially in the endemic areas like Bangladesh. In developing countries like Bangladesh, the predominant serotype of S. flexneri is 2a which is the most common strain in industrialized countries [1]. Efficacious first-line antimicrobial drugs such as sulphonamides, tetracycline, ampicillin, and trimethoprim-sulfamethoxazole have become largely ineffective against prevalent Shigella strains in many parts of the world and the recently reported emergence of ciprofloxacin and third-generation cephalosporin resistance, further narrows the choice of effective antimicrobials [2]. The American Academy of Pediatrics and the Infectious Disease Society of America recommend azithromycin as an alternative drug for the treatment of Shigellosis [3]. Plasmid-mediated multidrug resistance is a grave concern for the treatment of infectious diseases. Multiple plasmid-mediated mechanisms of resistance against the fluoroquinolones and aminoglycosides have been described, and the combination of plasmid-mediated resistance with chromosomally encoded resistance mechanisms of multiple drug classes results in strains that are resistant to all of the main classes of commonly used antimicrobial drugs [7]
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