Extended-Spectrum β-Lactamase-Producing Salmonella Species Isolated from Diarrhoeal Patients in Bangladesh: Characterization and Their Dissemination through Conjugation

Objective: Infections with Salmonella resistance to extended-spectrum cephalosporins threaten the efficacy of drugs for treating samonellosis. So the aim of this study was to characterize the ESBL (Extended Spectrum β-lactamase)-producing Salmonella species.
Methods: The ESBL-producers were extensively characterized using antibiogram, double disc diffusion synergy test, plasmid profiling, PCR, pulsed-field gel electrophoresis (PFGE) and conjugation experiments.
Results: Of the 200 Salmonella strains, n=8 were found to be ESBL-producers and these belonged to only two serogroups, Salmonella Group B (n=4) and Salmonella Group G (n=4). Most of the ESBL-positive strains were found to be resistant to third and fourth-generation cephalosporins and monobactams. Plasmid profiling indicated that n=6 and n=2 of the ESBL-producing strains harbored a 62-MDa and a 90-MDa plasmid, respectively. The PCR analysis revealed that blaTEM (β-lactamase Enzyme TEM Producing Gene) (n=6) was most predominant gene, followed by the blaOXA (β-lactamase Enzyme OXA Producing Gene) (n=4), blaSHV (β-lactamase Enzyme SHV Producing Gene) (n=2) and blaCTX-M-1 (β-lactamase Enzyme CTX-M-1 Producing Gene) (n=2) genes. Fifty percent (n=4) of the strains were positive for the int1 gene. The PFGE analysis revealed that almost similar clones were disseminated within the ESBL-producing strains and the non-ESBL-producing strains. The conjugation study revealed that the 62-MDa plasmid was transferred to E. coli K-12 and contained the blaTEM, blaOXA, blaCTX-M-1 and int1 genes.
Conclusion: The emergence of ESBL-producing Salmonella is of great concern and horizontal gene transfer plays an important role in the spread of ESBL.