Phenotypic and genotypic antimicrobial resistance patterns of Salmonella isolated from selected commercial layer farms in two districts of Bangladesh

Abstract

Salmonella spp. is a major foodborne pathogen in poultry. The rise of multidrug-resistant (MDR) strains poses a serious threat to both animal and public health. The current study evaluated the prevalence, antimicrobial susceptibility, and molecular resistance traits of Salmonella spp. from four commercial poultry farms in Bangladesh. The study screened 325 samples on Salmonella-Shigella (SS) agar and found 63 (19.38%) with presumptive Salmonella colonies. PCR targeting the invA gene confirmed 30 isolates, giving an overall prevalence of 9.23%. The distribution of isolates did not differ significantly across the four farms (p = 0.70). Antimicrobial testing revealed the highest resistance to oxytetracycline (86.67%), streptomycin (70.00%), and amoxicillin (63.33%). Resistance to florfenicol and chloramphenicol was lower (33.33% and 30.00%, respectively). All 30 confirmed isolates (100%) were multidrug-resistant. Their multiple antibiotic resistance (MAR) indices ranged from 0.375 to 0.625. Molecular screening showed tetA was the most common resistance gene (96.67%), followed by aadA1 (80.00%), blaTEM and qnrA (66.67% each), floR (40.00%), and cat1 (36.67%). We observed that genotypic detection often exceeded phenotypic resistance, especially for ampicillin/blaTEM and ciprofloxacin/qnrA. We detected the class 1 integron gene (intI1) in 40.00% of isolates. Every intI1-positive isolate showed an MDR phenotype and the highest MAR index (0.625). These results suggest that poultry farms act as critical reservoirs for MDR Salmonella. They also point to the farm environment as an active site for horizontal gene transfer, underscoring the urgent need for responsible antimicrobial use and ongoing molecular surveillance.