Evaluation of minimum inhibitory concentration and multi-drug resistance profile of <i>Salmonella </i>isolated from layer poultry farms in kaski district, Nepal

Salmonella species are the most prevalent bacterial pathogens infecting poultry. Antibiotics are administered as growth promoters or a treatment in poultry, raising the prospect of antibiotic resistance. There are very few baselines quantitative studies on antibiotic Minimum Inhibitory Concentration (MIC) against Salmonella of poultry origin. Thus, this study aims to evaluate the MIC and multi-drug resistance (MDR) profile of Salmonella isolated from the poultry farms of Kaski district of Nepal. Laboratory analysis was conducted to determine the minimum inhibitory concentration of nineteen different antibiotic molecules from various classes against Salmonella spp. isolated from poultry feces. The bacteria were identified using standard protocols and confirmed with polymerase chain reaction for the invA gene. Positively confirmed isolates were subjected to a MIC evaluation assay by the E-Test protocol, using Ezy MIC Strips following EUCAST guidelines and interpreted accordingly. Out of 610 samples collected, on screening using the invA gene for Salmonella, 48 isolates were confirmed as Salmonella spp., with an overall farm prevalence of 34.43% (n=61) and a sample-level prevalence of 7.8%. Salmonella spp. displayed the highest resistance to colistin (100%), ciprofloxacin (88%), norfloxacin (72%), ceftriaxone (72%), amoxicillin (77%), ampicillin (66%), erythromycin (66%), cefotaxime (66%), tetracycline (66%), doxycycline (55%), and chloramphenicol (55%). However, amikacin (94%) showed higher susceptibility. MIC analysis revealed elevated values for colistin, ciprofloxacin, amoxicillin, ampicillin, ceftriaxone, and doxycycline compared to epidemiological cut-offs. Multiple drug resistance was observed in 94% of Salmonella spp. isolates, each showing unique multiple antibiotic resistance (MAR) patterns. The study highlights the concerns of antibiotic resistance in commercial layer farms in Kaski district, possibly due to excessive antibiotic use by poultry farm owners. Urgent measures are needed to address this public health issue by regulating antibiotic usage in the poultry industry in the area.