Phenotypic and genotypic characterization of antimicrobial resistance and virulence profiles of Salmonella enterica serotypes isolated from necropsied horses in Kentucky

Salmonella is a foodborne pathogen that poses a significant threat to global public health. It affects several animal species, including horses. Salmonella infections in horses can be either asymptomatic or cause severe clinical illness. Infections caused by Salmonella are presently controlled with antibiotics. Due to the formation of biofilms and the emergence of antimicrobial resistance, the treatment has become more complicated. Our study focused on investigating the prevalence of Salmonella enterica in necropsied horses, assessing the capability for biofilm formation, and motility, determining the phenotypic and genotypic profiles of antibiotic resistance, and detecting virulence genes. A total of 2,182 necropsied horses were tested for the presence of Salmonella. Intestinal samples were enriched in selenite broth and cultured on hektoen and eosin methylene blue agar plates, whereas other samples were directly cultured on aforementioned plates. Confirmation of the serotypes was performed according to the Kauffmann–White–Le Minor Scheme followed by biofilm formation screening using crystal violet assay. The resistance profile of the isolates was determined by broth microdilution assay using the Sensititre Vet (Equine EQUIN2F). The genotypic antimicrobial resistance (AMR) and virulence profiles were detected using polymerase chain reaction (PCR). The overall prevalence of Salmonella was 1.19% (26/2182), with 11 different serotypes identified. Salmonella Typhimurium was the most prevalent serotype with 19.2% prevalence. All of the isolates were identified as biofilm producers and motile. Virulence genes related to invasion (invA, hilA, mgtC, and spiA), biofilm formation (csgA and csgB), and motility (filA, motA, flgG, figG, flgH, fimC, fimD, and fimH) of Salmonella were detected among 100% of the isolates. An overall 11.4% of the isolates were identified as multidrug-resistant (MDR), with resistance to gentamicin, amikacin, ampicillin, ceftazidime, ceftiofur, chloramphenicol, and trimethoprim/sulfamethoxazole. We found that beta-lactamase-producing genes bla_TEM, bla_CTXM, and bla_SHV2 were identified in 11.5% of the isolates, while only 3.8% carried the bla_OXA-9 gene. The presence of MDR pathogenic Salmonella in horses is alarming for human and animal health, especially when they have a high affinity for forming biofilm. Our study found horses as potential sources of pathogenic Salmonella transmission to humans. Thus, it is important to perform continuous monitoring and surveillance studies to track the source of infection and develop preventive measures. The study also investigates their motility, a factor that contributes to the spread of infection. The findings can improve treatment strategies for horses and help prevent the transmission of resistant bacteria to other animals as well as humans. Ultimately, the research could contribute to better management of antibiotic resistance in both veterinary and public health contexts, helping to safeguard animal welfare and public health.