Antibiotic Resistance and Chromosomal Variation in Equine Faecal Salmonella spp.

Aims: Equine hospital Salmonella spp. were investigated retrospectively using antibiotic resistance typing and macro-restriction pulsed field gel electrophoresis techniques.
Study Design: Retrospective study.
Place and Duration of Study: Department of Comparative Molecular Medicine and department of Animal and Population Health, School of Veterinary Science, University of Liverpool.
Methodology: Twenty four achieved Salmonella isolates of equine faecal origin, previously collected from a UK equine hospital, were serotyped, phagetyped and tested both for antimicrobial susceptibility, by disc diffusion (BSAC) and for genetic relatedness by XbaI I-PFGE.
Results: Most isolates were Salmonella typhimurium (n=21), including five DT104; two were Salmonella enteritidis, and one was untypeable. Sixteen isolates, including five Salmonella typhimurium DT104 isolates exhibiting the classic penta-resistance phenotype (ACSSuT), were characterized as multidrug resistant (MDR). Fourteen MDR isolates showed additional resistance to florfenicol, although no resistance to ciprofloxacin was detected. MDR isolates showed two dominant resistance phenotypes: ACTSSuFlo (N=8) resistant to ampicillin, chloramphenicol, tetracycline, trimethoprim, streptomycin, sulfamethoxazole and florfenicol and ACTTrSSuFlo (N=6) which is also resistant to trimethoprim. Dendrogram analysis identified eleven distinct genetic groups showing an overall similarity of 84%. The dominant resistance phenotypes were located mainly in two genetic groups: ACTSSuFlo isolates were all collected in the same year and were restricted to one clonal PFGE group showing >99.5% genetic similarity; ACTTrSSuFlo isolates were genetically more diverse with 4/6 clustering in a closely-related group showing 94% similarity.
Conclusion: These findings suggest that although some Salmonella infections were introduced to the hospital from outside, infections spreading within the equine hospital could also be a reservoir of MDR Salmonella zoonotic infections and play a role in the development and dissemination of antibiotic resistance.