Common plasmid encoding resistance to ampicillin, chloramphenicol, gentamicin, and trimethoprim-sulfadiazine in two serotypes of Salmonella isolated during an outbreak of equine salmonellosis.

This research study focuses on an outbreak of equine salmonellosis at the University of California where horses were discovered to be infected with a rare strain of Salmonella (S saint-paul). The findings highlighted the emergence of antibiotic resistance encoded by a common plasmid.

Background and Objective

• The research was triggered by an outbreak of equine salmonellosis between June 1981 and March 1982 at the Veterinary Medical Teaching Hospital, University of California, Davis.
• The primary objective of this study was to understand the reason behind the unusual resistance of the pathogen to certain antibiotics in comparison to initial strains detected at the beginning of the outbreak.

Important Observations

• During this outbreak, 44 horses were found to be infected with a rarely observed strain of Salmonella, known as S. saint-paul. This rarity indicates the strain was not commonly found in animals at the university prior to the outbreak.
• Scientists observed that unlike the initial isolates of S. saint-paul, the strains found towards the end of the outbreak were significantly resistant to a broad range of antibiotics, including ampicillin, cephalothin, chloramphenicol, gentamicin, kanamycin, sulfadiazine, trimethoprim, and trimethoprim-sulfadiazine.

Findings and Conclusion

• Upon investigation, researchers discovered a conjugal resistance plasmid (R-plasmid) that was responsible for this antibiotic resistance. This R-plasmid was identical to an R-plasmid found in S. krefeld, another strain of Salmonella. Interestingly, S. krefeld had been repeatedly isolated throughout the hospital. This led to the hypothesis that an in vivo transfer of the R-plasmid had taken place.
• These findings suggest that transfer of the resistance plasmid may have occurred from the donor organism, possibly S. krefeld, to S. saint-paul, leading to the antibiotic resistance.