A Retrospective Analysis of Salmonella Isolates across 11 Animal Species (1982-1999) Led to the First Identification of Chromosomally Encoded bla SCO-1 in the USA.

Antimicrobial resistance (AMR) in non-typhoidal Salmonella is a pressing public health concern in the United States, necessitating continuous surveillance. We conducted a retrospective analysis of 251 Salmonella isolates from 11 animal species recovered between 1982 and 1999, utilizing serotyping, antimicrobial susceptibility testing, and whole-genome sequencing (WGS). Phenotypic resistance was observed in 101 isolates, with S . Typhimurium, S . Dublin, S . Agona, and S . Muenster prevailing among 36 identified serovars. Notably, resistance to 12 of 17 antibiotics was detected, with ampicillin being most prevalent (79/251). We identified 38 resistance genes, primarily mediating aminoglycoside (n = 13) and β-lactamase (n = 6) resistance. Plasmid analysis unveiled nine distinct plasmids associated with AMR genes in these isolates. Chromosomally encoded bla SCO-1 was present in three S . Typhimurium and two S . Muenster isolates from equine samples, conferring resistance to amoxicillin/clavulanic acid. Phylogenetic analysis revealed three distinct clusters for these five isolates, indicating evolutionary divergence. This study represents the first report of bla SCO-1 in the USA, and our recovered isolates harboring this gene as early as 1989 precede those of all other reports. The enigmatic nature of bla SCO-1 prompts further research into its function. Our findings highlight the urgency of addressing antimicrobial resistance in Salmonella for effective public health interventions.