Prevalence and molecular characteristics of fosA3- and fosA7-positive Salmonella from food animals in Shandong Province of China

OBJECTIVE: To investigate the prevalence of the fosfomycin resistance genes fosA3 and fosA7 in Salmonella isolates from food animals in Shandong Province of China, between 2023 and 2024. METHODS: In this study, fosA1-10 genes and minimum inhibitory concentrations were determined in 124 Salmonella isolates from food animals. Conjugation experiments were performed on fosA-positive Salmonella isolates. Replicon types of the plasmids found in transconjugants were identified using polymerase chain reaction-based replicon typing. Whole-genome sequencing was performed for the fosA3-containing plasmid pS118 and the fosA7-positive Salmonella W126. RESULTS: Among the 124 isolates, 7 fosA3-positive (5.65%) and 14 fosA7-positive (11.29%) isolates were identified, all of which exhibited high fosfomycin resistance (minimum inhibitory concentrations ≥512 mg/L). The fosA3 gene could spread through horizontal gene transfer from all fosA3-positive Salmonella strains to Escherichia coli J53; however, the fosA7 gene failed to transfer. Three types of fosA3-positive plasmids were identified in transconjugants: F33:A-:B- (n = 5), F64:A-:B1 (n = 1), and IncHI2/ST3 (n = 1). Whole-genome sequencing revealed that the plasmid pS118 was highly similar to the pHNFP460-1 plasmid of E. coli, the pSE104-1 plasmid of Salmonella, and the pC252072-2 plasmid of Klebsiella aerogenes. The fosA7 gene was located on the chromosome of Salmonella W126. CONCLUSIONS: The high prevalence of fosA3 and fosA7 was identified in Salmonella isolates from food animals, which suggested that food animals are potential reservoirs of multidrug-resistant fosA3- and fosA7-positive Salmonella. The F33:A-:B-type plasmid is a potential epidemic vector mediating the dissemination of fosA3 among various bacterial strains in China.