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.