Genomic characteristics and antibiotic resistance profiles of monophasic Salmonella Typhimurium in Shaanxi Province, China

Our results revealed that all the strains belonged to the ST34 and did not carry virulence genes on pSLT (NC_003277). There were 12 strains carrying the <i>STM2757</i> gene. The isolates exhibited a considerable diversity in PFGE subtypes. Phenotypic antimicrobial resistance showed that the strains were most resistant to tetracycline (94.34%; 50/53) and ampicillin (94.34%; 50/53), followed by streptomycin (88.68%; 47/53) and ampicillin/sulbactam (64.15%; 34/53). Resistance gene prediction highlighted the presence of 64 distinct genes, with <i>aac(6')-Iaa</i> found in all strains (100%) and <i>tet</i>(B) in 93.1% of strains. Notably, the <i>floR</i> gene, relevant for resistance to phenicols, was observed in 44.83% of isolates. Genomic analysis revealed that 96.55% of strains were positive for the <i>sodC1</i> virulence gene, whereas only 10.34% carried the <i>sopE</i> gene. The most plasmid replicon was IncQ1 (84.48%; 49/58), followed by IncHI2 (32.76%; 19/58) and IncHI2A (32.76%; 19/58). Single nucleotide polymorphism analysis showed that 2 strains were clustered together with SRR17830210 (UK outbreak isolate) with a bootstrap value of 0.949. There were only 12 allelic differences between SNXiAn21SAL011 and the reference strain. Conclusively, the monophasic <i>Salmonella</i> Typhimurium ST34 strains in Shaanxi Province demonstrated unique genomic and antimicrobial resistance traits. This study may help to prevent outbreaks and rationalize salmonellosis antimicrobial therapeutics.