Outbreak of multidrug-resistant Salmonella typhimurium monophasic variant 1,4,[5],12:i:- sequence type 34 linked to food, in China.

The monophasic Salmonella Typhimurium variant (1,4,[5],12:i:-) is a growing global public health and food safety concern. This study analyzed 60 such isolates collected in Huzhou (2023-2025), including 16 outbreak strains. All isolates belonged to ST34 and carried 211 virulence genes. Antimicrobial susceptibility testing showed universal resistance to tetracycline (100%) and a high multidrug resistance rate (96.67% resistant to ≥ 3 antibiotic classes). Whole-genome sequencing identified 39 resistance genes; aac(6´)-Iaa was present in all strains, and 48.33% carried ≥ 4 resistance genes. Phylogenetic analysis revealed high overall homology with localized evolutionary traits. Notably, clinical outbreak and food-derived isolates were clonally identical (0 SNP difference).