Phylogenetic and genomic characterisation of the rare global Salmonella enterica serovar Wandsworth ST1498.

Salmonella Wandsworth, an uncommon serovar, has caused foodborne outbreaks and healthcare associated infections. Its underestimated prevalence and multidrug resistance (MDR) pose global public health risks. Genomic analysis is essential for investigating the evolution, diversity, antimicrobial resistance (AMR), and pathogenicity of foodborne pathogens. This study characterised two Salmonella Wandsworth isolates from a patient and a food sample, both collected in Fujian, China, in 2020. Antimicrobial susceptibility testing of clinical strain F2021 revealed resistance to 15 antimicrobials across 8 classes, correlating well with WGS-predicted resistance patterns. Complete genome sequencing uncovered a ∼ 58 kb chromosomal multidrug resistance region harbouring nine resistance genes in F2021. Furthermore, we characterised a novel ∼44 kb mosaic IncR-type plasmid in F2021 through S1-PFGE and complete genome sequencing. This plasmid maintains the IncR backbone but features unique structural organization and resistance gene clusters distinct from known plasmids. Coupled with 113 other global Wandsworth genomes from the NCBI database, all 115 analysed Salmonella Wandsworth genomes belonged to sequence type (ST) 1498. Core SNP-based phylogenomic analysis revealed isolates from different years, sources, and locations clustered together, forming three major clusters and ten clades based on SNP variation. Resistome analysis identified 26 AMR genes (spanning 10 classes) globally. Predominant genes (qnrS, dfrA, sul, tet, aadA) confer resistance to quinolones, trimethoprim, sulfonamides, tetracyclines, and aminoglycosides. Thirteen plasmid incompatibility groups were detected, including IncX2, IncHI2, IncHI2A, IncC, IncR, and ect. These findings underscore the importance of enhanced surveillance for rare Salmonella serovars that may emerge as significant threats due to evolving multidrug resistance mechanisms.