Comparison of ST19 Salmonella Typhimurium and ST34 Salmonella I,4,[5],12:i:-: a trade-off between plasmid-mediated antibiotic resistance and virulence

The increasing emergence of multidrug-resistant (MDR) Salmonella , especially ST34 S . I,4,[5],12:i:-, poses a great challenge to the prevention and control of salmonellosis. However, the reason for the global spread of ST34 S. I,4,[5],12:i:- is still largely unknown. Whole-genome sequencing (WGS) on 502 Salmonella isolates collected from animals in China to investigate the virulence profiles. And then, compared the difference of population structure, virulence/antibiotic resistance gene distribution, and pathogenicity between S. Typhimurium and S . I,4,[5],12:i:-. WGS analysis revealed that the distribution and prevalence of virulence genes was correlated with the serovars and STs of Salmonella isolated from animals. Of note, the number of virulence genes among ST19-Typhimurium-b, ST34 S. Typhimurium and ST34 S. I,4,[5],12:i:- strains were similar but lower than for ST19-Typhimurium-a. Phylogenomic analysis indicated ST34 S. Typhimurium/ S. I,4,[5],12:i:- strains clustered together and this group were primarily from in pigs. In comparison, most ST19 S. Typhimurium strains clustered together and this group mainly originate from chickens and ducks. Meanwhile, most ST19 S. Typhimurium strains carried a greater number of virulence genes including the marker genes of the pSLT2-like virulence plasmids and fewer ARGs than that of ST34 S. I,4,[5],12:i:-/ S. Typhimurium strains, which lacked all of the corresponding marker genes. Similar scenario was also found based on 15,289 publicly available genomes of S. Typhimurium and S. I,4,[5],12:i:- isolates from both animal and humans. The mouse infection model showed that ST19 S. Typhimurium strains bearing the pSLT2-like virulence plasmids exhibited higher pathogenicity than S. Typhimurium and S. I,4,[5],12:i:-. The high-level pathogenicity was strongly associated with the presence of the marker genes of the pSLT2-like virulence plasmids. Our findings revealed a trade-off between plasmid-mediated antibiotic resistance and virulence by comparison of ST19 S. Typhimurium and ST34 S. I,4,[5],12:i:- based on large WGS analysis. These findings could help to understand the genetic evolution of MDR S. I,4,[5],12:i:-, and possible help to curb the widespread spread of MDR Salmonella and bacterial drug resistance. • Dominant STs carry virulence genes highly similar to the corresponding serovars. • ST19 S. Typhimurium divided into two groups with different virulence gene numbers. • ST19 S. Typhimurium had more virulence genes and less ARGs than ST34 variants. • ST19 S. Typhimurium and ST34 variants have different hosts and pathogenicity. • This strong pathogenicity was linked to spvABCDR , pefABCD , mig-5 , rck and grvA.