Genomic epidemiology of ceftriaxone-resistant non-typhoidal Salmonella enterica strain in China

Non-typhoidal Salmonella (NTS) is one of the top causes of diarrhea worldwide. Ceftriaxone is commonly recommended as the initial treatment option for Salmonella infections due to its antibacterial effectiveness. The objective of this study was to investigate the molecular epidemiological characteristics of NTS and to compare the phenotypic and genotypic profiles of antimicrobial resistance in multidrug-resistant Salmonella strains by sequencing 329 NTS strains collected from a county-level hospital between 2018 and 2021. Multi-locus sequence typing (MLST), antimicrobial resistance genes and plasmid types were identified by BacWGSTdb 2.0 webserver. Phylogenetic analysis of all NTS strains was carried out using Snippy and Gubbins software. The transferability of ceftriaxone resistant plasmids was confirmed through plasmid conjugation assays, and verified by S1-PFGE-Southern blot assays. The predominant serotypes among all NTS strains were Typhimurium (161/329), Enteritidis (49/329) and London (45/329). The most common sequence type observed was ST34 (86/329), followed by ST19 (72/329) and ST11 (47/329). The antimicrobial resistance of Salmonella to a wide range of antimicrobials showed an overall increase. Out of these 37 (11.24%) ceftriaxone-resistant strains, with the majority of them (33/37) being bla_CTX-M. The predominant plasmid types identified were IncHI2 (14/21) and IncI1 (6/21), ranging in size from 70 kb to 360 kb. The conjugation efficiency was calculated with the high conjugation efficiency of 1.1 × 10^{- 5} to 9.3 × 10^{- 2}. The strains varied widely, ranging from 3 to 45,024 single nucleotide polymorphisms (SNPs). There are close linkages observed among the predominant lineage, with an average of 78 SNPs between each pair of ST34 strains. The findings contribute to our understanding of the transmission and resistance mechanisms of multidrug-resistant Salmonella, thereby facilitating the development of effective control strategies.