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Objectives: To reconstruct the genomic epidemiology and evolution of MDR Salmonella Indiana in China.Methods: A total of 108 Salmonella Indiana strains were collected from humans and livestock in China. All isolates were subjected to WGS and antimicrobial susceptibility testing. Phylogenetic relationships and evolutionary analyses were conducted using WGS data from this study and the NCBI database.Results: Almost all 108 Salmonella Indiana strains displayed the MDR phenotype. Importantly, 84 isolates possessed concurrent resistance to ciprofloxacin and cefotaxime. WGS analysis revealed that class 1 integrons on the chromosome and IncHI2 plasmids were the key vectors responsible for multiple antibiotic resistance gene (ARG) [including ESBL and plasmid-mediated quinolone resistance (PMQR) genes] transmission among Salmonella Indiana. The 108 Salmonella Indiana dataset displayed a relatively large core genome and ST17 was the predominant ST. Moreover, the global ST17 Salmonella Indiana strains could be divided into five distinct lineages, each of which was significantly associated with a geographical distribution. Genomic analysis revealed multiple antimicrobial resistance determinants and QRDR mutations in Chinese lineages, which almost did not occur in other global lineages. Using molecular clock analysis, we hypothesized that ST17 isolates have existed since 1956 and underwent a major population expansion from the 1980s to the 2000s and the genetic diversity started to decrease around 2011, probably due to geographical barriers, antimicrobial selective pressure and MDR, favouring the establishment of this prevalent multiple antibiotic-resistant lineage and local epidemics.Conclusions: This study revealed that adaptation to antimicrobial pressure was possibly pivotal in the recent evolutionary trajectory for the clonal spread of ST17 Salmonella Indiana in China. [ABSTRACT FROM AUTHOR]

Objectives: To investigate the prevalence and transmission of mcr-3 among Salmonella enterica serotype Typhimurium and 1,4,[5],12:i:-.Methods: A total of 4724 clinical Salmonella isolates were screened for the presence of mcr-3 in China during 2014-19. The clonal relationship of the mcr-3-positive isolates and their plasmid contents and complete sequence were also characterized based on WGS data from the Illumina and MinION platforms.Results: We identified 10 mcr-3-positive isolates, and all were MDR, mostly resistant to colistin, cefotaxime, ciprofloxacin, doxycycline and florfenicol. mcr-3 was co-present with blaCTX-M-55-qnrS1 on hybrid ST3-IncC-FII conjugatable plasmids (n = 6) and an ST3-IncC non-conjugatable plasmid (n = 1) and embedded into a pCHL5009T-like IncFII plasmid on the Salmonella chromosome (n = 3). Four distinctive genetic contexts surrounded mcr-3 and all but one were closely related to each other and to the corresponding region of IncFII plasmid pCHL5009T. IS15DI was most likely the vehicle for integration of mcr-3-carrying IncFII plasmids into ST3-IncC plasmids and the chromosome and for shaping the MDR regions. In addition, a phylogenetic tree based on the core genome revealed a unique Salmonella lineage (≤665 SNPs) that contained these 10 mcr-3-positive isolates and another 38 (33 from patients) mcr-3-positive Salmonella from five countries. In particular, most of the 51 mcr-3-positive isolates belonged to ST34 and harboured diverse antibiotic resistance genes (ARGs), including mcr-3-blaCTX-M-55-qnrS1, and possessed similar ARG profiles.Conclusions: Our findings revealed global clonal spread of MDR ST34 Salmonella from clinical isolates co-harbouring mcr-3 with blaCTX-M-55 and qnrS1 and a flexibility of mcr-3 co-transmittance with other ARGs mediated by mobile genetic elements. [ABSTRACT FROM AUTHOR]