Chromosomally and Plasmid-Located in Salmonella from Animals and Food Products in China.

This study aimed to investigate the prevalence and genomic characteristics of the colistin resistance gene in Salmonella enterica in China. In total, 445 S. enterica isolates from animals and food products were screened through PCR and sequencing for the presence of . The genes were detected in nine Salmonella strains (2.02%), with complete in S. enterica serovar Indiana ( = 1) and an Typhimurium monophasic variant ( 4,[5],12:i:-;  = 1), in S. enterica serovar London ( = 1), and an incomplete in . Indiana ( = 6). They exhibited MIC values of 0.25 to 8 mg/L to colistin and showed resistance to multiple antimicrobial agents. Whole-genome sequencing was performed on -positive Salmonella strains using Illumina HiSeq or PacBio single-molecule real-time sequencing. The complete gene was located on conjugative IncN1-IncHI2 plasmid and IncX4 plasmid, respectively, with high similarity to other -bearing plasmids belonging to the same incompatibility type. Together with an additional 13 antimicrobial resistance genes, the incomplete was embedded in an 81,442-bp multiresistance region on the chromosome in . Indiana YZ20MCS6. The Δ segment and a set of tellurite resistance determinants () in six . Indiana strains were similar to other IncHI2 plasmid backbones. The gene was located on an untyped plasmid pYULZMPS10. Although low prevalence of was observed in Salmonella, continuous surveillance of this gene in Salmonella is required. Plasmids play an important role in transmission, and , although incomplete, can be captured by chromosomes with the help of mobile elements. Colistin is a last-resort antibiotic for severe infections caused by multidrug-resistant (MDR) Gram-negative pathogens. Colistin resistance genes , particularly , have been found in around the world, mainly in Escherichia coli and Salmonella. Salmonella enterica is a major foodborne pathogen, with MDR Salmonella being considered a "Serious Threat Level pathogen" by the Centers for Disease Control and Prevention. Therefore, the prevalence of in Salmonella strains must be monitored. In this study, a low prevalence (2.02%) was observed in Salmonella strains from animals and food products, with plasmid-borne in S. enterica serovar Indiana and an Typhimurium monophasic variant ( 4,[5],12:i:-) and chromosomally located in Indiana. The gene was first identified in S. enterica serovar London associated with an untyped plasmid. Although this study reports a low prevalence in Salmonella, the transmission ability of -positive Salmonella strains to humans via the food chain is a public health concern.