Characteristics of Two mcr-1 -Harboring IncHI2 Plasmids from Clinical Salmonella Isolates in Jiaxing City.

Salmonella is a primary cause of foodborne diseases, and the increasing prevalence of mcr-1- carrying plasmids, which confer colistin resistance to Salmonella , poses significant global health concerns. As the frequency of occurrence of the mcr-1 gene is increasing globally, we studied the prevalence of mcr-1 in clinical Salmonella isolates by analyzing 195 clinical strains isolated in 2020. Of the 195 Salmonella isolates, 41 isolates were resistant to colistin. We found mcr-1 in two strains ( Salmonella Typhimurium ZJJX20006 and Salmonella Kentucky ZJJX20014), which we analyzed in detail via whole-genome sequencing and antibiotic susceptibility testing. Two strains displayed resistance to ampicillin, ampicillin-sulbactam, tetracycline, chloramphenicol, and cotrimoxazole, while ZJJX20006 displayed resistance to colistin and ZJJX20014 was sensitive. Genomic analysis revealed that these strains had plasmid-encoded mcr-1 in IncHI2 plasmids, which were not similar to the mcr-1 -IncX4 identified in 2016. These two strains also harbored other drug resistance genes, including bla OXA-1 and bla CTX-M-14 . Our findings may help clarify the molecular mechanisms of mcr-1 dissemination among Salmonella strains in Jiaxing City and offer insights into the evolution of mcr-1 in Salmonella .