Carriage and Transmission of mcr-1 in Salmonella Typhimurium and Its Monophasic 1,4,[5],12:i:- Variants from Diarrheal Outpatients: a 10-Year Genomic Epidemiology in Guangdong, Southern China

The banning of colistin as a feed additive for food-producing animals in mainland China in 2017 caused the decline in the prevalence of Escherichia coli-mobilized colistin resistance ( ) in China. Salmonella Typhimurium and its monophasic 1,4,[5],12:i:- variants are also the main species associated with the spread of ; however, the evidence of the prevalence and transmission of among Salmonella is lacking. Herein, the 5,354 Salmonella isolates recovered from fecal samples of diarrheal patients in Guangdong, Southern China, from 2009 to 2019 were screened for colistin resistance and , and -positive isolates were characterized based on whole-genome sequencing (WGS) data. Relatively high prevalence rates of colistin resistance and (4.05%/4.50%) were identified, and more importantly, the prevalence trends of colistin-resistant and -positive Salmonella isolates had a similar dynamic profile, i.e., both were first detected in 2012 and rapidly increased during 2013 to 2016, followed by a sharp decrease since 2017. WGS and phylogenetic analysis indicate that, whether before or after the ban, the persistence and cross-hospital transmission of are primarily determined by IncHI2 plasmids with similar backbones and sequence type 34 (ST34) Salmonella in specific clades that are associated with a high prevalence of IncHI2 plasmids and clinically important antimicrobial resistance genes, including - - genotypes. Our work reveals the difference in the prevalence rate of in clinical Salmonella before and after the Chinese colistin ban, whereas transmission was closely linked to multidrug-resistant IncHI2 plasmid and ST34 Salmonella across diverse hospitals over 10 years. Continued surveillance is required to explore the factors related to a sharp decrease in after the recent ban and determine whether the ban has affected the carriage of in Salmonella circulating in the health care system. Colistin is one of the last-line antibiotics for the clinical treatment of . However, the emergence of the mobilized colistin resistance ( ) gene has spread throughout the entire human health system and largely threatens the usage of colistin in the clinical setting. In this study, we investigated the existence of in clinical Salmonella from a 10-year continuous surveillance and genomic study. Overall, the colistin resistance rate and carriage of Salmonella in tertiary hospitals in Guangdong (2009 to 2019) were relatively high and, importantly, rapidly increased from 2013 to 2016 and sign