Emergence of mcr-1 among Nontyphoidal Salmonella Isolates in the United States

BACKGROUND: Colistin is considered a critically important antimicrobial for its role in the treatment of severe multidrug-resistant infections. Colistin resistance conferred by the plasmid-mediated gene mcr-1 has been reported in enteric pathogens globally since 2015, but remains rare in the United States. We describe the search for mcr-1 among nontyphoidal Salmonella (NTS) and the identification of the first human cases in the United States. METHODS: Whole genome sequencing (WGS) was performed on NTS isolates from humans by state health departments, from retail meat by the US Food and Drug Administration, and from food animals by the US Department of Agriculture. Sequences were uploaded to the National Center for Biotechnology Information and screened through their pathogen detection pipeline for the presence of resistance determinants (including mcr-1) beginning in late 2015; screening included some retrospective sequences. Isolates with the suspected mcr-1 gene were submitted to CDC for confirmatory PCR. Epidemiological information on human cases was collected from state health departments. RESULTS: Over 70,000 Salmonella isolates from humans, retail meat, and food animals were screened for mcr-1. No NTS with mcr-1 were identified in retail meat or food animals. Four human cases of NTS with mcr-1 were identified by WGS and three were confirmed by PCR (1 pending testing): Salmonella Corvallis in an 18-year-old man from Tennessee (isolation July 2014), Salmonella Enteritidis in a 55 year-old woman from Connecticut (isolation May 2016), Salmonella Typhimurium in a 57-year-old woman from Virginia (isolation November 2016), and Salmonella Enteritidis in a 47-year-old man from Minnesota (isolation April 2017). All patients traveled internationally in the 10 days prior to illness onset. CONCLUSION: NTS rarely contain mcr-1 in the United States. To date, all human cases have been linked to international travel, reflecting the higher prevalence of mcr-1 reported from other parts of the world. The absence of mcr-1 in NTS from US food animals and retail meat is likely because colistin has not been used in food animal agriculture in the United States, underscoring the importance of a One Health approach to combat antimicrobial resistance. DISCLOSURES: All authors: No reported disclosures.