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Background In Washington State, COVID-19 cases in long-term care facilities (LTCF) have accounted for less than 3% of all cases, yet 30% of all COVID-19 deaths. Understanding transmission patterns and outbreak epidemiology informs outbreak management. From April to October 2021, two large LTCFs experienced COVID-19 outbreaks. Whole genome sequencing and phylogenetic analysis were leveraged to explore transmission patterns and complement outbreak epidemiology. Methods Epidemiologic data was exported from the Washington Disease Reporting System. Sequences, retrieved from GISAID, were aligned to the Wuhan-1 reference genome using Nextalign version 1.11.0. Pairwise single nucleotide polymorphism (SNP) distance matrices were calculated using SNP-Dists version 0.8.2. Phylogenetic trees for each outbreak were generated using IQ-Tree multicore version 2.2.0-beta COVID-edition using the GTR+F+G4 nucleotide substitution model with 1000 bootstrap replicates. MicrobeTrace was used to visualize the phylogeny, SNP heatmap, and identify clusters among sequences. Results Weekly, LTCF A tested 162 residents and 800 staff, and LTCF B tested 60 residents and 144 staff. Of all cases in LTCF A (n= 119), 23% (n =27) were residents and 77% (n = 92) were staff, compared to 78% (n =28) residents and 22% (n =7) staff among total LTCF B cases (n=36). In LTCF A, 34% (n=40) of the cases had high-quality sequences available. Seven clusters of two or more genetically related sequences and thirteen genetically unrelated sequences were identified. Five of the clusters involved resident and staff cases, linked by unit. Two clusters and remaining unrelated sequences were among staff. In LTCF B, 40% (n=14) of the cases had high-quality sequences available. One cluster of genetically related sequences was identified, all from residents of two floors. The SNP differences between sequences from LTCF A ranged from 0 to 70, whereas SNP differences between LTCF B sequences ranged from 0 to 6. Conclusion Phylogenetic analysis of the two outbreaks confirms differences in disease transmission patterns. Multiple independent introductions of SARS-CoV-2 were identified in LTCF A, compared to a single introduction in LTCF B. Genomic epidemiology is a valuable resource for outbreak investigation and management. Disclosures All Authors: No reported disclosures.

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.ResultsOver 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.

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