20advances understanding of SE and the role mice may play in SE contamination and 44 spread among poultry population. Our data was able to identify SE isolates from 45 different farms or years of collection. Moreover, the annotations of clade-defining SNPs 46 provided information about possible protein functions among these SE isolates from 47 each subgroup. Clade-defining or farm-unique biomarkers were useful for rapid 48 detection and outbreak investigations. 49 homogeneous isolates (1, 7, 15). Application of WGS have also been useful in other 74 microorganisms, including E. coli (16), Vibrio cholera (17), and Staphylococcus aureus 75 (18). 76 Importantly, WGS can be also applied to historic isolates, some of which have been 77 stored for decades. Data from those historic isolates should allow us to understand the 78 origin and persistence of important traits. In this current project, we sequenced 91 SE 79 isolated from 82 mice at poultry farms during the 1990s, which lets us to compare both 80 site and host-adaptions with those of isolates from more recent sampling. Documenting 81 these genomes and fitting them into large-scale phylogeny projects such as 82 GenomeTrakr 83 (https://www.fda.gov/Food/FoodScienceResearch/WholeGenomeSequencingProgram 84 WGS/ucm363134.htm) and NCBI Pathogen Detection Isolates Browser 85 (https://www.ncbi.nlm.nih.gov/pathogens/) will refine our understanding of SE 86 contamination and spread in poultry facilities (19). Further, identifying and 87 characterizing biomarkers can facilitate the development of rapid and reliable tests that 88 could guide appropriate interventions during future outbreaks. 89 6 Materials and Methods 90 Bacterial isolates 91 Ninety-one SE isolates from mouse spleens (n=62) and intestines (n=29), collected 92 from 15 poultry farms in Pennsylvania during 1995-1998, are listed in Table 1. Among 93 these isolates, eight pairs were isolated from the spleen and intestine of the same 94 mouse; these were designated as m1 through m8. These isolates are archived under 95 Bioproject Number PRJNA186035 (https://www.ncbi.nlm.nih.gov/bioproject/186035). 96
Whole genome sequencing and assembly 97Genomic DNA was extracted after incubation of culture for 16 hours at 37 ºC in 98Trypticase Soy Broth (TSB) using the DNeasy Blood and Tissue Kit (Qiagen Inc, 99