These files contain data that support hypotheses presented in Ceres et. al. Evolutionary genomic analyses of canine E. coli infections identifies a relic capsular locus associated with resistance to multiple classes of antimicrobial drugs. In Ceres et. al. we found: Escherichia coli is the leading cause of death attributed to antimicrobial resistance (AMR) worldwide, and the known AMR mechanisms involve a range of functional proteins. Here we employed a pan-GWAS approach on over 1,000 E. coli isolates from sick dogs collected across the US and Canada and identified a strong statistical association of AMR, involving a range of antibiotics, to a group 1 capsular (CPS) gene cluster. This cluster included genes under relaxed selection pressure, had several loci missing, and pseudogenes for other key loci. It is widespread in E. coli and Klebsiella infections across multiple host species. Earlier studies demonstrated that the octameric CPS polysaccharide export protein Wza can transmit macrolide antibiotics into the E. coli periplasm. We suggest the CPS in question, and its highly divergent Wza, functions as an antibiotic trap, preventing drug penetration. We also highlight the high diversity of lineages circulating in dogs across all regions studied, overlap with human lineages, and regional prevalence of resistance to multiple drug classes.