Genomic content typifying a prevalent clade of bovine mastitis-associated Escherichia coli

E. coli represents a heterogeneous population with capabilities to cause disease in several anatomical sites. Among sites that can be colonised is the bovine mammary gland (udder) and a distinct class of mammary pathogenic E. coli (MPEC) has been proposed. MPEC are the principle causative agents of bovine mastitis in well-managed dairy farms, costing producers in the European Union an estimated €2 billion per year. Despite the economic impact, and the threat this disease presents to small and medium sized dairy farmers, the factors which mediate the ability for E. coli to thrive in bovine mammary tissue remain poorly elucidated. Strains belonging to E. coli phylogroup A are most frequently isolated from mastitis. In this paper, we apply a population level genomic analysis to this group of E. coli to uncover genomic signatures of mammary infectivity. Through a robust statistical analysis, we show that not all strains of E. coli are equally likely to cause mastitis, and those that do possess specific gene content that may promote their adaptation and survival in the bovine udder. Through a pan-genomic analysis, we identify just three genetic loci which are ubiquitous in MPEC, but appear dispensable for E. coli from other niches.