E. coli is one of the most important causes of mastitis in dairy cattle in the UK. Traditionally, E. coli mastitis has been associated with peracute mastitis but it is now more commonly associated with subacute mastitis without obvious systemic signs and with persistence in the udder after treatment. This suggests that the E. coli strains causing mastitis may have become udder-adapted. However evaluation of phylogeny and the possession of virulence factors suggest that E. coli infection of the udder is simply opportunistic infection by commensal stains of the bacteria which are not specifically adapted to invade or persist in the udder.