The causal organism of mastitis in cattle is a bacterium. Mastitis is an inflammation of the mammary gland in cows which can be caused by various bacteria, but the most common ones are Staphylococcus aureus, Streptococcus agalactiae, and Escherichia coli. Bacteria can enter the udder through the teat canal and cause an infection which results in swelling, pain, and the production of abnormal milk.