Mastitis is an infection of the udder in cows that, although not dangerous, causes them to give poor-quality milk  Most cases of mastitis are caused by the bacterium Staphylococcus aureus, against which antibiotics are ineffective  However, a related bacterium, Staphylococcus simulans, quickly clears up even severe infections of S  aureus simply by displacing the bacteria  S
simulans is nevertheless unlikely to become the preferred treatment for cases of mastitis, since
the current resistance of S. aureus to antibiotics derives in part from a past pattern of overuse of those antibiotics