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 milk from cows infected with S. aureus is generally not of such poor quality as to be entirely unusable