Varieties of staphylococcus aureus that are resistant to the drug methicillin already existed in the population before methicillin was developed. They were strongly selected for as methicillin became widely used to treat bacterial infections. The bacteria is resistant to the antibiotic methicillin and other drugs in the same class. Staph infections may be treated with oral, topical, or intravenous antibiotics, depending upon the type of infection.