Antibiotic drugs are used against infections caused by microorganisms, like fungi or bacteria. There are a lot of classes of antibiotics, and also a lot of ways they can be administered: orally, intravenous (for more serious cases) but also eye drops or ointments. There are two main variations on the way antibiotics work: they can either be bacteriostatic or bactericidal. While bacteriostatic antibiotic drugs only inhibit the growth of bacteria, limiting its reproduction, bactericidal antibiotics kill the bacteria itself. Penicillin is the best known bactericidal substance. Other examples include nitrofurantoin (Furadantin, Macrodantin), vancomycin (Vancocin) quinolones (Zagam, Uroflox, Floxin, Tarivid, etc).