Penicillin treats middle ear infections, pneumonia, urinary tract infections, bronchitis and laryngitis, according to MedicineNet. The Free Dictionary claims that penicillin also treats skin infections, respiratory ailments and scarlet fever.
MedicineNet mentions that penicillin destroys a variety of bacteria, including N. gonorrhea, H. influenzae and E. coli. Marry Bellis for About.com reports that penicillin comes from penicillin mold, and it is used to destroy harmful organisms in the body. Sir Alexander Fleming first discovered that penicillin could kill the bacterium Staphylococcus aureus in 1928.
The Free Dictionary notes that penicillin interrupts the cell structures of vulnerable organisms, causing the cell walls to burst due to osmotic pressure. This interferes with an organism's ability to replicate.
Wikipedia adds that several forms of penicillin exist to destroy various bacterial colonies. For instance, antipseudomonal penicillin is more potent than other forms. Even though there are bacterial strains resistant to penicillin, it is still widely used to treat bacterial infections that remain susceptible.
The Free Dictionary notes that penicillin is particularly harmful to gram-positive bacterium, which includes staphylococci and streptococci. Certain forms of gram-negative bacteria are also vulnerable to penicillin exposure. There are also newer forms of penicillin that attack organisms that produce penicillinase. Wikipedia reports that gram-negative bacteria do not entirely lose cell walls, and they are called spheroplasts after being exposed to penicillin.