The therapeutic efficacy of penicillin can be determined accurately in vitro. It is effective when administered locally or systemically.
Because of its high antibacterial activity, one injection of penicillin will protect mice against a light infecting dose of pneumococci while repeated treatments are effective against larger doses. Well-established local and systemic pneumococcal infections in rabbits can be successfully treated by repeated intravenous injections of penicillin.
While penicillin is rapidly excreted, demonstrable amounts are present in the urine of mice as long as eight hours after injection.
Sulfonamide-resistant strains of pneumococci (4, 13, 14) are susceptible to the action of penicillin.
Strains of staphylococci (5, 18), pneumococci, and streptococci can be rendered resistant in vitro to penicillin. Loss of virulence accompanies this increased resistance.
A strain of Staphylococcus aureus has been rendered more virulent and for this reason more resistant to penicillin by serial passage in mice treated with penicillin. Susceptibility in vitro was unchanged by such passage.
Penicillin, even in impure form, is many times less toxic for animals than any other well-known antibiotic substance.