It was in vitro research, as shown here, that allowed Fleming to observe penicillin as it killed bacteria. Today this same type of research allows doctors to select appropriate antibiotics for patients with bacterial infections and even aids in discovering new antibiotics.
Didn't penicillin come from animal experimentation?
Actually, it's a fact that animal tests significantly sidetracked development of this important drug. In 1929, Alexander Fleming observed penicillin as it killed bacteria in a Petri dish. Intrigued, he administered the compound to bacteria-infected rabbits, hoping it would do the same thing.
"And it almost didn't come at all."
Unfortunately, penicillin was ineffective against the rabbit's infection. Disappointed, Fleming set the drug aside for a decade, as the rabbits had "proved" the drug was useless as a systemic medication.
Years later, he administered the drug in desperation to a dying patient, for whom all other treatments were ineffectual. The penicillin performed a miracle, and the rest is history.
Fleming might have thrown penicillin away had he done his initial tests on guinea pigs or hamsters, as it would have killed those species. Fleming later admitted that misleading results from animal testing almost prevented discovery of the entire field of antibiotics.