In 1928, bacteriologist Alexander Fleming was studying Staphylococcus, a type of bacteria that can cause serious infections. He grew the bacteria in small containers called

petri dishes. One day he noticed that mold had accidentally been introduced into one of

the petri dishes and was startled to see that there were no bacteria around the moldy

area. Fleming correctly deduced that the mold had had an antibacterial effect; somehow

it had prevented the bacteria from growing. Fleming identified the mold as the strain

Penicillium notatum and called its antibacterial ingredient-which he was unable to

successfully extract-penicillin. Fleming's discovery of penicillin's unique effect led to the

eventual finding that it could be used to treat infections. Several years after his initial

observation, scientists built on his work by extracting penicillin from mold and

demonstrating its effectiveness as a treatment.