How was penicillin discovered?

Penicillin was discovered by Alexander Fleming in 1921 by accident. Alexander Fleming once returned from a vacation to find that one of his staphylococcus culture plates had accidentally gotten contaminated with mould. The mould inhibited the growth of staphylococcus culture. This observation gave him an idea and the antibiotic was isolated from the fungus which was called Penicillium notatum.