Penicillin comes from molds of the genus Penicillium. The scientist Sir Alexander Fleming discovered penicillin accidentally, but he was never able to obtain a pure sample of the organism; however, he did find that penicillin was potent against bacterial growth.
Penicillin is an antibiotic, an agent that stops the growth of other organisms. The first Penicillium mold prevented a culture of Staphylococcus aureus from growing. All types of penicillin kill bacteria by preventing bacteria from synthesizing their cell walls, which are vitally important to bacteria because the cell wall regulates the flow of water into the cell. If water flowed freely, it would cause the cell to swell and eventually burst.