In his laboratory, he noticed that in some of his bacteria colonies, that he was growing, were some clear spots. He realized that something had killed the bacteria in these clear spots, which ended up to be a fungus growth. He then discovered that inside this mold was a substance that killed bacteria. It was the antibiotic, penicillin. Penicillin became the most powerful germ-killer known at that time.
Antibiotics kill disease-causing bacteria by interfering with their processes.
Penicillin kills bacteria by attaching to their cell walls. Then it destroys part of the wall. The cell wall breaks apart and bacteria dies. After four years, when drug companies started to mass produce penicillin, in 1943, the first signs of penicillin-resistant bacteria started to show up. The first bacteria that fought penicillin was called Staphylococcus aureus.