Alexander Fleming accidently discovered antibiotic penicillin from a mould, but he was working on          

Alexander Fleming accidentally discovered penicillin while working on Staphylococcus bacteria. He noticed mold from the Penicillium genus inhibiting bacterial growth around it. Fleming named the substance produced by the mold "penicillin" and recognised its potential as an antibiotic. Ernst Chain and Howard Florey later purified and realized penicillin's antibiotic properties, leading to its mass production and Nobel Prize award in 1945.