Polarised light micrograph of crystals of the antibiotic drug penicillin. Penicillin was the first antibiotic to be discovered. It was isolated from the mould Penicillium chrysogenum in 1940 by Alexander Fleming. Most penicillin today is synthetic. It is a broad-spectrum antibiotic used to treat many bacterial infections including pneumonia, tonsillitis, syphilis and meningococcal meningitis.