Penicillin G. Computer model of a molecule of the antibiotic drug penicillin G, also known as benzylpenicillin (formula: C16.H18.N2.O4.S). The atoms are shown as spheres and are colour-coded: carbon (green), hydrogen (white), nitrogen (blue), oxygen (red) and sulphur (yellow). This was the first penicillin that Alexander Fleming isolated from the mould Penicillium chrysogenum in 1940. Most penicillin today is synthetic. Penicillin G is a broad-spectrum antibiotic used to treat many bacterial infections including pneumonia, tonsillitis, syphilis and meningococcal meningitis.