Penicillin G molecule. Animation of a molecule of the antibiotic drug penicillin G, also known as benzylpenicillin. 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. The atoms are represented as spheres and are colour_coded: carbon black, hydrogen white, nitrogen blue, oxygen red and sulphur yellow.