An antibiotic derived from the mould Penicillium notatum; specifically it is known as penicillin G and belongs to a class of similar substances called penicillins. They are all active against a wide variety of bacteria, producing their effects by inhibiting cross-linking of peptidoglycans in the bacterial cell wall, especially Gram-positive bacteria. Penicillins are used to treat a variety of infections caused by these bacteria, although many bacterial strains have developed resistance.