Penicillin was the first antibiotic to be used clinically in 1941. It is a miracle that the least toxic drug of its kind was the first to be discovered. It was originally obtained from the fungus Penicillium notatum, but the present source is a high yielding mutant of P. chrysogenum. The penicillin nucleus consists of fused thiazolidine and β-lactam rings to which side chains are attached through an amide linkage. Penicillin G (PnG), having a benzyl side chain at R (benzyl penicillin), is the original penicillin used clinically. The side chain of natural penicillin can be split off by an amidase to produce 6-aminopenicillanic acid. Other side chains can then be attached to it resulting in different semisynthetic penicillins with unique antibacterial activities and different pharmacokinetic profiles.
