Penicillin is a naturally occurring antibiotic, containing a fused thiazolidine and beta lactam ring, interfering with the growth or metabolism of many microorganisms of harmful nature. The substance was first observed by A. Fleming in 1929, however the chemical structure was determined in 1945 and the first chemical synthesis was performed by Sheehan only in 1957. Penicillin has a very low toxicity for mankind. Penicillin being acid labile, is having restricted therapeutic use. However, Penicillin is the starting compound for many higher molecular weight derivatives of common therapeutic use like amoxicillin, ampicillin, cephalexin etc. Two Penicillins are mainly used: Penicillin G and Penicillin V (phenylacetic acid side chain and phenoxy acetic acid side chain respectively).
