Penicillins (penams) are a group of β-lactam antibiotics originally obtained from Penicillium sp. The penam bicyclic skeleton consists of a β-lactam ring fused with a thiazolidine ring. The penicillins have their origin in the pioneering work of Fleming, University of Oxford in 1928 and remain one of the most important classes of antibiotics today. Penicillin kills susceptible bacteria by inhibiting the transpeptidase that cross-links cell wall peptidoglycan. The Penicillin Metabolite Set contains benzylpenicillin (Pencillin G), phenoxymethylpenicillin (penicillin V) as the natural products and the key semi-synthetic penicillins developed subsequently, including the orally effective ampicillin and its prodrug, hetacillin, and the β-lactamase penicillins.