Thienamycin, the first of the carbapenems, was isolated from Streptomyces cattl eya. Because of its
extremely intense and broad-spectrum antimicrobial activity as well as its ability to inactivate β-lactamases,
it combines in one molecule the functional features of the best of the β-lactam antibiotics as well as the
β-lactamase inhibitors. It differs structurally in several important respects from the penicillins and
cephalosporins. The sulfur atom is not part of the 5-membered ring but, rather, has been replaced by a
methylene moiety at that position. Carbon is roughly half the molecular size of sulfur. Consequently, the
carbapenem ring system is highly strained and very susceptible to reactions cleaving the β-lactam bond. The
sulfur atom is now attached to C-3 as part of a functionalized side chain.
