any of a class of chemical substances, some of which have therapeutically useful antibacterial activity, whose structure contains a beta-lactam ring fused to a six-membered ring containing a sulfur and a nitrogen atom. The first of the series, cephalosporin C, was discovered by G. Brotzu in 1955 in the culture broth of a Cephalosporium species found off the coast of Sardinia. Other cephalosporins have been found to be produced by species of soil bacteria (actinomycetes). Many semisynthetic analogs have been tested for antibacterial effect, and several of them have found use as important clinically useful antibacterial agents, some of which may be taken orally for treatment of bacterial infections. The cephalosporins are the second class of beta-lactam antibiotic to be discovered, the first being the penicillins and more recent classes being the thienamycins and sulfazecins. The cephamycins are a variant of cephalosporins with a methoxyl group on the beta-lactam ring, rendering them more resistant to penicillinases. Among the cephalosporins which have been found clinically useful are cephalexin, cephaloridine, and cephalothin.  [PJC]