The beta-lactam antibiotics stand out among As discussed below, this ring may be that part antimicrobial agents for-their wide spectrum of of the molecule which reacts with the penicillin activity and for their remarkably low toxicity to target. The side chains attached to the nucleus animals. These substances are composed of two are of importance because of their ease of chemical modification and the profound effects 'Present address: Department of Biology, Massachusetts which they exert on the properties of the mole