Penicillin refers to any member of beta-lactam antibiotics group. These agents are identified by a beta-lactam ring within their molecular structure. As the most widely used group of antibiotics available, beta-lactams are used for the treatment of bacterial infections usually caused by gram-positive organisms. Beta-lactam antibiotics are bactericidal, functioning to inhibit the synthesis of the peptidoglycan layer of bacterial cell walls. Bacterial penicillin-binding proteins and beta-lactamases constitute a large family of serine proteases that perform essential functions in the synthesis and maintenance of peptidoglycan cell wall. Notably, beta-lactamases cleave beta-lactams, therefore providing the bacteria with resistance to the antibiotic. Homologues of beta-lactamases occur in many species, including human, rat, cow, rabbit, pig, xenopus, zebrafish, and C. elegans. The human homologues, LACTB and LACTB2, are active-site-serine enzymes thought to be involved in metabolism.