Beta-lactam in a heteroatomic ring structure, consisting of three carbon atoms and one nitrogen atom (Fig. 1). The beta-lactam ring is part of several antibiotics, such as penicillin, which are therefore also called beta-lactam antibiotics. These antibiotics work by inhibiting the bacterial cell wall synthesis. This has a lethal effect on bacteria, especially on gram positive ones. Bacteria can become resistant against beta-lactam antibiotics by expressing beta-lactamase.
Figure 1 : Penicillin-G. Beta-lactam ring marked blue.