A number of antibiotics produced by fungi of the genus Cephalosporium have been identified. These antibiotics called cephalosporins contain, in common with the penicillins, a β-lactam ring. In addition to the numerous penicillins and cephalosporins in use, three other classes of β-lactam antibiotics are available for clinical use. These are the carbapenems, the carbacephems, and the monobactams. All β-lactam antibiotics have the same bactericidal mechanism of action. They block a critical step in bacterial cell wall synthesis.