An antibiotic is an agent that either kills or inhibits the growth of a microorganism. This definition excluded substances that kill bacteria but that are not produced by microorganisms (such as gastric juices and hydrogen peroxide). It also excluded synthetic antibacterial compounds such as the sulfonamides. Many antibacterial compounds are relatively small molecules with a molecular weight of less than 2000 atomic mass units. most modern antibacterials are semisynthetic modifications of various natural compounds.These include, for example, the beta-lactam antibiotics, which include the penicillins, the cephalosporins, and the carbapenems