Selman Waksman defined the term antibiotic in 1942 to describe any substance produced by a microorganism that is antagonistic to the growth of other microorganisms in high dilution. While this definition was coined it meant in the same way that substances that kill bacteria, but are not produced by microorganisms were excluded. Not included were synthetic antibacterial compounds such as the sulfonamides, too. Thanks to proceedings in medicinal chemistry, most of today's antibacterials chemically are semisynthetic modifications of various natural compounds.These include, for example, the beta-lactam antibacterials, which include the penicillins (produced by fungi in the genus Penicillium), the cephalosporins, and the carbapenems In general many antibacterial compounds are relatively small molecules with a molecular weight of less than 2000 atomic mass units.

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