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An antibiotic is a chemical compound that inhibits or abolishes the growth of microorganisms, such as bacteria, fungi, or protozoans. The original meaning of antibiotic includes any agent with biological activity against living organisms; however, the term is commonly used to refer to substances with anti-bacterial, anti-fungal, or anti-parasitical activity. The first antibiotic compounds used in modern medicine were produced and isolated from living organisms, for example, the penicillin class produced by fungi in the genus Penicillium, or streptomycin from bacteria of the genus Streptomyces. With the advent of organic chemistry many antibiotics are now also obtained by chemical synthesis, such as the sulfa drugs. Many antibiotics are relatively small molecules with a molecular weight less than 2000 Da.