In common usage, an antibiotic is a substance or compound (also called chemotherapeutic agent) that kills or inhibits the growth of bacteria.

The term "antibiotic" (from the  – anti, "against" and  – bios, "life") was coined by Selman Waksman in 1942 to describe any substance produced by a micro-organism that is antagonistic to the growth of other micro-organisms in high dilution. This original definition excluded naturally occurring substances, such as gastric juice and hydrogen peroxide (they kill bacteria but are not produced by micro-organisms), and also excluded synthetic compounds such as the sulfonamides (which are antimicrobial agents). Many antibiotics are relatively small molecules with a molecular weight less than 2000 Da.Facts: date=February 2009

With advances in medicinal chemistry, most antibiotics are now modified chemically from original compounds found in nature, as is the case with beta-lactams (which include the penicillins, produced by fungi in the genus Penicillium, the cephalosporins, and the carbapenems). Some antibiotics are still produced and isolated from living organisms, such as the aminoglycosides; in addition, many more have been created through purely synthetic means, such as the quinolones.Facts: date=February 2009

History of antibiotics

Although potent antibiotic compounds for treatment of human diseases caused by bacteria (such as tuberculosis, bubonic plague, or leprosy) were not isolated and identified until the twentieth century, cures for infection were described in ancient Chinese medicine using plants with antibiotic-like properties over 2,500 years ago. Many other ancient cultures, including the ancient Egyptians, ancient Greeks and medieval Arabs already used molds and plants to treat infections. Quinine became widely used as a therapeutic agent in the 17th century for the treatment of malaria, the disease caused by protozoan parasites of the genus Plasmodium.

Originally known as antiobiosis, antibiotics were drugs that had actions against bacteria. The term antibiosis which means ‘against life' was introduced by the French bacteriologist Vuillemin as a descriptive name of the phenomenon exibited by these drugs.Calderon CB, Sabundayo BP (2007). Antimicrobial Classifications: Drugs for Bugs. In Schwalbe R, Steele-Moore L, Goodwin AC. Antimicrobial Susceptibility Testing Protocols. CRC Press. Taylor & Frances group. ISBN 0-8247-4100-5 (Antibiosis was first described in 1877 in bacteria when Louis Pasteur and Robert Koch observed that an airborne bacillus could inhibit the growth of Bacillus anthracis.). These drugs were later renamed antibiotics by Selman Wakeman, an american microbiologist in 1942.