Any substance produced by a microorganism that inhibits the growth of another microorganism. Antibiotics are widely used as drugs to combat bacterial diseases. Examples are penicillin, obtained from the mould fungus Penicillium notatum and active against staphylococcal infections and many other gram-positive bacteria, and streptomycin, obtained from the actinomycete bacterium Streptomyces griseas and used to treat tuberculosis. Biosynthesis of antibiotics may be from amino acids (e.g. penicillin), sugars (e.g. streptomycin), or from acetate or propionate (e.g. tetracyclines). Commercial production is usually by large scale culture of the appropriate organism though some simple antibiotics, e.g. chloramphenicol, are cheaper to produce by artificial synthesis.
Antibiotics have proved useful research tools. Those that inhibit protein synthesis have been used to investigate ribosome structure and function. Most antibiotics inhibit protein synthesis on the 70S ribosomes of prokaryotes but not the 80S ribosomes of eukaryotes. The susceptibility of mitochondrial and chloroplast ribosomes to antibiotics is taken as further evidence that these organelles are derived from endosymbiotic prokaryotic organisms (see serial endosymbiotic theory
). Some antibiotics, e.g. cycloheximide, inhibit protein synthesis by 80S ribosomes but not 70S ribosomes. See also phytoalexin