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Auxotrophy (Gr. αὐξάνω "to increase"; τροφή "nourishment") is the inability of an organism to synthesize a particular organic compound required for its growth (as defined by IUPAC). An auxotroph is an organism that displays this characteristic; auxotrophic is the corresponding adjective. Auxotrophy is the opposite of prototrophy, which is characterized by the ability to synthesize all the compounds needed for growth.
The method of replica plating implemented by Esther Lederberg included auxotrophs that were temperature-sensitive; that is, their ability to synthesize was temperature-dependent. (Auxotrophs are usually not temperature-dependent. They can also depend on other factors.) Multiple auxotrophs can also coexist at the same time, within the same organism.
In genetics, a strain is said to be auxotrophic if it carries a mutation that renders it unable to synthesize an essential compound. For example, a yeast mutant with an inactivated uracil synthesis pathway gene is a uracil auxotroph. (E.g., if the yeast Orotidine 5'-phosphate decarboxylase gene is inactivated, the resultant strain is a uracil auxotroph.) Such a strain is unable to synthesize uracil and will only be able to grow if uracil can be taken up from the environment. This is the opposite of a uracil prototroph, or in this case a wild-type strain, which can still grow in the absence of uracil. Auxotrophic genetic markers are often used in molecular genetics; they were famously used in Beadle and Tatum's Nobel prize-winning work on the one gene-one enzyme hypothesis.
Researchers have used strains of E. coli auxotrophic for specific amino acids to introduce non-natural amino acid analogues into proteins. For instance cells auxotrophic for the amino acid phenylalanine can be grown in media supplemented with an analogue such as para-azido phenylalanine.
Many living things, including humans, are auxotrophic for large classes of compounds required for growth and must obtain these compounds through diet (see vitamin, essential amino acid, essential fatty acid).
See also 
- "Regulation of endosomal clathrin and retromer-mediated endosome to Golgi retrograde transport by the J-domain protein RME-8" - The EMBO Journal
- "Pleiotropic effects of purine auxotrophy inRhizobium meliloti on cell surface molecules" - Springerlink
- http://mic.sgmjournals.org/cgi/reprint/16/1/286.pdf "Auxotrophy and Organic Compounds in the Nutrition of Marine Phytoplankton"