 a xenobiotic is a foreign chemical substance found within an organism that is not normally naturally produced by or expected to be present within that organism. it can also cover substances which are present in much higher concentrations than are usual. specifically, drugs such as antibiotics are xenobiotics in humans because the human body does not produce them itself, nor are they part of a normal food. natural compounds can also become xenobiotics if they are taken up by another organism, such as the uptake of natural human hormones by fish found downstream of sewage treatment plant outfalls, or the chemical defenses produced by some organisms as protection against predators. the term xenobiotics, however, is very often used in the context of pollutants such as dioxins and polychlorinated biphenyls and their effect on the biota, because xenobiotics are understood as substances foreign to an entire biological system, i.e. artificial substances, which did not exist in nature before their synthesis by humans. the term xenobiotic is derived from the greek words Î¾Î­Î½Î¿Ï (xenos) = foreigner, stranger and Î²Î¯Î¿Ï (bios, vios) = life, plus the greek suffix for adjectives -ÏÎ¹ÎºÏÏ, -Î®, -Ï (tic). ==xenobiotic metabolism== the body removes xenobiotics by xenobiotic metabolism. this consists of the deactivation and the excretion of xenobiotics, and happens mostly in the liver. excretion routes are urine, feces, breath, and sweat. hepatic enzymes are responsible for the metabolism of xenobiotics by first activating them (oxidation, reduction, hydrolysis and/or hydration of the xenobiotic), and then conjugating the active secondary metabolite with glucuronic acid, sulphuric acid, or glutathione, followed by excretion in bile or urine. an example of a group of enzymes involved in xenobiotic metabolism is hepatic microsomal cytochrome p450. these enzymes that metabolize xenobiotics are very important for the pharmaceutical industry, because they are responsible for the breakdown of medications. organisms can also evolve to tolerate xenobiotics. an example is the co-evolution of the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. in this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of resistance in the snake.{{cite journal |author=brodie ed, ridenhour bj, brodie ed |title=the evolutionary response of predators to dangerous prey: hotspots and cold
Microbiology_terms