Editing Ames test (section)

== General procedure ==
The Ames test uses several strains of the bacterium ''[[Salmonella typhimurium]]'' that carry mutations in genes involved in [[histidine]] synthesis. These strains are [[auxotrophy|auxotrophic]] mutants, i.e. they require histidine for growth, but cannot produce it. The method tests the capability of the tested substance in creating mutations that result in a return to a "prototrophic" state, so that the cells can grow on a histidine-free medium.

The tester strains are specially constructed to detect either [[Translational frameshift|frameshift]] (e.g. strains TA-1537 and TA-1538) or [[point mutation|point]] (e.g. strain TA-1531) [[mutation]]s in the genes required to synthesize histidine, so that mutagens acting via different mechanisms may be identified. Some compounds are quite specific, causing reversions in just one or two strains.<ref name="ames 1">{{cite journal | vauthors = Ames BN, Gurney EG, Miller JA, Bartsch H | title = Carcinogens as frameshift mutagens: metabolites and derivatives of 2-acetylaminofluorene and other aromatic amine carcinogens | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 69 | issue = 11 | pages = 3128–32 | date = November 1972 | pmid = 4564203 | pmc = 389719 | doi = 10.1073/pnas.69.11.3128 | bibcode = 1972PNAS...69.3128A | doi-access = free }}</ref> The tester strains also carry mutations in the genes responsible for [[lipopolysaccharide]] synthesis, making the [[cell wall]] of the bacteria more permeable,<ref name="ames 2">{{cite journal | vauthors = Ames BN, Lee FD, Durston WE | title = An improved bacterial test system for the detection and classification of mutagens and carcinogens | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 70 | issue = 3 | pages = 782–6 | date = March 1973 | pmid = 4577135 | pmc = 433358 | doi = 10.1073/pnas.70.3.782 | bibcode = 1973PNAS...70..782A | doi-access = free }}</ref> and in the [[Nucleotide excision repair|excision repair system]] to make the test more sensitive.<ref name="ames 3">{{cite journal | vauthors = McCann J, Spingarn NE, Kobori J, Ames BN | title = Detection of carcinogens as mutagens: bacterial tester strains with R factor plasmids | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 72 | issue = 3 | pages = 979–83 | date = March 1975 | pmid = 165497 | pmc = 432447 | doi = 10.1073/pnas.72.3.979 | bibcode = 1975PNAS...72..979M | doi-access = free }}</ref>

Larger organisms like mammals have metabolic processes that could potentially turn a chemical considered not mutagenic into one that is or one that is considered mutagenic into one that is not.<ref>{{cite book | first1 = Leland | last1 = Hartwell | first2 = Michael | last2 = Goldberg | first3 = Leroy | last3 = Hood | first4 = Ann | last4 = Reynolds | first5= Lee | last5 = Silver | name-list-style = vanc | title = Genetics: from genes to genomes|date=2011|publisher=McGraw-Hill | isbn = 978-0-07-352526-6 |edition=4th|location=New York|oclc=317623365}}</ref> Therefore, to more effectively test a chemical compound's mutagenicity in relation to larger organisms, rat liver enzymes can be added in an attempt to replicate the metabolic processes' effect on the compound being tested in the Ames Test. Rat liver extract is optionally added to simulate the effect of [[metabolism]], as some compounds, like [[benzo(a)pyrene|benzo[''a'']pyrene]], are not mutagenic themselves but their metabolic products are.<ref name="ames">{{cite journal | vauthors = Ames BN, Durston WE, Yamasaki E, Lee FD | title = Carcinogens are mutagens: a simple test system combining liver homogenates for activation and bacteria for detection | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 70 | issue = 8 | pages = 2281–5 | date = August 1973 | pmid = 4151811 | pmc = 433718 | doi = 10.1073/pnas.70.8.2281 | bibcode = 1973PNAS...70.2281A | doi-access = free }}</ref>

The bacteria are spread on an [[agar]] plate with a small amount of histidine. This small amount of histidine in the growth medium allows the bacteria to grow for an initial time and have the opportunity to mutate.
When the histidine is depleted only bacteria that have mutated to gain the ability to produce its own histidine will survive. The plate is incubated for 48 hours. The mutagenicity of a substance is proportional to the number of colonies observed.