Editing Ames test (section)

==Fluctuation method==
[[File:Wiki muta2.png|300px|right|thumb|Fluctuation method: [[96-well plate]]]]
[[File:Ames MPF 384 plate.jpg|thumb|300px|Fluctuation method: 384-well plate]]
The Ames test was initially developed using agar plates (the plate incorporation technique), as described above. Since that time, an alternative to performing the Ames test has been developed, which is known as the "fluctuation method". This technique is the same in concept as the agar-based method, with bacteria being added to a reaction mixture with a small amount of [[histidine]], which allows the bacteria to grow and mutate, returning to synthesize their own histidine. By including a pH indicator, the frequency of mutation is counted in [[microplate]]s as the number of wells which have changed color (caused by a drop in pH due to metabolic processes of reproducing bacteria). As with the traditional Ames test, the sample is compared to the natural background rate of reverse mutation in order to establish the genotoxicity of a substance. The fluctuation method is performed entirely in liquid culture and is scored by counting the number of wells that turn yellow from purple in 96-well or 384-well microplates.

In the 96-well plate method the frequency of mutation is counted as the number of wells out of 96 which have changed color. The plates are incubated for up to five days, with mutated (yellow) colonies being counted each day and compared to the background rate of reverse mutation using established tables of significance to determine the significant differences between the background rate of mutation and that for the tested samples.

In the more scaled-down 384-well plate microfluctuation method the frequency of mutation is counted as the number of wells out of 48 which have changed color after 2 days of incubation. A test sample is assayed across 6 dose levels with concurrent zero-dose (background) and positive controls which all fit into one 384-well plate. The assay is performed in triplicates to provide statistical robustness. It uses the recommended OECD Guideline 471 tester strains (histidine auxotrophs and tryptophan auxotrophs).

The fluctuation method is comparable to the traditional pour plate method in terms of sensitivity and accuracy, however, it does have a number of advantages: it needs less test sample, it has a simple colorimetric endpoint, counting the number of positive wells out of possible 96 or 48 wells is much less time-consuming than counting individual colonies on an agar plate. Several commercial kits are available. Most kits have consumable components in a ready-to-use state, including lyophilized bacteria, and tests can be performed using multichannel pipettes. The fluctuation method also allows for testing higher volumes of aqueous samples (up to 75% v/v), increasing the sensitivity and extending its application to low-level environmental mutagens.<ref>{{cite journal | vauthors = Bridges BA | title = The fluctuation test | journal = Archives of Toxicology | volume = 46 | issue = 1–2 | pages = 41–4 | date = November 1980 | pmid = 7235997 | doi = 10.1007/BF00361244 | bibcode = 1980ArTox..46...41B | s2cid = 23769437 }}</ref>