Overview of Ames Test

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The AMES test also known as bacteria reversed mutation assay is used to evaluate the mutagenic properties of test articles. The test was first developed by Bruce Ames in 1974 (Krebsfaenger). The amino acid dependent strain of S. typhimurium and E. coli are used in this experiment where in the absence of the external histidine source, the cells cannot grow to form colonies. Specifically these strains of Salmonella are defective in 1.) Repair of mutations (uvrB) and 2.) A rfa mutation (eliminating a portion of lipolysaccharide (a coating of outer bacterial surface)). The rfa mutation here fulfills two purposes: 1.) Helps Salmonella in growing in presence of sodium desoxycholate or crystal violet. and 2.) Increases the cell permeability allowing more mutagen to enter the cell. The lack of uvrB gene in the decreases the rate of repair mechanism of mutations occurring resulting in the increased incidences of occurring mutations. These auxotrophic strain cannot grow on the media without histidine and biotine (due to uvrB).

If these organisms are allowed to grow on the media lacking both of these, the strain get converted to prototroph resulting the organisms to grow on the mutagenic chemicals to be tested in the media. If the chemical being tested is mutagenic, the organisms will grow as some substance are capable of causing mutations in the cells at same site or at nearby sites resulting in restoring gene's function and these mutations in the cells can revert back the gene regaining its function (Tejs). These revertant cells are then able to grow on the media which does not contain histdine as it can now synthesize histidine on its own. This mutation causes the cells to divide continuously. If there is no further mutation occurring in the cell, the cells will die out like normal cell. But if any further mutation occurs which allows the cells to grow for many generations, then the cancerous cells will be formed.

The principle of Ames test is specifically based...
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