The Ames test is a screening test that is used to help identify chemicals that affect the structure of DNA. The test exposes Salmonella bacteria to chemicals and looks for changes in the way bacteria grow. These changes result from mutations that occur when the structure of DNA is altered in certain places. Many chemicals that cause mutations can cause cancer in animals or in people. When the test was developed, it was thought that most of the chemicals that produce results in the Ames test could also cause cancer. It was hoped that this simple test would be an easy way to find cancer-causing chemicals. Over time, the test was found to be a less reliable predictor of carcino-genesis than had been hoped. Some chemicals that are known to cause cancer do not test positive in the Ames test and some chemicals that test positive do not cause cancer. Nonetheless, the test is still considered an important part of assessing the safety of new chemicals.
The Ames test uses strains of Salmonella that have been altered to make them more susceptible to mutation than normal Salmonella. To perform the test, the altered Salmonella strains are combined in a test tube with the chemical of interest. Because Salmonella bacteria lack the enzymes that animals use to metabolize chemicals, animal liver enzymes are often added to the test tube. That way, the test is able to detect what might happen if the chemical entered a human body. The Salmonella are then transferred to a petri dish to grow for one or two days. The altered Salmonella used for the test require the amino acid histidine to grow, and a positive result in the test is indicated when, in response to mutation, the Salmonella no longer require histidine to grow.
A positive result in an Ames test does not indicate by itself that a particular chemical is capable of causing cancer. It does suggest that a chemical can produce mutations and that more extensive testing is needed to determine whether the chemical is likely to produce cancer in humans. The test is useful as a screening tool for setting priorities because it is an inexpensive and quick way to help single out chemicals that should be targeted for further testing. It is also used in industry as a primary preventive approach to eliminate potential carcinogens early in the process of developing new commercial chemicals.
The test is named for its creator, Dr. Bruce Ames. Its development depended upon basic scientific advances in understanding the role of mutagenesis in chemical carcinogenesis, and its use was fundamental in the understanding of the mechanisms of carcinogenesis.