The Ames test is a bioassay that determines the likelihood of mutagenic activity in chemicals. Bacteria, specifically Salmonella typhimurium, is used to perform the test. A positive result indicates the chemical may be a carcinogen and requires further investigation. The test is named after its developer, Bruce Ames.
The Ames test is a type of bioassay used to determine the degree of probable mutagenic activity that can occur in the presence of one or more chemicals. Basically, it’s a quick and cheap method of predicting whether or not a chemical poses a cancer risk by causing mutations in genetic material, including DNA. However, it is also used to illustrate that a chemical is not harmful. Indeed, manufacturers of cosmetics and pharmaceuticals use Ames tests to initially “prove” that their products do not cause cancer in humans.
Bacteria is the preferred guinea pig for the Ames test, namely Salmonella typhimurium. Although several strains of this species are used, they are all altered so that they are more receptive to genetic mutation. This is usually achieved with exposure to liver enzymes of animal origin, such as rats. The reason for this is because bacteria naturally lack the enzymes needed to facilitate the metabolism of introduced chemicals. This setup also more closely mimics the cellular composition of human tissue.
To perform the Ames test, the modified bacterium is combined with the test chemical in a test tube. Then, the sample is added to a glass slide that contains agar and histidine, an amino acid in proteins. The purpose of this chemical cocktail is twofold: the agar provides nutrients to feed the bacterium, while the presence of histidine allows for chemical metabolism. The sample is then left to grow under these conditions for 24-48 hours in isolation.
During this time, the volume of histidine is depleted, which would normally create a condition in which the bacterium could not survive. However, if the sample continues to thrive, it indicates that the bacterium is no longer dependent on the presence of enzymes to colonize. In short, this means that the bacterium has mutated. In this case the Ames test gave a positive result.
Does this mean that the chemical tested causes cancer in humans? Not necessarily. Indeed, a positive result simply demonstrates that the chemical in question is capable of causing genetic mutations. However, the practical conclusion to be drawn from this type of finding is that the chemical should be further investigated as a possible carcinogen.
The Ames test is named after Bruce Ames, who developed this screening method in the 1950s while researching cancer at the University of California. Prior to its inception, suspected chemicals were tested on animals. In addition to promoting the suffering and ultimate death of these animals, the method was expensive and took months or years to produce results. Conversely, growing bacteria for the same purpose provided a significantly faster and cheaper alternative.
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