What is an Ames Test?

The Ames test is a type of bioassay used to determine the degree of probable mutagenic activity likely to occur in the presence of one or more chemicals. Basically, it’s a fast and inexpensive method of predicting whether or not a chemical poses a cancer risk from causing mutations in genetic material, including DNA. However, it is also used for the purpose of illustrating that a chemical substance is not harmful. In fact, manufacturers of cosmetics and pharmaceuticals utilize Ames testing to initially “prove” that their products do not cause cancer in humans.

Bacteria is the guinea pig of choice for the Ames test, namely Salmonella typhimurium. While 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 from an animal source, such as rats. The reason for this is because bacteria naturally lack the enzymes needed to facilitate metabolism of the chemicals being introduced. This configuration also more closely simulates the cellular makeup of human tissue.

To perform the Ames test, the modified bacterium is combined with the chemical being tested in a test tube. Then, the sample is added to a glass plate that contains agar and histidine, a protein-based amino acid. The purpose of this chemical cocktail is two fold: The agar provides nutrients to feed the bacterium, while the presence of histidine allows chemical metabolism to take place. The sample is then permitted to grow in these conditions for 24 to 48 hours in isolation.

During this time, the volume of histidine is depleted, which would normally create a condition in which the bacterium couldn’t survive. However, if the sample does continue to thrive, it indicates that the bacterium no longer depends on the presence of enzymes to colonize. In short, this means that the bacterium has mutated. In this event, the Ames test has produced a positive result.

Does this mean that the chemical tested causes cancer in humans? Not necessarily. In fact, 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 kind of outcome is that the chemical should be further investigated as a possible cancer-causing agent.

The Ames test is named after Bruce Ames, who developed this screening method in the 1950s while engaged in cancer research at the University of California. Prior to its inception, suspect chemicals were tested on animals. Aside from promoting the suffering and ultimate death of these animals, the method was expensive and took months or years to obtain any results. In contrast, cultivating bacteria for the same purpose provided a significantly faster and cost-effective alternative.