Question

Recall that in the Ames Test, potential mutagens are tested for their ability to revert His− auxotrophs to His+. Any particular his- mutation present in a His− auxotrophic bacterial strain would be able to be reverted only by some mutagens and not others. Yet, the purpose of the Ames Test is to determine whether or not a specific substance is a mutagen of any type. How is the Ames Test actually performed to circumvent this problem?

Answer 1

The Ames test checks for mutagenicity of chemical compounds using strains of bacteria with specific mutations; the presence of revertant colonies indicates mutagenic potential. The inclusion of liver extract simulates metabolism, enhancing detection of mutagens that are activated metabolically. Mutagenicity suggests potential carcinogenicity, prompting further testing.

Step-by-step explanation:

The Ames test is a biological assay designed to evaluate the mutagenic potential of chemical compounds. By using a strain of Salmonella typhimurium that requires histidine to grow (his- auxotrophs), the test measures the rate at which exposure to a chemical induces mutations by counting the number of revertant colonies that can grow without histidine. To test for a wide array of mutagens, the Ames test includes different strains of bacteria, each with mutations in different parts of the histidine synthesis pathway. This diversity increases the chances that a mutagen will cause a reversion in at least one strain, thus revealing its mutagenic properties.

To account for chemicals that require metabolic activation to become mutagenic, a liver extract (often from rats) is added to mimic mammalian metabolism. The number of colonies that grow on a histidine-free medium after exposure to the chemical (with and without liver extract) is compared to the number of colonies that grow without the chemical exposure, providing an indication of the substance's mutagenicity. If the chemical increases the mutation rate, it may also be a potential carcinogen since many carcinogens are mutagens. Additional tests, including animal models, are used to further assess the carcinogenic risk of compounds identified as mutagenic in the Ames test.

Answer 2

To work around this problem, the Ames test is done as follows:

A petri dish is seeded with salmonella colonies, moreover, in that petri dish a little chemical is added in the center. A culture medium is also added to the plate where agar is one of the ingredients and biotin and histidine have been mixed.

Only bacteria that show some kind of mutation will grow on these agar plates. This is because histidine has the function of allowing cell divisions necessary only for cases in which mutagenesis is necessary to occur.