Ames Test

A biological assay to assess the mutagenic potential of chemical compounds

Ames Test
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Purpose	Assessing mutagenic potential of chemical compounds
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The Ames test is a widely used assay to determine the mutagenicity of chemical compounds. Named after its developer, Bruce Ames, the test uses strains of the bacterium Salmonella typhimurium that carry mutations in genes involved in histidine synthesis. These strains are unable to grow on a medium lacking histidine unless a mutation occurs that reverses the original mutation, allowing the bacteria to synthesize histidine again.

Principle

The Ames test is based on the principle that mutagenic compounds can induce reverse mutations in the DNA of the test organism. The test organism, typically a strain of Salmonella typhimurium, carries a mutation that renders it unable to produce the amino acid histidine. When exposed to a mutagenic substance, some bacteria may undergo a reverse mutation that restores their ability to synthesize histidine, allowing them to grow on a histidine-free medium.

Procedure

The procedure involves the following steps:

1. Preparation of bacterial strains: Strains of Salmonella typhimurium with specific mutations are prepared. These strains are auxotrophic for histidine, meaning they cannot grow without an external source of histidine.

2. Exposure to test substance: The bacterial strains are exposed to the chemical compound being tested. This is often done in the presence of a liver extract (S9 mix) to simulate the metabolic activation of the compound in a mammalian system.

3. Plating on selective media: The bacteria are plated on agar plates that lack histidine. Only bacteria that have undergone a reverse mutation can grow and form colonies.

4. Counting revertant colonies: After incubation, the number of colonies on each plate is counted. An increase in the number of colonies compared to a control indicates that the test substance is mutagenic.

Interpretation

The results of the Ames test are interpreted by comparing the number of revertant colonies on the test plates to the number on control plates. A significant increase in revertant colonies suggests that the test compound is a mutagen. The test is often used as an initial screen for potential carcinogens, as many mutagens are also carcinogens.

Limitations

While the Ames test is a powerful tool for detecting mutagenic compounds, it has limitations:

- It primarily detects point mutations and may not identify compounds that cause other types of genetic damage. - The test uses bacterial cells, which may not metabolize compounds in the same way as human cells. - Some compounds require metabolic activation to become mutagenic, which may not be accurately replicated in the test.

Applications

The Ames test is used in various fields, including:

- Pharmaceuticals: Screening drug candidates for mutagenicity. - Environmental monitoring: Testing environmental samples for mutagenic pollutants. - Food safety: Assessing the safety of food additives and preservatives.

Also see

• Mutagenicity
• Carcinogen
• Genotoxicity
• Salmonella typhimurium
• Bruce Ames

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