How the Ames Test Works

Ames testing utilizes specific bacterial strains with mutations preventing histidine or tryptophan synthesis. These strains require these amino acids for growth. When exposed to a potential mutagen, bacteria may undergo reverse mutations, regaining the ability to synthesize histidine or tryptophan. This reversion to histidine or tryptophan independence is quantified to determine mutagenic potential.

Enhanced Ames Test

The enhanced Ames test is a modified version of the classic Ames test, specifically modified to detect N-nitrosamines, a class of compounds that can be challenging to detect using the standard method due to limited metabolism.

Scientist preparing a petri dish for an Ames assay.

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Purpose and Context of the Ames Test

  • The enhanced Ames test is designed to address the limitations of the standard Ames test to detect N-nitrosamines
  • N-nitrosamines are impurities found in certain drugs and other products. Many are potent mutagens in vivo and carcinogens
  • Regulatory authorities have recognized the need for improved sensitivity in detecting N-nitrosamines and have issued guidance on enhanced testing conditions

ASTM Details and Variations in Ames Assays

The ASTM test method is a modification of the standard Ames assay. By using this modified method there is a good correlation between the mouse skin-painting bioassay specifically for test materials obtained from raw and refined lubricating oil process streams.

Ames Test Treat and Plate Method for Histidine or Tryptophan Containing Test Materials

The Treat and Plate method is an adaptation of the conventional Ames test, tailored to assess the mutagenicity of substances containing histidine or tryptophan. These amino acids can interfere with the accuracy of the test by causing bacterial overgrowth. The modified approach of the Ames mutagenicity test prevents these issues and ensures a more reliable evaluation of mutagenic potential.

Scientist preparing a petri dish for an Ames assay.

EMGS Scientific Poster - Ames Assay
Scientists reviewed concerns of bacterial reverse mutation test and OECD TG4711. See the comparison of historical negative control data from the Ames Assay.
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Testing Gas/Volatile Test Materials in an Ames Study

Testing gas or volatile substances in an Ames study presents unique challenges due to their physical properties. Special adaptations are used to deliver and contain the test material in closed to ensure adequate exposure of the bacterial cultures to the test material.

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Frequently Asked Questions (FAQs) for the Ames Test

  • What is the Ames test and why is it important?

    The Ames test, (i.e., Salmonella typhimurium and/or Escherichia coli reverse mutation assay) offered at Charles River is a bacterial short-term test for the identification of carcinogens that measures mutations in the DNA in bacteria. Since its development, Ames testing has been widely used to assess the mutagenic and carcinogenic risks of many chemicals as it is rapid and inexpensive.

  • What do Ames studies determine?

    The Ames test is a rapid and reliable bacterial assay used to evaluate a chemical's potential genotoxicity by measuring its ability to induce reverse mutations at selected loci of several bacterial strains.

  • What is the difference between the plate incorporation and pre-incubation method in an Ames assay?

    In the plate incorporation method, the test item is mixed with the top agar and bacteria and is immediately plated. In the pre-incubation method, the bacterial strains are mixed with the test item in the presence or absence of S9-mix for 20-30 minutes at 37 °C (with shaking). Thereafter, this mixture is added to top agar and plated. The pre-incubation method is generally considered the most sensitive method for detecting mutagens but is also more susceptible to cytotoxicity.

  • How are toxic effects of Ames testing item evaluated?

    Cytotoxicity of the test item is normally indicated by the partial or complete absence of a bacterial background lawn or a substantial dose-related reduction in revertant colony counts, compared with the lower dose levels and concurrent vehicle control accounting for the laboratory historical control range.

  • Why are five tester strains required for mutagenicity testing in the Ames assay?

    Every bacteria strain detects a certain type of damage. To cover a wide range of mutational events, five tester strains are included in the Ames mutagenicity test.

    StrainDetects
    TA98frameshift
    TA100
    TA1535    		base substitution
    base substitution
    TA1537,
    TA97, or
    TA97a    		frameshift
    frameshift
    frameshift
    TA102,
    WP2 uvrA, or
    WP2 uvrA (pKM101)    		base substitution, x-linker
    base substitution
    base substitution

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