In an Ames test using his⁻ Salmonella bacteria a researcher determines that adding a test compound plus the S9 extract produces a large number of his⁺ revertants but mixing the his⁻ strain plus the test compound without adding S9 does not produce an elevated number of his⁺ revertants.
What is the reason for the different experimental results described?

Verified step by step guidance

1. Understand the Ames test: The Ames test is used to assess the mutagenic potential of a compound. It involves using a strain of bacteria (e.g., his⁻ Salmonella) that cannot synthesize histidine due to a mutation. If the compound causes mutations that revert the bacteria to his⁺, it indicates mutagenic activity.
2. Recognize the role of the S9 extract: The S9 extract is a mixture of enzymes derived from the liver of mammals (often rats). It mimics the metabolic processes that occur in the liver, which can activate or detoxify compounds. Some compounds are not directly mutagenic but become mutagenic after metabolic activation by the S9 extract.
3. Analyze the experimental results: In the experiment, the test compound plus the S9 extract produced a large number of his⁺ revertants, indicating that the compound becomes mutagenic after metabolic activation. Without the S9 extract, the compound did not produce an elevated number of his⁺ revertants, suggesting it is not directly mutagenic in its original form.
4. Conclude the reason for the difference: The test compound requires metabolic activation by the enzymes in the S9 extract to become mutagenic. Without the S9 extract, the compound remains inactive and does not induce mutations in the his⁻ Salmonella strain.
5. Summarize the importance of the S9 extract: The inclusion of the S9 extract in the Ames test is crucial for identifying compounds that are pro-mutagens (inactive until metabolized). This highlights the importance of simulating metabolic processes in mutagenicity testing.

Key Concepts

Ames Test

The Ames test is a widely used method for screening potential carcinogens by assessing their mutagenic effects on specific strains of bacteria, typically Salmonella. In this test, bacteria that require histidine (his⁻) for growth are exposed to a test compound, and the number of revertants (his⁺) that can grow without histidine is measured. A significant increase in revertants indicates that the compound may cause mutations that restore the bacteria's ability to synthesize histidine.

S9 Extract

S9 extract is a metabolic activation system derived from the liver of rodents, containing enzymes that can convert pro-mutagens into active mutagens. In the Ames test, the addition of S9 extract allows researchers to simulate the metabolic processes that occur in living organisms, which can enhance the mutagenic potential of certain compounds. This is crucial for identifying substances that may not be mutagenic in their original form but become so after metabolic activation.

Mutagenesis and Reversion

Mutagenesis refers to the process by which genetic mutations are induced, leading to changes in the DNA sequence. In the context of the Ames test, reversion is the process where a his⁻ mutant bacterium undergoes a mutation that restores its ability to synthesize histidine, resulting in the his⁺ phenotype. The presence of S9 extract can facilitate this reversion by activating compounds that cause mutations, explaining the observed difference in revertant numbers when S9 is included versus when it is not.