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 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?

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

The 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, adding S9 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 assessing the effects of substances that require metabolic activation to exert their mutagenic effects.

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 occurs when a his⁻ mutant bacterium undergoes a mutation that restores its ability to produce histidine (his⁺). The presence of S9 extract can facilitate this process by activating compounds that cause mutations, resulting in a higher number of revertants when compared to tests without S9.