Question

A chemist has synthesized a novel chemical, which he suspects to be a potential mutagen. Name and explain a popular test that can be used to test the mutagenicity of this product in bacteria.

Short answer

Answer: The Ames test is a widely used bacterial test to evaluate the mutagenic potential of chemicals. It utilizes strains of the bacterium Salmonella typhimurium with specific mutations that prevent the synthesis of the amino acid histidine (His). These bacteria are exposed to various concentrations of the chemical being tested, and the increase in the number of colonies reverting to the ability to synthesize histidine (His+) indicates the mutagenic potential of the chemical. The test can be performed with and without a liver extract (S9 fraction) to evaluate if the metabolites of the chemical are mutagenic.

Step-by-step solution

Name of the popular test

The "Ames test" is a widely used bacterial test to evaluate the mutagenic potential of chemicals. It was developed by Bruce Ames and his colleagues in the early 1970s.

Explain the principle of Ames test

The Ames test is based on the use of strains of the bacterium Salmonella typhimurium that carry specific mutations in genes responsible for the synthesis of the amino acid histidine (His). These His- strains cannot synthesize histidine, thus they are auxotrophic for histidine, meaning they need histidine supplementation to grow. If a substance is mutagenic, it can cause reverse mutations (reversions) in these bacteria, allowing them to regain their ability to synthesize histidine (His+) and grow in a histidine-free media.

Outline the procedure of Ames test

1. Prepare a series of dilutions of the suspected mutagenic chemical. 2. Inoculate the His- bacteria into a minimal culture medium, which lacks histidine. 3. Add a particular concentration of the chemical to each culture, along with a positive control (a known mutagen) and a negative control (no mutagen). 4. Incubate the cultures for a suitable time (e.g., 48 hours) at an appropriate temperature (e.g., 37°C). 5. Observe and count the number of colonies that grow in each culture. The number of colonies is proportional to the number of mutant bacteria that had reverted back to being able to synthesize histidine (His+). 6. Compare the number of revertant colonies in each test culture to the number in the negative control culture. An increase in the number of colonies compared to the negative control suggests that the tested chemical is mutagenic.

Evaluation of results

The Ames test gives a qualitative result (i.e., whether a substance is mutagenic or not) and also a quantitative indication of the mutagenic potential of the substance, as the number of revertant colonies is usually correlated to the potency of the mutagen. The test can be performed with and without a liver extract (known as the S9 fraction), which contains enzymes that can metabolize substances into their active (or inactive) forms. This allows the researcher to observe whether the metabolites of the novel chemical are themselves mutagenic.