person wearing medical gloves holding petri dish

Bacterial Reverse Mutation Test: Test, Procedure, and Analysis

Bacterial Reverse Mutation test also popularly known as the Ames test which plays a pivotal role in understanding genetic mutations. AMES Test has the ability to assess the potential mutagenic properties of various compounds. With the integration of this test into our scientific practices, we have taken significant strides in genotoxicity testing, pharmaceutical research and even environmental analysis.

The Bacterial Reverse Mutation Test: A Vital Tool in Mutagenicity Testing
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The Origin of the Bacterial Reverse Mutation Test

In the 1970s, Bruce Ames, an American molecular biologist, developed a method to identify potential mutagens – substances that could induce genetic mutations. His test, now recognized as the Ames test or the Bacterial Reverse Mutation Test and that has become a standard in evaluating the mutagenicity of different substances from drugs to chemicals found in our daily lives.

The Basic Principle: Reverse Mutation

At the heart of the bacterial reverse mutation test lies a fascinating biological process called “reverse mutation,” or “back mutation.”

Simply put, this process can revert a mutated gene back to its original, non-mutated state. Reverse mutations can often occur naturally, but in the context of the Ames test, these are typically induced by the potential mutagen under examination.

The primary bacteria used in this test is Salmonella typhimurium, which has been genetically altered to make it more susceptible to mutations.

This particular strain of bacteria is unable to synthesize the amino acid histidine, which is essential for their growth. When exposed to a mutagenic substance, the bacteria’s mutated histidine synthesis gene can experience a reverse mutation, allowing it to synthesize histidine again and hence grow on a histidine-free medium.

Procedure- Bacterial Reverse Mutation Test

A typical bacterial reverse mutation test involves placing the Salmonella typhimurium bacteria on a medium lacking histidine along with the compound under investigation. The petri dish is then incubated for a specified period, typically around 48 hours.

Following this, the colonies are examined for growth. If the substance being tested has caused reverse mutations in the bacteria’s DNA, they will be able to grow on the histidine-free medium, indicating that the substance is potentially mutagenic.

This straightforward, yet effective, test helps identify substances that can cause genetic mutations. It provides vital clues to whether or not a substance could pose genotoxic risks to humans or the environment.


Preparation of Test Substances and Bacterial Cultures

The first step in conducting the bacterial reverse mutation test is preparing the test substance and bacterial cultures. The substance under investigation is typically dissolved in an appropriate solvent. The bacteria, usually Salmonella typhimurium strains with specific mutations, are cultured in a nutrient broth.

Plate Incorporation Method

One of the most common methods of carrying out the Ames test is the Plate Incorporation method. This involves incorporating the test substance directly into the agar medium.

  1. Prepare the Agar Plates: The agar plates are prepared with a minimal glucose agar medium which lacks histidine. This means that only bacteria that have undergone a reverse mutation will be able to grow.
  2. Mix Test Substance with Bacteria: The test substance is mixed with an overnight culture of the bacteria and a rat liver extract, which contains enzymes that can metabolize the test substance, mimicking the conditions in a mammalian body.
  3. Spread Mixture on Agar Plates: The entire mixture is then spread onto the agar plates. Multiple plates are prepared for each dose of the test substance to ensure reliable results.
  4. Incubation: The plates are inverted and incubated at 37°C (approximately human body temperature) for 48 hours.

Observation and Analysis

After the incubation period, the plates are observed for the presence of bacterial colonies. Growth on the histidine-free medium indicates a reverse mutation caused by the test substance. The number of colonies in the control plates (those without the test substance) is compared with the number in the test plates.

  1. Observation: Examine each plate for the presence of bacterial colonies. The colonies will be visible as tiny spots on the surface of the agar.
  2. Scoring: Count the number of colonies on each plate. This can be done manually or with the aid of an automated colony counter.
  3. Interpretation: Compare the number of colonies on the test plates with the control plates. A significant increase in the number of colonies on the test plates indicates that the test substance is mutagenic.

It’s important to note that, because the bacterial reverse mutation test uses a simplified model system, any positive results would need to be confirmed with further testing in mammalian systems to account for the complexity of higher organisms.

Moreover, false positives and negatives can occur due to various factors, hence the importance of conducting the test in a controlled environment and repeating the process for validation.

In conclusion, the bacterial reverse mutation test is an invaluable tool in preliminary screenings for potential mutagens. Its simplicity, cost-effectiveness, and relative speed make it an excellent first step in the journey to ensure the safety of new substances entering our environment.

It’s a testament to the ingenious methodologies that are rooted in our understanding of fundamental biological processes such as mutation and metabolism.

The Role of the Bacterial Reverse Mutation Test in Modern Science

The Ames test has become a cornerstone in various areas of research and industry. It’s a standard preliminary screening tool for potential drugs, food additives, and other chemicals that may have genotoxic properties. If a substance shows mutagenic properties in the Ames test, it’s typically subjected to further in-depth testing using more complex eukaryotic models.

One key advantage of the bacterial reverse mutation test is its simplicity and high-throughput capabilities. Given its quick turnaround time and relative inexpensiveness compared to tests using mammalian cells or live animals, it is often the first line of defense in identifying potentially hazardous substances.

Final Thoughts

The bacterial reverse mutation test is an essential part of the genotoxicity testing toolkit. It provides scientists and researchers a first glance at the potential mutagenic properties of a substance, guiding their way towards creating safer drugs, chemicals, and environmental practices.

Despite being developed over five decades ago, the test’s relevance and applicability have remained intact. It stands as a testament to the importance of understanding our genetic building blocks and how they interact with the world around us.

Understanding the bacterial reverse mutation test is crucial, not only for scientists but for anyone interested in the journey our everyday substances take from their creation to their eventualuse.

This test forms an essential part of this journey, helping to ensure that the substances we interact with, consume, or use are safe for us and our environment. In the ever-advancing world of science, the bacterial reverse mutation test remains a trusted and widely-used tool in our collective arsenal against genetic mutations and the damage they can cause.


In essence, the bacterial reverse mutation test serves as a beacon, casting its light on potential genotoxic substances and allowing us to navigate safely in a world where new chemicals are continually being developed and used.

Its simplicity, combined with its robust and accurate results, makes it an invaluable tool in genetic studies and mutagenicity testing. Thus, the bacterial reverse mutation test, or the AMES test, continues to contribute to our better understanding of mutagenic properties, shaping a safer future for us all.