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The Ames Test: Using Bacteria to Test for Carcinogens

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  • 0:07 Mutations
  • 1:15 Mutagens and Carcinogens
  • 2:21 The Ames Test
  • 6:06 Lesson Summary
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Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

We often think of bacteria as the bad guys. But in the Ames test, special mutant bacteria can help us avoid getting cancer! Take a look at how they do that in this lesson and test your knowledge with a quiz.

Mutations

A lot of the time, we think of bacteria as harmful little creatures that cause infections. But sometimes they can really help us out! Would you believe that bacteria can help prevent us from getting cancer? Well, kind of. In this lesson, we'll learn about the Ames test, which is a test that uses bacteria as guinea pigs to determine whether a chemical can cause cancer.

First, we need to understand what mutations are. Mutations are changes to a DNA sequence. As you know, cells have tons of information stored as DNA sequences. The exact sequences are really important because they are basically the cell's instruction manual for life. They tell cells what to do, when and how.

Now, you may know of some superheroes in comics and movies that can get special powers by being mutants, but in real life, mutations are often bad news for cells. If mutations happen in the DNA, a cell's instructions can get messed up and certain things in the cell won't work properly. If many mutations happen, cells' normal functions can get so messed up that they begin to divide out of control, which can cause cancer.

Mutagens and Carcinogens

You may be asking yourself, 'how do mutations happen and how can I avoid them?' Mutations can happen spontaneously because of mistakes and errors that occasionally happen inside of cells. These mistakes are kind of like typos that can occur while the cell is making a copy of its DNA. Or they can be caused by radiation, such as x-rays and the ultraviolet light that comes from the sun.

Finally, they can be caused by mutagens. Mutagens are chemicals that cause changes in DNA sequences. They can do this in a variety of ways, and we don't need to go into the details here. If a cell encounters a mutagen, its DNA sequence will be changed at random locations throughout the genome.

Some genes are particularly important for a cell to behave itself and divide at a reasonable rate. If these genes get mutated, a cell can start to divide out of control, which can cause cancer. A substance that causes cancer is a carcinogen. Mutagens are often carcinogens because they can induce DNA sequence changes that lead to cancer.

The Ames Test

The bottom line is it is important to know whether the chemicals that we use in our food, cosmetics, medicines, household products and so on are safe for us. Are they mutagens? Can they cause cancer? To find out, we can use a special test called the Ames test. The Ames test uses Salmonella bacteria as guinea pigs to test whether a chemical is a mutagen.

Let's see how this works. To do the Ames test, you start off with a mutant strain of Salmonella bacteria. Wait a minute - the bacteria are already mutants? How the heck is that supposed to work? If they're already mutated, how can we tell if a chemical causes mutations? Here's the key: In the Ames test, we're actually testing whether a chemical can mutate the mutation back to normal. What?! That probably doesn't make sense yet, but stay with me here.

The mutant Salmonella bacteria that are used in the Ames test can't make their own histidine. Histidine (His) is an essential amino acid that the bacteria need to live. So being a so-called 'His' mutant would be kind of like you not knowing how to make your own cheeseburgers. That is, if cheeseburgers were essential for life, which is still open for debate.

In the lab, His mutant Salmonella can only grow on plates that have histidine added to them. Similarly, if you didn't know how to make your own cheeseburgers, you could only live in places where cheeseburgers were readily available to you. So you take these His mutant Salmonella and spread them on a plate that has no histidine. What do you expect will happen if you just leave these bacteria to grow? They won't grow, right? Because they are missing an essential nutrient that they need to survive.

Here's where your suspected mutagen comes in. You take your chemical that you want to test - let's call it Chemical X - and you mix it with some rat liver enzymes. This step is because sometimes harmless chemicals become mutagens after being processed in our bodies. Then you put Chemical X onto the plate where the His mutant Salmonella are. Then you incubate the plate overnight to see whether anything grows.

When you get your plate out of the incubator, there are two possible results. First, there could be no Salmonella colonies, or very few. We assume that each colony forms from one bacterial cell growing into a population that is big enough to see with the naked eye. Alternatively, there could be lots of colonies. How could there be lots of colonies of His mutant Salmonella on a plate without histidine? They're not supposed to grow without histidine.

Here's the thing: if Chemical X was a mutagen, it could have randomly caused a DNA sequence change that mutated the His gene back to normal. It mutated the mutant! Now the Salmonella can make histidine, so they can grow! This would be like if you suddenly got mutated and magically knew how to make cheeseburgers. Now you could live wherever you want - you'd be free at last!

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