Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons
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Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.
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.
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 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!
So the plate that had no colonies or only a few did not contain a mutagen. And the plate that had lots of colonies contained a mutagen that reverted the mutation in the His gene back to normal. Now we know not to put that chemical into our food, cosmetics, medicines, household products and so on!
The Ames test relies on a major assumption. The assumption is, if a chemical can cause mutations in Salmonella, it could cause mutations and/or cancer in animals. This assumption is correct about 90% of the time. It makes sense because DNA in bacteria and animals has the same structure and function, so chemicals that can change its sequence in bacteria should be able to do the same thing in animals.
In this lesson, we've learned how bacteria can help us prevent cancer by serving as guinea pigs to test whether chemicals can cause mutations. We learned that mutations are changes to a DNA sequence, and they can mess up the normal functions of a cell and even cause cancer.
We learned that mutations can be caused in a few different ways. They can happen spontaneously because of mistakes and errors that happen occasionally inside of cells. They can also be caused by radiation, such as x-rays and UV light from the sun. Finally, they can be caused by chemicals called mutagens. Many mutagens are carcinogens, or substances that cause cancer.
We learned about how the Ames test works to test whether a chemical is a mutagen. The test uses Salmonella bacteria that have a mutation that prevents them from making their own histidine. They can't grow on plates that don't have histidine added to them unless a mutagen causes random DNA sequence changes that mutate the mutant gene back to normal.
So in an Ames test, when a lot of Salmonella colonies grow near a chemical, it's a very good indication that that chemical is a mutagen and can possibly cause cancer - definitely a good thing to know so that we can avoid being exposed to carcinogens as much as possible.
When this lesson is completed, you should be able to understand how the Ames test is used to test for mutated cells which could be carcinogens. You could gain additional knowledge about the substance that the Ames test uses and detail what happens during the testing process.
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Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons