Understanding antibiotic resistance
Understanding antibiotic resistance

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Understanding antibiotic resistance

3.3 Experimentally evolving antibiotic resistance

Most animals evolve over millions of years. However, because bacteria grow and evolve so rapidly, scientists can study the evolution of antibiotic resistance in the laboratory. You will now look at an experiment which shows how bacteria adapt to survive increasingly higher doses of antibiotic (Baym et al., 2016).

Activity 4 Evolution in action

Allow about 30 minutes

Watch the following video taken from an episode of the BBC’s Horizon programme. Here, researchers from Harvard University and Technion-Israel Institute of Technology describe an experiment to evolve antibiotic-resistant bacteria in the laboratory.

Download this video clip.Video player: Video 4
Skip transcript: Video 4 Experimental evolution of antibiotic resistance.

Transcript: Video 4 Experimental evolution of antibiotic resistance.

NARRATOR
Scientists are now trying to understand exactly how superbugs have gained resistance, and ultimately how we can defeat them. Here at Harvard University scientists are investigating why some of our antibiotics are failing. It's an experiment that happens in Professor Roy Kishony's lab. Here they are deliberately trying to create superbugs.
ROY KISHONY
So this is a new device that we have developed. We call it the "morbidostat."
NARRATOR
Using the morbidostat they are going to produce a highly-resistant version of a harmless strain of a bacteria we all have in our gut, E. coli.
ROY KISHONY
In the beginning you have bacteria just going happily in the tube. They have enough food, so they're going fast.
NARRATOR
They start by trying to kill the E. coli by dripping in a low concentration of antibiotic. But as the millions of bacteria have been multiplying in the tubes, some, by chance, will have developed mutations that allow them to be resistant to the antibiotic.
ROY KISHONY
This mutant would start replicating faster than everyone else. Ultimately it would take over on the whole population.
NARRATOR
So now they try to kill this new mutant strain. They up the strength of the antibiotic. Again, most of them die. But a new mutation appears that can survive the even stronger antibiotic.
ROY KISHONY
And then see another step. Now we can go in even higher drug concentrations. So we keep iterating this process over and over and over.
NARRATOR
This experiment shows that bacteria become resistant by being exposed to low levels of the very thing we use to protect us – antibiotics. Now the team have created a new experiment to find out exactly what is happening in these mutant bacteria to allow them to be resistant. It starts with what is, in effect, a giant Petri dish.
ROY KISHONY
We're setting an experiment, really, for the first time in which we're going to let bacteria swim against an ever-increasing concentration of an antibiotic and see what happens.
NARRATOR
The jelly contains food for the bacteria to grow. But each slab is infused with an increasing concentration of antibiotic, which should act as a barrier, killing the bacteria.
ROY KISHONY
The first slab is no drug. Then about the amount that's needed to kill the bacteria. Then 10 times more, 100 times more, and 1000 times more.
NARRATOR
The experiment begins with a tiny drop of E. coli.
ROY KISHONY
They're certainly going to spread when there is no drug. But we want to see, can they actually go to the place where there is an antibiotic?
End transcript: Video 4 Experimental evolution of antibiotic resistance.
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Video 4 Experimental evolution of antibiotic resistance.
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In the next video you will watch what happens when bacteria grown on a plate containing increasing concentrations of antibiotics like the one described in Video 4. This plate is known as a Microbial-Evolution and Growth Area (MEGA) plate. Before you watch the next video, note down what you think will happen to the bacteria as they grow on the MEGA plate.

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Discussion

As bacteria grow to fill the area of the MEGA plate, with no antibiotic they begin to compete for resources, meeting one of Darwin’s conditions for evolution – a struggle for existence.

At this point, mutations occur which allow some bacteria to survive in the area of the plate containing antibiotic. These bacteria have a survival advantage over the antibiotic-sensitive bacteria and grow and reproduce to cover the area of the plate containing a low antibiotic dose. As they fill this area of the plate, they also begin to compete for resources and the cycle of mutation, selection and growth repeats.

Now click on the following link to watch a video showing a time-lapse recording of bacteria growing on the MEGA plate [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] . Then answer the questions below.

1 How many different mutants have reached the 1000× antibiotic concentration at the end of the experiment? (Hint: you will need to watch until the end of the video.)

Answer

Using the coloured tree diagram at the end of the video, we counted 16 different mutants that reached the 1000× antibiotic concentration at the end of the experiment.

2 Would you expect the first mutant bacteria that appear (those that occur at the no antibiotic:1× antibiotic boundary) to grow on the 1000× antibiotic region on the plate?

Answer

It is unlikely that the resistance mutations that allowed bacteria to survive on the 1× dose of antibiotic would be sufficient for bacteria to survive on the 1000× dose. It is more likely that bacteria would require multiple antibiotic resistance mutations to survive on the 1000× dose.

3 Did your predictions from the first part of this activity match the experimental results?

Discussion

You may not have exactly predicted what would happen in the experiment but you may have been able to make some suggestions about how bacteria acquire mutations in order to cross the no antibiotic:1× antibiotic boundary.

Now that you have watched the experiment, you may want to reread the discussion from the first part of this activity.

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