GEOSC 10
Geology of the National Parks

Evolution on a Plate

Evolution on a Plate

Start by watching this one-minute-and-fifty-five-second video showing the evolution of antibiotic resistance. The medicines that saved the lives of your grandparents from nasty diseases often are completely useless for you, because the disease organisms have evolved. As explained in the video, the researchers prepared a simple demonstration of this important process. They started bacteria growing in good food for bacteria, but with nearby bands of this food containing low and then higher concentrations of an antibiotic. The bacteria initially grew rapidly in the food without antibiotic, but as the bacteria spread, they encountered a low concentration of antibiotic. You will see the growth of the bacteria stop as the antibiotic killed them… until first one and then another cell evolved a defense against the antibiotic, allowing the offspring of those special new bacteria to grow rapidly into the antibiotic-bearing food. The process repeated as the bacteria encountered higher and still higher concentrations of antibiotic. In 11 days, the bacteria were rapidly growing in the highest concentration of antibiotic, and were essentially immune to that medicine. (The researchers did destroy these bacteria after the experiment, but dealing with similar processes involving bacteria, antibiotics, and people is a very important issue.)

The evolutionary changes shown in this video were very small, although obviously important. But, the time used in the experiment was a tiny, tiny fraction of the billions of years of life on Earth. Evolution is real, and knowledge of it is really useful. So, let’s go visit a fascinating National Monument, and get started on evolution.

Video: The Evolution of Bacteria on a “Mega-Plate” Petri Dish (Kishony Lab) (1:55)

Click here for a transcript of The Evolution of Bacteria video.

So what we ended up building was basically a Petri dish, except that it's 2 feet by 4 feet. And the way we set it up is that there are nine bands, and at the base of each of these bands we put a normal Petri-dish-thick agar with different amounts of antibiotic. On the outside there's no antibiotic; just in from that there's barely more than the E. coli can survive; inside of that, there's 10 times as much; a 100 times; and then finally the middle band has a thousand times as much antibiotic. And then across the top of it we pour some thin agar that bacteria can move around in. The background is black because there's ink in it, and the bacteria appear as white. First you see they spread in the area where there's no antibiotic up until the point that can no longer survive. Then a mutant appears on the right. It's resistant to the antibiotic. It spreads until it starts to compete with other mutants around it. When these mutants hit the next boundary, they too have to pause and develop new mutations to make it into 10 times as much antibiotic. And then you see the different mutants repeat this at 100, and after about 11 days, they finally make it into 1000 times as much antibiotic as the wild-type can survive. And so we can see by this process of accumulating successive mutations that bacteria which are normally sensitive to an antibiotic can evolve resistance to extremely high concentrations in a short period of time.

Credit: Harvard Medical School "The Evolution of Bacteria." YouTube. Sep 9. 2016.