Survival of the Fittest— Battling Bacteria

Setting the Scene

Bacteria have cell walls made of peptidoglycan, which is made of carbohydrates and proteins. Some bacteria have more proteins in their cell walls and some have more carbohydrates. The combination of protein and carbohydrates allows bacteria to survive in various environments.

The Experiment

The research team studying the bacteria hypothesized that the color of the bacteria’s cell wall and the strength of the cell wall are related. To test the color and cell wall strength hypothesis, your team will conduct an experiment in which a brown bacteria and an orange bacteria are crushed together.

You have 10 brown and 10 orange candies in your evolution materials so that you can perform the experiment below. Watch the video to see the procedure for red vs blue bacteria. 

 

 

Procedures

Make a chart as shown in the image to the right (or use the copy from your evolution materials. Select one brown and one orange bacteria from your O. glucosi containers and place them as a pair on the appropriate circles on Chart 1: O. glucosi before and after crushing. Repeat until all 10 pairs are be in place on the chart. To determine which bacteria has the stronger cell wall, pick up the first pair of BROWN and ORANGE bacteria. Stack one on top of the other as shown in in the video. Hold the two bacteria so that your thumbs are on the bottom surface and your index fingers are placed securely on the top. Evenly apply pressure to the top and bottom of the stack. As soon as one of the cell walls cracks, stop. Examine the two specimens and determined which one cracked first. (see the video for a demonstration) Indicate the survivor by coloring in the circle in the "Strongest Cell Wall" column with the appropriate colored pencil/marker (brown or orange). Place the uncrushed and crushed bacteria in the waste container. Repeat the above procedures until you have “crushed” all the pairings. Count the number of orange survivors to find the total. Count the number of brown survivors to find the total. Enter your data into the "Battling Bacteria Data" worksheet. Make sure you are on the correct sheet for your period. At the bottom of the data table you will find the total and average for whole class.

 

Questions - Answer in your notebook.

1. Evolution is a change in the allele frequency of a population over time. (Remember, and allele is just a version of a gene.) We started with equal numbers of brown and orange bacteria. Based on the definition of evolution, did our population of bacteria evolve during the course of this experiment?
2. Explain why it is important to use class results and not just the results obtained by an individual team.
3. Based on class results, are color and cell wall strength related? Support your answer using data. 

4. Many antibiotics work by limiting the bacteria's ability to build a strong cell wall. Based on class data, which color bacteria are most likely to survive a treatment of antibiotic so that they can go on to reproduce? Explain.
5. Assuming the trait for color and cell wall strength are genetic (meaning they can be passed from one generation to the next through reproduction), describe what you think will happen to the population of bacteria after 20 generations?