Bacterial Evolution

In this activity, you will model how a bacteria population is affected by exposure to an antibiotic. Bacteria are single-celled, small, simple organisms and are often used to model life cycles and evolution because they go through many generations in a relatively short period of time. Normally, bacteria die when exposed to an antibiotic, such as penicillin. However, sometimes bacteria develop mutations that make them resistant to antibiotics. The bacteria with beneficial mutations will survive longer and reproduce more offspring (ie. Natural Selection!) We will use different colored paper clips to represent the two types of bacteria. Typical bacteria (plastic-coated paper clips) will die when exposed to antibiotics. Mutated bacteria (silver paper clips) are antibiotic resistant. Model Assumptions Typical bacteria have a 1-in-6 chance of surviving exposure to an antibiotic. Mutated bacteria have a 5-in-6 chance of surviving exposure to an antibiotic. Both typical and mutated bacteria produce offspring of the same type. This means that typical bacteria will produce typical bacteria and mutated bacteria will produce mutated bacteria. Procedure Start with a population of 20 bacteria (18 typical and 2 mutated). Record the starting bacteria population for both typical and mutated bacteria in Table 1.3 in the column labeled At start of generation. The entire population of bacteria will be exposed to an antibiotic. You will simulate this event by rolling the die for each individual bacterium (paper clip) to see if the bacterium survives antibiotic treatment. For typical bacteria, survival and reproduction happen only when a 1 is rolled. Any other roll (2-6) will result in death. For mutated bacteria, survival and reproduction occurs when 1-5 is rolled. Death only occurs when a 6 is rolled. Begin the experiment. For each individual bacterium, roll the die. Determine if the bacterium survives by consulting Table 1.2. When a bacterium dies, remove it from the population by setting it aside. Record the number of bacteria that died after antibiotic treatment in the Dead column in Table 1.3. Record the number of bacteria that survived after antibiotic treatment in the Survivors column in Table 1.3. The surviving bacteria reproduce. Bacteria divide in half when they reproduce, so each surviving bacterium becomes two bacteria. In the Reproduction column for each generation, use the number of survivors to calculate and record the total number of bacteria after each surviving bacterium reproduces (Hint: Multiply the number of surviving bacteria by two.) Copy the number of bacteria in your Reproduction column at the end of generation 1 into the column At start of generation for generation 2. Repeat steps 24, filling in Table 1.3 for another four generations.