Exploring Our Fluid Earth

Bacteria are single-celled, small, simple organisms. They do not have specialized compartments (organelles) inside their cells. However, bacteria do perform essential roles in the environment, from decomposition in the soil to digestion in the human gut (Fig. 1.2 D; Fig. 1.5). Bacteria are often used to model life cycles and evolution because they go through many generations in a relatively short time. In this activity, you will model how a bacteria population is affected by exposure to an antibiotic.

Fig. 1.5. (A) Scanning electron microscope image of a disease-causing bacteria, Escherichia coli

Image courtesy of National Institute of Allergy and Infectious Diseases (NIAID)

Fig. 1.5. (B) Compound microscope image of blue-green bacteria or cyanobacteria

Image courtesy of Matthew J. Parker, Wikimedia Commons

Bacteria normally die when exposed to an antibiotic, such as penicillin. However, some bacteria have developed heritable traits that make them resistant to antibiotics. The primary cause of antibiotic resistance in bacteria is genetic mutation. When a mutation allows a bacterium to survive in the presence of an antibiotic, the surviving bacteria pass on their antibiotic resistance mutation to their offspring when they reproduce.

In this activity, the starting bacteria population includes (1) typical bacteria that die when exposed to antibiotics and (2) mutated bacteria that are antibiotic resistant. The mutated bacteria have a higher chance of survival when exposed to an antibiotic. The paper clips in this activity represent bacteria. The term phenotype is used to describe the physical traits displayed by an organism. All phenotypes are the expression of genetic information in an individual’s DNA molecules. The plastic-coated paper clips represent the typical bacteria phenotype, and the silver paper clips represent bacteria that have undergone a mutation that gives them antibiotic resistance.

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.

Materials

• Table 1.4
• Fig. 1.6
• Paper clips
  • 50 plastic-coated (typical bacteria)
  • 50 regular silver (mutated bacteria)
• Six-sided die
• Pens or pencils (two colors)

Fig. 1.6. Number of typical and mutated bacteria over generations

Procedure

1. Start with a population of 20 bacteria, 18 typical and two mutated. Record the starting bacteria population for both typical and mutated bacteria in Table 1.4 in the column labeled “At start of generation.”
    
2. 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.
   
   1. For typical bacteria, which have a 1-in-6 chance of surviving exposure to an antibiotic, survival and reproduction happen only when a 1 is rolled. Any other roll will lead to death (see Table 1.3).
   2. For mutated bacteria, which have a 5-in-6 chance of surviving exposure to an antibiotic, survival and reproduction occurs in rolls of 1–5. Death only occurs when a 6 is rolled (see Table 1.3).

3. Predict the number of typical and mutated bacteria that will constitute your population of bacteria at the end of five generations.
    
4. For each individual bacterium, roll the die.
   
   1. Determine if the bacterium survives by consulting Table 1.3.
   2. When a bacterium dies, remove it from the population by setting it aside.
   3. Record the number of bacteria that died after antibiotic treatment in the “Dead” column in Table 1.4.
   4. Record the number of bacteria that survived after antibiotic treatment in the “Survivors” column in Table 1.4.
       
5. The surviving bacteria reproduce. Bacteria divide in half when they reproduce, in a process called binary fission. Each surviving bacteria becomes two bacteria. In Table 1.4, use the number of survivors from generation 1 to calculate and record the total number of bacteria after each surviving bacteria reproduces in the “Reproduction” column in Table 1.4. (Hint: Multiply the number of surviving bacteria by two.)
    
6. Write the number of bacteria in your “Reproduction” column at the end of generation 1 in the column “At start of generation” for generation 2.
    
7. Repeat steps 2–4, filling in Table 1.4 for another four generations.
    
8. Graph your results for both typical and mutated bacteria in Fig. 1.6. Use the numbers in the “At start of generation” column. Use a different color for each type of bacteria.
    
9. Optional Part A: If there is time, run the experiment again.
    
10. Optional Part B: If there is time, continue to model the experiment through another five generations. Before you start, predict the number of typical and mutated bacteria that will be present at the end of ten generations.