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Microevolution and Antibiotic-Resistant Bacteria

The discovery and mass production of antibiotics is one of the greatest achievements of biotechnology. However, the widespread use of antibiotics has had an unintended consequence; the evolution of antibiotic-resistant bacteria, or bacteria that can no longer be killed by a specific antibiotic. If a person is infected by these bacteria, several different types of antibiotics may need to be tried before the infection is cleared up-if it can be treated at all.

Antibiotic-resistant bacteria have evolved through natural selection because of pressures that humans have out on bacterial populations. In a bacterial population, a bacterium may possess a mutation that makes it resistant to an anti-biotic. In the presence of the antibiotic, this bacterium has an advantage. While the others are killed, the antibiotic-resistant bacteria is generated.

In this exercise you will work with a model bacterial population. You will examine how the population changes over time and in response to environmental pressures.


How do environmental pressures and mutations affect a population of bacteria?


  1. Count out 20 white chips. This group of chips represents a bacterial population; each chip is an individual bacterium. The white chips are the non-resistant bacteria--they are all susceptible to being killed by an antibiotic A. The blue colored chips represent antibiotic-resistant bacteria-- those that are not killed by antibiotic A.

  1. Roll the two number cubes and add the resulting numbers together. For example, if you roll a 4 and a 3, then your total number is 7. Look up the total in the model key in Table 1 and find the corresponding event description and effect on the population.


Sum of Number Cubes

Event description

Effect on Population

4, 5, or 6

Nutrients are plentiful

The population doubles.


Nutrients are low

If there are fewer than 10 bacteria, the population is not affected. If there are more than 10, two-thirds of the population dies.


The bacteria come into contact with a cleaning product

Ninety percent of the bacteria die.


Antibiotic A is put into the environment

All non-resistant bacteria in the colony are killed.


One bacterium mutates to that it is resistant to Antibiotic A

One non-resistant bacterium (white) is replaced by an antibiotic-resistant bacterium (colored)


An antibiotic A-resistant bacterium in the population shares DNA with 10% of the other bacteria.

If there are antibiotic resistant bacteria in your population, 10% of the non-resistant bacteria are converted to anti-biotic-resistant bacteria.

7,8,or 9

No change in the environment

No change occurs in the population.

  1. Figure out how you would change your population to reflect the event. For example, if a bacterium is “killed,” you would remove it from your population. If one bacterium develops antibiotic resistance, you would exchange it for a colored chip. Always round the number of bacteria that are affected by an event to the nearest whole number. For example, if you have 20 bacteria in your population and two-thirds of them due, that would mean 13.3 die. You would then round the number to 13 and remove 13 chips from your population.

  1. Record your results in Table 2, describing the event and the effect on your population.

  1. Continue to roll the number cubes and change your population accordingly. For each roll, fill in the rows in Table 2 describing the event and modifications to your population. Stop after 20 rolls. If your population is wiped out before 20 rolls of the number cubes, do the activity again. Keep track of your population as you did before.

  1. Total the number of bacteria in your final population and determine what percentage is resistant of antibiotic A. For example, if you have 20 chips and 2 of them are blue colored, 2/20 or 10% of the population is resistant to antibiotic A.

  1. Add your data  this form number of chips (bacteria) per 1, 5, 10, 15, 20 generation numbers.


Table 2. Number of Individual Types per Generation

Roll of Number of Cubes (Generation)

Number of Cube Total


Number of White Chips

Number of Blue Colored Chips





















Analysis & Conclusion Questions:

  1. What changes occurred in your bacteria population? Would you expect that all the same changes happened to your classmates’ bacteria?

  1. Compare the % of your population resistant to antibiotic-A to your classmates.  What reason could explain the differences between % resistant?  

                % resistant =  (number resistant bacteria/total number of bacteria) x 100

  1. Imagine that all of the classrooms populations were grouped into one large                      population. What percentage of this population would have antibiotic-resistant bacteria?

  1.  Antibiotics are often found in the environment- in the soil, for example. How might antibiotics be introduced  into the environment?

  1. Misuse of antibiotics can encourage development of resistant bacteria. Misuse includes taking antibiotics when one is not sick, taking them for viral infections (they kill only bacteria, not viruses), and not taking them for a long as prescription says to. Knowing all this, what do you think the public can do to protect against the development and spread of antibiotic-resistant bacteria?

  1. Unfortunately, the ability of bacteria to become resistant to antibiotics is not a pretend issue.  Read this article from The Atlantic about a new bacterial strain resistant to nearly all antibiotics and answer the following questions (A.S. Questions):
  1. How does this situation represent natural selection?
  2. Why is this situation such a potential problem?

  1. IBM has recently helped develop hydrogels to fight drug-resistant superbugs.  Read the article found here.  Does this provide a fix to the problem of evolving bacteria resistant to antibiotics?

  1. Using the class data for the activity, found here, create a graph to present the average change of data over the course of the generations.  Error bars are not necessary for this activity.  Print and staple your graph to this packet for turn-in.


Classroom Skills

Exceeds Proficiency

Meets Proficiency

Not Proficient

Data Collection & Processing: Aspect 3

Exceeds Proficiency

Meets Proficiency

Not Proficient