Unit A: Continuity of Life

Why do penicillin and other antibiotics become less useful over time?

1. Get Set to Explore


  • antibiotics: Drugs people use to kill disease-causing bacteria.
  • bacterial fission: A process by which bacteria reproduce in which one cell splits into two cells.
  • evolve: To undergo genetic changes at the species level over time.
  • mutation: Any permanent change in a gene or a chromosome.
  • resistant: Able to fight off successfully.

Building Background

  • Ask students if they've ever taken antibiotics, reviewing the definition of the term if necessary. Elicit names of illnesses and diseases that antibiotics are used to fight.
  • Go over the definitions of the other vocabulary words with the class.
  • Make a KWL chart on the board and ask students what they already know about how bacteria become resistant to antibiotics. List the facts they state in the Know (K) column of the chart. Then have volunteers write what they want to learn about bacterial resistance to antibiotics in the What (W) column of the chart. Leave the chart on the board, so that the class can fill in the Learn (L) column later.
  • Present the Discover! question and direct students to formulate possible answers.

2. Guide the Exploration

  • Tell students to launch the Discover! Simulation. Explain that the circle is a drawing of a Petri dish that contains bacteria represented by dots.
  • Students should listen closely to the narration and take notes on what is happening in the simulation. When directed, they should click each of the labeled bottles to apply a particular antibiotic to the Petri dish. During the simulation, three different antibiotics will be applied.
  • Before students do Step 3 of the simulation, hold a class discussion and let students explain what the simulation shows. Guide students to realize that every time bacteria developed a mutation giving them resistance to the current antibiotic, the population of bacteria with that mutation grew. Return to the Discover! question and encourage students to refine their answers.

3. Review/Assess

  • Go over Step 3's Wrap-up text. Review how bacteria with mutations were able to survive the application of each antibiotic and let students describe how this affected the bacteria population. You may wish to underscore the idea that mutations happen by chance and that very few mutations are actually beneficial. The simulation focuses solely on mutations that confer resistance to antibiotics.
  • Encourage students to share observations. Let them fill in the L column of the KWL chart to summarize what they learned.
  • Hold a brainstorming session for students to answer the Extension question: What other adaptations could the bacteria develop by mutating? Following are some possible answers: Bacteria could become able to live in a different type of host; bacteria could adapt to live in environments with a different temperature or pH; bacteria could become resistant to antibodies released by the host; bacteria could adapt to live in different parts of the host. Accept any answers that show understanding of life processes.

If time permits, present children with the following questions:

  • Critical Thinking Evaluate Some scientists say that household use of antibacterial soaps does more harm than good. Why do they think this? Answer: In general, most people do not need to kill the common bacteria in their homes to remain healthy; so applying antibacterial soaps in the home only gives the bacteria more opportunity to adapt to the bacteria-killing substances. This means that these substances may not be effective when and where they actually are needed.
  • Inquiry Skill Predict What would happen if bacteria took a long time to reproduce rather than a short time? Answer: Since bacteria populations become resistant to antibiotics only when some bacteria pass on their ability to resist these antibiotics to their offspring, with a longer time span between generations, bacteria would still become resistant to antibiotics, but resistance wouldn't develop as quickly.

4. Reaching All Learners

On Level: Logical/Mathematical Learners

Encourage these students to make a line graph showing what happened to each of the different groups of bacteria during the course of the simulation. Students can represent bacterial populations using the same colors the simulation uses. They should note on the graph each time an antibiotic is applied. If possible, let students complete their graph before the class goes on to Step 3 of the simulation. Students can present their graphs in class discussion to reinforce the main ideas of the simulation. If class time is short, post the graphs on a bulletin board.