99.99% Antibacterial Products and Natural Selection

Jennifer Welborn through NSTA, Science Scope
Type Category
Instructional Materials
Activity , Lesson/Lesson Plan
This resource, vetted by NSTA curators, is provided to teachers along with suggested modifications to make it more in line with the vision of the NGSS. While not considered to be “fully aligned,” the resources and expert recommendations provide teachers with concrete examples and expert guidance using the EQuIP rubric to adapted existing resources. Read more here.


This activity is a hands-on simulation using Skittles and mini-marshmallows to show how natural selection can act as a mechanism to increase the presence of antibacterial resistance in a population. Students simulate the effect of hand sanitizer on a population of bacteria, collect, record, graph and analyze their data. The bacteria that are affected by the selective pressure decrease and the population of bacteria evolves to be one that is largely populated by bacteria that are unaffected by the selective pressure. To begin the lesson students are given background information about natural selection and discuss how this can relate to their lives. Then students are given Skittles and mini-marshmallows and simulate the effect of hand sanitizer on bacteria. Since most of the “bacteria” are soft marshmallows they are caught more easily by a toothpick (the hand sanitizer) than the hard shelled Skittles. After a specified period of time the round ends and the remaining “bacteria” reproduce by fission for the next generation. The process continues for three generations. Possible follow-up lessons include watching videoclips and reading about antibiotic resistance having student role play scenarios to illustrate how antibiotic resistance happens in our world.

Intended Audience

Educational Level
  • Grade 8
  • Grade 7
  • Grade 6
  • Middle School
Access Restrictions

Available for purchase - The right to view, keep, and/or download material upon payment of a one-time fee.

Performance Expectations

MS-LS4-4 Construct an explanation based on evidence that describes how genetic variations of traits in a population increase some individuals’ probability of surviving and reproducing in a specific environment.

Clarification Statement: Emphasis is on using simple probability statements and proportional reasoning to construct explanations.

Assessment Boundary: none

This resource is explicitly designed to build towards this performance expectation.

Comments about Including the Performance Expectation
As written, the activity does not have students construct an explanation. The activity helps students gain an understanding of the Disciplinary Core Idea of natural selection for this Performance Expectation, but more needs to be done to have them construct an explanation. Writing a Darwinian Explanation (see the tip for Aligning to the Practice below for a more detailed explanation) or an explanation in the Claim/Evidence/Reasoning format would help students achieve this goal. The model presented seems to be absolute. The toothpicks (representing the hand sanitizer) are 100% effective against the marshmallows (normal bacteria) and 0% effective against the skittles (drug resistant bacteria). The result of this model is eventually having a population that is nothing but skittles (drug resistant bacteria) so that the entire population is 100% resistant to the toothpicks (hand sanitizer). If this were how things worked in real life, it would make no difference whether a person continued to take the full prescription of his or her antibiotics because the antibiotics (toothpicks) have no effect on the skittles. In real life, the antibiotics can still kill the drug-resistant bacteria, it just isn’t as effective as it is against normal bacteria. This difficulty with the activity could be turned into a strength if teachers noted the problem with the simulation and asked students how they could refine the simulation to better fit the actual phenomena. Another suggestion would be to use harder gumdrops like Dots instead of skittles. Gum drops can be grabbed with a toothpick, but it’s harder to grab a gumdrop with a toothpick than it is to grab a skittle. You could also use hard and soft gumdrops rather than marshmallows and skittles. This change would more closely model antibiotics resistant bacteria.

Science and Engineering Practices

This resource was not designed to build towards this science and engineering practice, but can be used to build towards it using the suggestions provided below.

Comments about Including the Science and Engineering Practice
The multiple choice homework questions ask students to explain, but they are not a fully constructed scientific explanation. One way to have students fully address the practice would be to have them write a Darwinian Explanation that describes the variation in the population at the start of the simulation, the selective advantage, which variation is more likely to reproduce and pass on their genes and what the variations are like in the end population. An example would be: There were more marshmallow bacteria at the beginning of the activity than Skittles bacteria. When the hand sanitizer was added most of the marshmallow bacteria died and the Skittles survived. Since they were alive they could reproduce and make offspring that were Skittles bacteria. Over generations there were mostly Skittles bacteria in the population although a few marshmallow bacteria were present. An alternate way to formulate an explanation is to have students make a claim, back it with evidence from the simulation and then state reasoning that ties the evidence to the claim.

Disciplinary Core Ideas

This resource is explicitly designed to build towards this disciplinary core idea.

Comments about Including the Disciplinary Core Idea
You may want to include more questions about harmful, beneficial and neutral traits. Writing a scientific explanation with the tips mentioned above would be very helpful to students to tie the practice of constructing an explanation using models or representations to the disciplinary core idea of natural selection.

Crosscutting Concepts

This resource appears to be designed to build towards this crosscutting concept, though the resource developer has not explicitly stated so.

Comments about Including the Crosscutting Concept
The addition of hand sanitizer (cause) led to the increase in the population of “skittles” bacteria (effect). The selective advantage influences the effect of the cause since a hard shell on the “skittles” bacteria was responsible for their survival while the soft outside of the marshmallow influenced the number that were killed by the hand sanitizer. A teacher could add questions explicitly asking about cause and effect in the simulation. This could include "The hand sanitizer caused what type of effect on the variation in the population of bacteria?"

Resource Quality

  • Alignment to the Dimensions of the NGSS: The two disciplinary core ideas of natural selection and adaptation are addressed. More can be done for students to help them construct explanations based on evidence. The crosscutting concepts of cause and effect and structure and function could be made more explicit.

  • Instructional Supports: Having students write Darwinian Explanations or an explanation in the Claim/Evidence/Reasoning format may help them to see the connection between what they did in the simulation and what happens in real life more clearly. Questions about the patterns and cause and effect could help students tie these crosscutting concepts into what they did in class. Also, ELLs and students who read below grade level will find the homework questions too difficult. Many of the questions are too long and are of a much higher reading level. Sentence starters and sentence frames would be of great benefit.

  • Monitoring Student Progress: There is a formative assessment question that is designed to assess understanding of natural selection. It includes distractors that highlight common student misconceptions. Student responses to this question help the teacher to identify areas to target instruction. The summative-assessment question is designed so that students apply what they have learned about evolution by natural selection to a current phenomenon: the increasing numbers of tuskless elephants. The practice of constructing an explanation is not explicit. The question should be expanded to have students include the practice of writing an explanation in the form of a Darwinian Explanation or an explanation in the Claim/Evidence/Reasoning format. There is also a remedial activity that helps students who having trouble understanding the concept.

  • Quality of Technological Interactivity: - none -