BioFilm Experiment

While studying microbiology might not be on the top of your middle school or high school kid’s to-do list, you can entice them into the topic by talking about controlling the growth of microbes in swimming pools, or even closer to home, on the surface of their teeth! Colgate even has an article about biofilms!

Essentially biofilms are microscopic bacteria that adhere and grow on a surface. This You Tube video does a good job of explaining how biofilms form. I’ve designed this lab to have two parts. The first component is an engineering challenge where your students will design an apparatus to collect biofilm from a body of water. Then, they will coat plastic surfaces with different substances in an attempt to decrease the amount of biofilm growth. Two weeks later, they complete the biology portion of the lab when they collect their specimens and determine which coated surfaces controlled the bacterial growth best. Engineering & scientific process skills students will practice while conducting this lab.

• Designing an apparatus to collect bacteria
• Developing a research design to test an idea
• Predict outcome; make a hypothesis
• Collect data: categorizing results to make comparison easier
• Organizing data: constructing bar graphs
• Describing and drawing observations
• Verifying predictions
• Determining reasons for results
• Extrapolate results for what it may mean for society, and what additional experiments could follow

You will need to find a body of water in which to place the apparatus your kids will make. The apparatus need to remain submersed for two full weeks, so seek permission if you are going onto private property. If you don’t have pond water easily  accessible, you can recreate your own pond. This is what we did. I placed two shovels full of dirt from a bare patch in the lawn into a large plastic garbage can and then filled it with water. You need to let this sit for at least a week before starting your experiment, so the organisms in the soil have time to take up housekeeping in their new home! Part 1: Designing the biofilm apparatus What you’ll need:

• Clean, empty soda bottles (one for each child or pair)
• Single hole punch
• Scissors
• Twine or rope
• rubber bands
• weights (rocks or pebbles)
• Large plastic jug
• Substances to coat the plastic such as; paint, oil, peppers, cayenne pepper, etc…
• foam brushes
• Other items requested by your kids!

PreLab Discussion To ensure a high-level inquiry in this lab, it is important that you allow your kids to take the lead. Read with them the first page of the lab, as seen below. After your kids understand the goal and the task, work with them as they brainstorm ideas for their apparatus. While they can refer to the ladder graphic in their lab, they should not be limited, or boxed in by it. If they have another idea that they’d like to try, let them. I’m hesitant to post my photos, in fear you’ll lead your own kids to replicate the apparatus that my kids constructed. Therefore, as you plan for this lab, plan more for the discussion you’ll have with them rather then organizing the materials. After the discussion and brainstorming, your kids can help gather and obtain the materials before they begin building. Here are few a questions you may consider asking while they construct their bacteria collection apparatus.

• How might the depth of the pond where we are placing the appartus change our design?
• What size do you think would be appropriate?
• How do you think the space between the steps of the ladder might influence how well the bacteria grows on the surface?
• How important is it to keep the steps of the ladder the same size?
• Will your rungs stay separated when they are placed in water? Maybe consider a trial to see how it behaves submersed.
• What’s the best way to keep the ladder stable, and less likely to be influenced by any water currents?
• How should you attach it to the large plastic jug?
• What sort of surface coatings might microbes be more likely stick? Less likely?

Once they’ve spent some time thinking about design, let them dig right in. It is normal for the design to change as they work. My best piece of (friendly) advice is to keep your mouth shut. Your kids will most likely ask direct questions, seeking your guidance, however, answer their probes with more questions! Show restraint. It’s hard, especially if you have an idea that will help their design. But it’s not your project, it’s theirs. So what if they have a design that doesn’t collect as much bacteria as the one you could have built, allow them to take ownership of their apparatus. Once your kids have constructed the apparatus, they must decide with what substances they will coat the ladder rungs. Again, it’s best if you don’t have materials sitting out (a great advantage for the homeschooling teacher). Instead have them brainstorm, do a little research, and determine which substances they believe will decrease bacterial growth. They should not coat the 5th, or top rung with any substance so it will act as the control. As your kids coat the surfaces of their ladder, also have them complete the table where they predict which rungs will have the most and least amount of bacterial growth. They will need to include a weight at the bottom and a way to attach it to the plastic jug near the surface of the water. Before setting the apparatus into the water, they should write a formal hypothesis predicting what substance will inhibit the bacterial growth the best. Part 2: Microbiology After two weeks are up, retrieve the apparatus, being as gentle as you can, and keeping the ladder wet if the transport is further than just a few feet. We had great success in the growing of biofilm, not only on our ladders, but also on our jugs. Here are our apparatus in our sink waiting for observations. I had two groups’ ladders attached to each jug, and they were a tangled mess after two weeks. If at all possible keep the ladders separate. You may want to have students wear gloves while handling the biofilm apparatus. Students should be gentle when handling the biofilm apparatus as they want to preserve the state in which the bacteria was living when it was in the “pond.” One of the first tasks students are asked to complete is to compare the amount of biofilm growth on each rung. The directions say to record the top (control) rung as a “FIVE.” This allows them to rate each rung as more or less than the control. My kids had some above and some below the control. This is categorizing qualitative data in a way that makes it easier to compare. By looking at the graph, you should be able to tell which rungs your kids thought had the most, least, and which ones had similar amounts of growth. But this graph won’t account for color differences, thickness or other aspects of the bacterial growth. Hopefully as your kids graph this data, they will come to this conclusion themselves. Depending on the level of your student, you can choose how detailed they should be recording their qualitative data. I had my students draw lab drawing of each rung, and then a microscope drawing as well. But the drawings ended up all looking the same. What is more important is the description of bacterial growth. You can never emphasize enough the importance of making observations with emerging investigators. Observations include all the senses not just sight. Encourage your kids to really use the “English-y” side of their brain when writing what they observe.

As with all my labs, I have post-lab questions that help students reinforce the scientific method process as they look back over their lab experience. There’s questions on extraneous variables (other outside influences that may have affected the results), a question on how confident they are that the results are reliable. I also want student to really think about why this matters. Can we control bacterial growth by changing or modifying a surface? Does painting help? What sort of industries might be affected by bacterial growth on water surfaces? Biofilm.org may help students make some of these applications.

Some student groups had more obvious differences in the amount of biofilm growth on their rungs. This is a great experiment to perform more than once. Allow students to tweak their ladders, application method etc… until they are confident that it is the substance they’ve added to the rung that inhibits bacteria growth, and not some extraneous variable.

I’d like to close with the warning to not let students complete this microbe lab with an “icky” attitude toward microbes. Bacteria get a bad wrap, but are absolutely essential to human existence. To hound this point home, one of my favorite TED talks is by Dr. Bonnie Bassler. She talks about the importance of bacteria, and gets a little into how they communicate with one another. The kids will like the first 2 minutes, you’ll enjoy all of it!  My favorite quote from this TED talk is, “While you think of yourself as human beings, I think of you as 99 percent bacterial.”

This lab was modified from an activity titled, Defend Your Surface from the book; Meet the Microbesthrough the MicrobeWorld Activities pg. 39-43.

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Darci is a traditionally trained science (and English) high school teacher, having spent 16 years in public education, but is now a new homeschooling mom. Darci’s passion is helping kids of all ages learn about the process of science, rather than the facts.She provides professional development for teachers wanting to implement student research,and is considering offering research experience for high schoolers in the online environment, who otherwise might not get the opportunity.