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Run your tongue over your teeth. If it's been a while since you last brushed, you may feel a filmy or fuzzy coating on your teeth. What's there is similar to the slimy coating you might feel if you stuck your finger down into a sink drain or that you might see coating the sides and bottom of a swimming pool that hasn't been kept clean. These are all examples of biofilms.
Biofilms are communities of microbes. They form when bacterial cells able to make large amounts of sticky, slimy substances called polysaccharides (polly-sack-uh-rides) attach themselves to a surface. The slimy coating they make holds the cells to the surface they've settled on and captures other bacteria who live and grow off the waste products produced by the first bacteria. Layers keep being added, creating a complex community.
In this activity, you'll explore whether biofilms can form on all possible surfaces and which they grow best on. You'll also learn something about how we can prevent biofilm formation.
Print out these pages and follow the directions to do this activity at home. When you're done, come back to this page to test your newfound knowledge by answering the questions below. (No fair peeking at the answers before you do the activity!)
Note: This is a long-term activity that will stretch over a several days.
Note: Be careful handling sharp scissors.
1. Create an artificial pond: Fill your large bucket with tap water, place it on newspaper and let it sit for 24 hours. Pour 2 cups of dirt or mud into the bucket and stir in. Make sure the dirt is as free of chemicals as possible (e.g. don't use dirt from a garden treated with weed or insect killers). Or if you have a stream or pond nearby, you could just fill a bucket with water and dirt from there. Be very careful around bodies of water that are or may be deep. Fill your bucket near the edge and don't go into the water. Wash your hands when done.
2. Using the scissors, cut the top and bottom off your soda bottle about 5 cm from each end. Throw away the top and bottom.
The remaining part of the bottle looks like a cylinder. Cut through it from top to bottom to create a rectangular piece.
Flatten the piece and then cut into 5 equal strips.
Punch a hole into each end of each strip.
3. Set one strip aside. Spread newspaper or paper towels on a table. Working over the newspaper, coat each of the other 4 strips, front and back, with a different coating. Use only one coating substance for each strip. Set them on the newspaper and let them dry (drying time will vary for each coating substance used).
4. Arrange your 5 strips in a row so that they are parallel. Thread a piece of string through the hole in one end of one strip and pull it through, leaving about about 7 cm hanging. Tie the strip to the string. Thread the long end of the string through one hole of the next strip and tie it off about 3 cm from the first strip.
Continue to thread the strips onto the string, keeping them 3 cm apart. Using your second string, repeat this process on the other side of the the 5 strips. Tie whatever you're using as weights (washers, fishing weights, stones) to the 7 cm ends of the strings. Tie the other ends of your strings together. Your setup should look like the picture here.
5. Hang your setup from the stick or dowel and lay the stick across the top of your artificial pond so that the setup hangs down into the water. All strips should be submerged, but they shouldn't be resting on the bottom. Wash your hands. Leave the bucket and setup undisturbed for two weeks (check occasionally to make sure your top strips are still covered by water).
6. Working carefully around your setup so you don't jostle it badly, remove a few cups of water from your artificial pond and pour into your shallow pan or small bucket placed on newspaper. Carefully move your setup to this pan or bucket so that it doesn't dry out while you're working with it.
You'll likely see green, brown, pink or reddish spots. Rate the amount of growth on each strip on a scale of 1 to 5 (1 being the least growth). Give the uncoated strip a 3 rating and rate the other strips compared to it. Create a bar graph with the different coatings labeled on the horizontal axis and the growth ratings on the vertical axis. Wash your hands after touching strips.
If you want more details about biofilms and to see an animation showing how biofilms form, visit this Web site.
This experiment is based on an activity developed by the National Association of Biology Teachers.