July 19th, 2018

Praxis: Scum, slime and science

By Mo Cranker on August 5, 2017.

I think it would be an understatement to say that this summer has been a hot one! Personally, I cannot recall so many hot days in a row. A sure sign of the heat is definitely the scum and slime I see on many of the local ponds as I drive by. What exactly is this biofilm? Well, let’s investigate further!

*Remember to ask an adult before doing this experiment.


– measuring cups

– missing spoon

– chicken broth

– dirt

– food colouring

– water

– long flat plastic container

– warm location

– several days to do the experiment

– science notebook and pen


1. Measure 250 mL (1 cup) of chicken broth and pour into the plastic container.

2. Take a pinch of dirt and sprinkle it evenly over the top of the chicken broth. Stir to distribute it evenly.

3. Place the container in a warm location.

4. Check your container every day and make your observations. Write these observations down in your science notebook. Make note of the temperature where the container is stored as well as anything else you may notice.

5. When does the biofilm begin to form on top of the liquid?

6. When the liquid begins to look cloudy, the biofilm is beginning to form. Note: this may take several days.

7. The day you observe the biofilm forming, dump the liquid out.

8. Rinse the container with water — do not scrub it.

9. Gently squeeze a few drops of food colouring into the container. Swirl it around.

10. Gently rinse out the container.

11. Observe what is left behind.

12. The food colouring will adhere (stick) to the biofilm that was formed.

13. Wash your hands with warm soapy water once you are finished this experiment.

What is going on?
You should have observed that the biofilm formed a ring around the inside of the container. This happens because bacteria forms in the liquid you have in the container. By adding the dirt to the chicken broth, you are introducing bacteria. As little as one teaspoon of dirt can contain between 100 million and 1 billion bacteria! You provided food for the bacteria in the form of the chicken broth. As the bacteria grow, they form a colony of bacteria living on the surface of the liquid in your container, feeding on the liquid below and ultimately forming the biofilm that you observe. I know it is kind of disgusting. Make sure you wash your hands well!

Patty Rooks is senior scientific consultant at PRAXIS, “Connecting Science To The Community.” Contact Praxis at praxis@praxismh.ca, http://www.praxismh.ca, Tweet or follow us @PraxisMedHat, or friend us on Facebook.

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