Microbes

Our body carry organisms that are too small to see. These creatures, called microbes, can be used to identify whether a suspect wore certain clothing or whether he or she was at the scene of a crime.

Materials

- ½ cup (125 ml) tap water
- saucepan
- 4 packages of unflavored gelatin
- timer
- clean 1-quart (1-liter) jar with lid
- trash can
- tennis shoes
- cotton swab
- rubber gloves
- dishwashing liquid
- adult helper

Procedure

NOTE: This is an outdoor activity and requires adult help.

1. Have your adult helper prepare the gelatin by boiling water in the saucepan and dissolving the gelatin packages in the boiling water.
2. Allow the gelatin to cool until it is enough to handle but not solid (about 5 minutes).
3. When the gelatin has cooled, pour it into the jar.
4. Hold the jar over the trash can. Tip the jar on its side and let the excess gelatin pour out.
5. Set the jar on its side and leave it undisturbed for 4 hours.
6. Put the tennis shoes on without socks and go play outside for at least 30 minutes.
7. Remove the shoes. Take the cotton swab and rub it between all your toes.
8. Reach into the jar and carefully brush the gelatin with the cotton swab so that you makle a wavy line in the gelatin as shown.
9. Wash your hands and feet.
10. Place the lid on the jar and put the jar in a warm, dark location. Leave it there for 4 days.
11. After 4 days, observe the gelatin in the jar. What do you see?
CAUTION: Do not touch the gelatin! Don’t keep the jar longer than four days.
12. When you are done with the experiment, put on the rubber gloves, fill the jar with hot water, let it soak for 5 minutes, and then wash the jar. The gelatin will dissolve and can be washed down the sink.
CAUTION: Be sure to wash your hands thoroughly when you are finished.

Explanation

The side of your shoes is warm, dark, and damp from sweat. This is a perfect environment for microbes to grow, as they like warm, dark, damp places. Your jar had a similar environment. The gelatin provided food for the microbes that were on your feet. Having food and a nice place to live allowed the microbes to eat and reproduce.

After four days, you should have been able to see grooves in the gelatin where the microbes had eaten it. Sometimes you might have seen different colors, depending on the type of microbe that was living on your feet and on the gelatin. If you could look at microbes under a microscope, you would see that they have unique characteristics, such as size, shape, color, and reaction to certain chemicals.

Microbes can be found not only on the inside of shoes, but on the soles of shoes and your skin as well. Forensic scientists compare microbes found at a crime scene to known microbes and to those found on a suspect. If the microbes match, they can be used to place the suspect at the crime scene.