Mold
Mapping deaths and thinking like an epidemiologist is helpful in understanding the spread of disease. However, grasping the concept of invisible organisms is difficult. “Germ” is still an abstraction. In this exploration, students find evidence for microorganisms by using bread to make "germs" visible. Students can see indirect evidence of microorganisms by growing them on unpreserved bread.
Materials Required
• Plastic baggies
• Bread, ideally without preservatives
• Warm place
Procedure
1. Cut enough small squares of bread so the student has at least four pieces. Keep the bread away from contamination in a folded napkin until ready for use.
2. “Look” for microorganisms around the room or house by moistening the bread with a few drops of clear water and then wiping the bread on a surface such as a sink, floor, shoe, etc. Do this with three pieces of bread. Keep one piece of bread untouched.
3. Place each bread piece in a separate baggie. Label each baggie with the name of the collector and the location where the bread was wiped. Label the untouched piece as “control.”
4. Place the baggies in a warm place.
5. Examine the bread after several days of growth. Describe the findings. Make drawings or take photos.
Discussion
Dr. Snow’s belief in germ theory was not a popular view. Most people thought that life could arise from nonliving sources and that disease was more about catching an evil air or demon than becoming infected with a microbe. It was common to believe, for example, that if one placed rice or wheat grains in an old rag and allowed it to sit in a dark corner for 21 days, mice would appear. There were specific "rules" that dictated these events. Dew on plants would give rise to aphids. Garbage would produce fleas and flies. Rotting logs would transform into crocodiles and clumps of mud into frogs. So strong were the ideas about the abiotic origin of life that it took a great deal of proof to put that belief to rest. Many simple experiments helped disprove the ideas of spontaneous generation. In 1668, Francesco Redi, an Italian physician, observed that when meat is left in the open, maggots develop. It was believed that the maggots spontaneously developed from the meat. To disprove this, he conducted a simple experiment. He put meat in three conditions: one was a closed container, one had cheesecloth covering the meat and the third had the meat open to the air. The meat samples in the closed container and covered with cheesecloth were maggot free, while the meat in the open developed maggots. For the most part, this kind of experiment could be used to prove the idea that visible living things had their origins only from previous living things of the same sorts.
Extension
The experiments of Redi, Spallanzani and Pasteur can be repeated. Students can design their own controls for testing the idea of spontaneous generation of life in different media.
A good summary of the Redi experiment can be seen here: