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Modified from instructions developed by Susan Fleming for the Cell Lab at the Science Museum of Minnesota, 2001, a project supported by the National Science Foundation
In recent years there has been a huge increase in production of household goods that have antibacterial properties. This includes not just soaps and other cleansers, but also such things as sponges, cutting boards, and toys. But are all these products really necessary to keep the average person healthy? Do they work? And do these products pose a risk by artificially selecting for stronger strains of bacteria? For most people, washing their hands with soap and water is good enough to remove most pathogens from the skin. In terms of selecting for "super bugs", there is some evidence of this happening, but not everyone agrees on the degree of danger that this presents to the public. It is important to keep in mind that we share our world (and our bodies) with a lot of bacteria that are not harmful and that are sometimes necessary for us to function well.
This experiment tests how well soaps, hand gels, and cleansers kill bacteria. The bacterium used in this experiment is Bacillus megaterium ('big beast'), one of the largest bacteria known. It is a rod-shaped bacterium found in dirt and soil. B. megaterium will not make you sick but can cause 'wet wood' disease in elm trees. It is able to survive harsh conditions by forming spores, which protect it from cold, heat and dryness. Other bacteria can be used for this experiment, but it works much better with B. megaterium.
The indicator solution contains fluorescein diacetate (FDA). FDA is sometimes used in cancer research to detect the presence of live cancer cells in a culture. In this activity FDA shows the presence of live bacteria. In cells that are alive, enzymes (called esterases) convert the colorless FDA into a green fluorescent compound called fluorescein. This makes the cells glow green under an ultraviolet (UV) light. When cells are dead and the enzymes are not active, the FDA doesn't get changed to fluorescein and the solution remains colorless.
Important concepts and links to other topics:
Instructions for making indicator solution:
The concentration of fluorescein diacetate used in this experiment is 1mg/ml in acetone. To make 100ml of indicator solution, dissolve 100mg of FDA in 100ml of acetone.