Testing the levels of bacteria in common items is an interesting, if gross, experiment to undertake. Students typically grow the bacteria on agar, a gel-like substance that feeds the germs. Taking a few simple steps gives those microbes their best chance to grow on the agar, making the experiment more successful.
Type of Agar
Several types of agar exist, and most of them will grow bacteria. Some of these varieties, though, are dangerous for student use, and some are not optimal for growth of bacteria though they may grow other microbes. According to Science Buddies, the best type of agar to use is a nutrient agar, like an LB agar, which will not grow one type of bacteria over another. If you purchase a bacteria growth kit for your science experiment, you may find that you don't have a choice of agar but the company will use one that is effective but safe.
In order to continue to be effective, the agar must stay relatively moist. If you leave it out in the open, not only will this create a potentially dangerous situation, it will dry the agar out. Instead, carefully close the lid on the petri dish and place the dish in a clear, sealed plastic bag. This will allow you to view the bacteria growth, with an extra layer of contamination-protection.
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Bacteria prefer to grow in warm areas. Science Buddies states that the optimal temperature for growth is around 90 degrees Fahrenheit, but Steve Spangler Science warns not to go over 98 degrees. If you're not in the middle of a hot summer, you'll need an incubator to keep the petri dishes at the right temperature. If you don't have a laboratory incubator, you can build a makeshift one by putting a small lamp with a 75-watt bulb in an aquarium with a plastic covering on top.
Bacteria are tiny cells, invisible to the naked eye. When you're growing bacteria, the colonies you see are actually millions of cells grouped together. Though you might begin to see the growth after a day or two, some types may need more time. For the best results, you need to give the bacteria time to grow. If you're not satisfied with your results after two days, let it sit in the incubator for a few more days to see if this spurs growth. If not, you may have an example of something in the home that does not have a large amount of bacteria. Repeat the experiment with that item to see if you get the same results.