|MadSci Network: Microbiology|
The easiest way to count bacteria is to do exactly what you are doing. What will happen is that the bacteria will grow up overnight, or more likelly in a couple of days and form colonies on the plate. Each colony is started by one bacteria, and so by counting the number of colonies you know how many bacteria there are. So what happens when there are too many colonies for you to count? The best thing to do, after you have done the experiment once to see what happens, is to plate dilutions. You can dilute the bacteria by 10 fold dilutions, by taking one milliliter of the water from the toothbrush and adding it to 9 milliliters of water. You would then plate some of that dilution. It may be necassary to make several dilutions before it is dilute enough for you to count. Then depending on which dilution is countable you add that many zeroes after the number you get with that dilution. How this works. Let's say you put your toothbrush in 10 mls of water. Mix it arround to make sure it is good and mixed, and then plate 1/10 of a ml of that. You have plated then only 1 in 100 of the toothbrush-water, so what ever number you get you have to multiple by 100. Now if that gave you too many bacteria, so then you repeat the experiment over again. This time you plate additional dilutions. You make say a series of three diltutions, by taking one milliliter of toothbrush-water and adding it to nine milliliters of fresh water. This is called the 10-1 dilution. Then you take one milliliter of the 10-1 dilution and add it to 9 milliliters of fresh water. This is called the 10-2 dilution, and so on, until you think you have enough dilutions. Now what happens when you plate 1/10 of a milliliter of water of these dilutions? toothbrush-water 1/10 100 10-1 1/10 1,000 10-2 1/10 10,000 10-3 1/10 100,000 10-4 1/10 1,000,000 10-5 1/10 10,000,000 This is what happens when you plate 1/10 of a milliliter. NOTE: be careful about your water source. If your water is not sterile then there will be a certain number of bacteria in the water that will grow on the plate. It is best to use sterile water. Best Wishes, David Beck
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