MadSci Network: Microbiology

Re: Tooth brush cleaning

Date: Tue Jan 26 17:49:54 1999
Posted By: David Beck, MadSci Admin
Area of science: Microbiology
ID: 916193889.Mi

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 

Now what happens when you plate 1/10 of a milliliter of water of these 

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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