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Name: Emily W.
Status: Student
Age: 17
Location: N/A
Country: N/A
Date: September 2002


Question:
Hi again, I have already used this facility before and found it very useful. Now I have one more question. Now that I have found out how to cultivate my own skin bacteria, how do I 'count' or measure the amount of bacteria present. I have to do a comparative report so I need to compare growth of bacteria. How do i measure bacteria growth? Thank you again. You have already been a big help!



Replies:
Hi Emily,

The number of bacteria present in a sample are normally given as number of colony forming units, as each living bacterial cell can form a single colony. In order to count such colonies, you have to assure that the bacterial cells present in your sample are well spread over the agar plates on which they grow. That is normally done by making a suspension of your sample.

I suggest the following: take a sample of your skin bacteria with a swap and standardize the area of skin you sample. Dip the swap in a sterile solution of physiological salt solution (make this by adding 0.9 gram to 100 ml of water and sterilize this). Keep the volume in which you dissolve your swap as small as possible, and, again, standardize this, say exactly 1 ml.

Now take as exact as you can a sample to put on the agar plate. 50 or 100 microliter with a micropipette would be optimal, otherwise use drops from a glass pipette. With a spreader you spread this drop onto the complete surface of your agar plate. That would ensure even spread of all colony forming units over the plate. Incubate till you see colonies and count them. Calculate back how many colony forming units were on the skin area, taking into account your dilution factor in the salt solution.

Good luck with your experiment.
Trudy Wassenaar


Dear Emily,

The biggest trick to quantifying the amount of bacteria present is to determine the amount on a specific unit of skin surface area. Since you appear to already know how to cultivate bacteria from your skin, I would suggest measuring and marking specific areas of, perhaps, 1-2cm x 1-2cm (try several different sizes) on the skin surface. Then scrape the bacteria from these specific regions by whatever means you have used before and apply each to an individual culture plate. The bacterial colonies that grow, each one of which derives from a single bacterium, can then be counted and used to determine the number of bacteria per square cm of skin surface area. You will be able to generate easily countable numbers of bacterial colonies by varying the size of the skin surface area harvested.

Best of luck with your experiment,

Jeff Buzby, Ph.D.
Children's Hospital of Orange County




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