Culturing and Counting Skin Bacteria
Name: Emily W.
Date: September 2002
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!
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
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
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.
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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Update: June 2012