Bacteria Growth Results
Country: United States
Date: January 2007
I am doing a science project on bacteria. I wanted
to see which was more effective -- Germ-X hand sanitizer or
Soft Soap Antibacterial Soap. I had my dad and brother put their
dirty fingers on two different blood agar plates and then had my
brother clean his hands with Germ-X and my dad with Soft Soap. They
then placed their clean hand onto another set of agars. I let the
agars grow for three days. My brothers clean hand had just a few
very large colonies of bacteria (about the size of a pencil eraser
maybe a little larger where my dad had a bunch of very tiny
My question is since there a like a million bacteria in
a colony whose hand was cleaner? Like I said, my brother's dish
had 5 rather large spots plus a large spot of fungus, my dad's had
all pinhead size bacteria colonies, but there were a lot of
them. He didn't have any fungus on his though.
There are so many factors that go into what populations of microbes we
carry, one simple experiment is difficult to quantitate. Your Dad and
brother have different microecosystems on their hands: different
biochemistries, pH, amount of oil, they have been in different places that
day and picked up different organisms. I would compare the amount of
bacteria before and after for each person and look for a decrease in number
between time 1 and time 2, not just after, using different soaps. Or, I
would have each person touch their left hand to one plate, their right to
another before washing and then wash each hand in a different soap and
repeat with touching the agar. Also, the different soaps might have
different chemicals which are active against different organisms, or have
different concentrations of the same ingredient.
It's a more complicated problem than it appears.
This experiment is not properly controlled. Once the dirty fingers have
been rubbed on a plate, the number of bacteria on the fingers has been
reduced so you don't know whether a reduced bacterial count is due to the
effect of the anti-bacterial agent or to the loss of bacteria caused by the
first rubbing. Another problem is that fingers cannot by reproducibly rubbed
on the plate in exactly the same manner. A controlled experiment would be to
thoroughly clean both hands of a person with soap and isopropyl alcohol,
then contaminate both hands by dipping them in contaminated water.
Then have another person clean one hand with antiseptic soap or wipe. Then
dip the clean hand in a bowl of sterile water and the other, uncleaned hand
in a separate bowl of sterile water. Then spread 1 mL from each bowl on
separate plates and compare the number of colonies formed on the two plates.
Ron Baker, Ph.D.
You started with a problem of "initial conditions". You don't know what
the initial conditions of your subjects were to start with. Your father's
"dirty" hands and your brother's "dirty" hands may have had very different
colonies even before you started the experiment. You don't know that. You
might want to revise the procedure. Have each rub each of their hands
together. This presumably will make the microbes on the left and right hands
the same. Then you wash one hand using latex gloves so you don't contaminate
the hand, and compare the left and right hand of each. This should average
out the possible differences in microbes each individual has. The next day
you might want to reverse left and right hand treatments to average out the
right vs. left hand difference. Comparing the cultures from subject A and
subject B is far more complicated because you can't define the microbial
history of A vs. B.
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Update: June 2012