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Name: Erika
Status: Student
Grade:  6-8
Location: MD
Country: United States
Date: January 2007


Question:
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 colonies.

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.



Replies:
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.

vanhoeck


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.

Vince Calder



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