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Name: Josh R.
Status: Student
Age: 14
Location: N/A
Country: N/A
Date: 2002


Question:
I am doing a science fair project. Which soap kills the most bacteria. I am using 4 different kinds of soap. I have swabbed my hand with a sterile swab and put it in NS solution, and used a syringe and dropped 5-6 drops on a blood agar plate. Next I took 15mlOf distilled water/15 ml of soap and put a half disk in the soap and then on one side of the plate. My question is I can not tell which one is best. If you turn the plate over you can see a dark area were the half disk was placed. Some disk are really dark and some not so dark . What does this dark area represent?



Replies:
It is hard to tell what you are doing form your description but (what is NS?). This would be my approach:

Control groups:

Control 1] sterile agar plate should be incubated [unused...uncontaminated] for the same periods of time as swabbed plates
Control 2] swab agar plate with sterile swab wetted with sterile water.
Control 3] wet sterile swab with sterile water, wipe unwashed hands with swab and streak the entire agar plate with swab

Experimental groups

Experimental group 1a] wet sterile swab and with the swab wipe both hands that were washed with soap 1, then streak the blood agar plate with this swab. Incubate at room temp and observe and record results ever 24 hrs for 5 days.

Experimental group 1b] wet sterile swab and with the swab wipe both hands that were washed with soap 2, then streak the blood agar plate with this swab. Incubate at room temp and observe and record results ever 24 hrs for 5 days.

Experimental group 1c] wet sterile swab and with the swab wipe both hands that were washed with soap 3, then streak the blood agar plate with this swab. Incubate at room temp and observe and record results ever 24 hrs for 5 days.

Experimental group 2a] wet sterile swab and with the swab wipe both hands that were washed with soap 1, then streak the nutrient agar plate with this swab. Incubate at room temp and observe and record results ever 24 hrs for 5 days.

Experimental group 2b] wet sterile swab and with the swab wipe both hands that were washed with soap 2, then streak the nutrient agar plate with this swab. Incubate at room temp and observe and record results ever 24 hrs for 5 days.

Experimental group 2c] wet sterile swab and with the swab wipe both hands that were washed with soap 3, then streak the nutrient agar plate with this swab. Incubate at room temp and observe and record results ever 24 hrs for 5 days.

Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy


I am assuming that when you said you put 5-6 drops of the solution from your hand on the plate, you spread the drops over the whole surface of the plate. When you put the soap discs on the plate you should see a "zone of inhibition" where the soap kills the bacteria. This is an area where the bacteria don't grow. This area may appear darker than the rest of the plate because no bacteria are growing there. Standard procedure is to measure the radius, from the disc to the edge of the zone of inhibition, in millimeters. The soap solution that has the largest zone of inhibition works the best at inhibiting the growth of bacteria.

vanhoeck


Hi Josh,
It is always difficult to interpret results like this without seeing it for real but I will try. You have tried to put bacteria from the surface of your hand onto an agarplate (with the swab and syringe) and then tried to kill the bacteria with soap, after they were already on the agar plate.You dipped the plate in water or soap, is that correct? By doing so, my fear is that the plate became too wet. Bacteria may not form nice round colonies on a wet agar plate, because their growth is spreading onto the surface and you can not recognize colonies. A colony is the offspring of one living bacterial cell. By counting colonies you can tell how many living bacteria there were. But if you don't get colonies but a spread growth (we call this confluent growth) there is nothing to count. The coloring on your plate could be such confluent growth, but it could also be a coloring of the agar contents due to the dipping in the soap.

I suggest you repeat the experiment in a slightly different way. I suggest two ways of doing it:

1. Before the experiment, rub your hands tightly. Wash 1 hand only in soap, the other with water. Then take a sterile swab from hand 1, put in NS, put the drops on an agar plate as you did before. take a sterile swab from hand 2 and do the same on a second plate. Comparing the nr. of colonies on the two plates will hopefully tell if soap or water was better killing the bacteria on your hands.

2. You can also take two sterile swabs of your hands, wet the sterile swab with either soap solution or water (best would be to drip a few drops on the swab but let it not get so wet that it drips off), let it work for 30 sec or so, and then dip the swab in your NS solution. Now you are testing the effect of soap or water on bacteria that are on the swab, not on your hands.

In fact, it would be interesting to do both and compare the results. Soap/water may not only kill bacteria, it may also release bacteria from your hand surface that were deep in the pores of your skin. In other words, it will be a surprise to see which of your plates has the most bacteria.

One other hint: don't let all bacteria grow on a small surface of your agar plate. If you put 5 drops with your syringe, tilt the plate so that it runs 'tears'. This increases the surface on which colonies can form and it will improve your counting.

Good luck with it!
Trudy Wassenaar



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