Ask A Scientist Molecular Biology Archive

name         Josh R.
status       student
age          14

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?
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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
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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
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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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