MadSci Network: Microbiology

Re: Growing bacteria

Area: Microbiology
Posted By: Eric Clambey, MadSci Admin
Date: Mon Apr 28 09:55:52 1997
Area of science: Microbiology
ID: 861989640.Mi
A couple thoughts on your experiments and then some suggestions for your 

Household products such as 409 and Lysol are classified as disinfectants.  
Disinfectants are chemicals used on non-living surfaces and objects to decrease
the number of microbes on that surface.  The effectiveness of a disinfectant
depends on a couple of things:  the length of exposure of the surface to the
disinfectant, and the concentration of the disinfectant (some household products
recommend that you actually dilute the product before using it).  Another 
important thing to realize is that some disinfectants actually kill microbes. 
These are called bactericidal (or microbicidal) agents.  Other disinfectants
inhibit the growth of bacteria.  These are called bacteriostatic agents.

Now that that has been said, here are a couple suggestions for your experiment:

1.  If you really want to test the effectiveness of Lysol, 409, etc,... be sure
to read the label to discover how they are supposed to be used.  Should you
leave the disinfectant on the surface of the object for a short while or for 
10 minutes?  Should you use the disinfectant at full strength or should you
make the disinfectant less strong (by diluting it in something like water)?  
Any household product should have these directions on it.

As for your actual experiment method, in a microbiology lab that I helped 
instruct we were interested in some of the same things that you were and this 
is how we did them. 

1.  For testing the effectiveness of lysol we took a tube full of lysol and 
combined that with a culture of already growing bacteria.  We then compared 
how the bacteria grew on an agar plate depending on how long the bacteria were
exposed to the lysol.  We did an agar plate where the bacteria were not exposed
to the lysol at all, an agar plate where the bacteria were exposed to the lysol
for 5 minutes and one for 10 minute lysol exposure.  

Unfortunately, this might be difficult for you to do since we used a bacterial 
culture that was already growing and therefore we used a lot of bacteria in the
experiment.  This is something you might not have access to.  

2.  A person can also take a small piece of paper in a circular shape and spray 
it with the household product of choice.  After swabbing the blood agar plate
with the swab of bacteria you can then carefully put the piece of paper directly
in the center of the blood agar plate.  Try not to handle the paper with your 
hand if possible.  You might use a tweezers to pick up the paper and put it on
the plate.  If the chemical either kills the bacteria or inhibits their growth
you should actually see a larger circle surrounding the paper disk where no 
bacteria grew.  The bacteria should have grown on the rest of the plate.  IT IS
VERY IMPORTANT that you make sure you swab the whole plate with your swab of 
bacteria!  If you do not this technique will not work very well, since this 
technique depends on the fact that there is bacteria spread across the entire 
blood agar plate.  

3.  In terms of your suggested experiment, where you wanted to spray the surface
and then swab it again, there might be a couple of problems.  If the 
disinfectant is bacteriostatic (it inhibits but does NOT kill bacteria) then 
once you swab the "disinfected" surface and put the bacteria on the plate, the
bacteria are no longer exposed to the chemical that is inhibiting their growth.
Because of this, your technique will only work if the disinfectant actually 
KILLS the bacteria.  Give it a try, but don't be surprised if your disinfectant
doesn't actually kill bacteria but just inhibits their growth.  

As for your second suggestion about how to do your experiment, I think that 
my suggestion number 2 is a variation of this.  I would swab out the bacteria
on the plate and then put the piece of paper with the disinfectant on it on 
the plate.  

One last thing.  Be careful not to touch things like the blood agar plate and 
the paper disc with your bare hands.  There are lots of bacteria on your hands!

Good luck and I hoped that helps.

Some experiments were taken from 
Johnson, T.R. & C.L. Case.  "Laboratory Experiments in Microbiology".  
     Brief Edition.  3rd Edition.  Benjamin/Cummings Publishing Company, Inc.

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