MadSci Network: Microbiology

Re: How can agar plates show you the germs on food?

Date: Thu Feb 24 15:11:17 2005
Posted By: Edward Richter, Faculty, Food Microbiology
Area of science: Microbiology
ID: 1107910187.Mi

Food is typically analyzed for microorganisms (bacteria, yeast & molds) 
by preparing an aqueous homogenate and then placing a portion of that 
homogenate (or dilution) of that homogenate into microbiological growth 
medium.  The agar plates are then incubated at various times and the 
subsequent colonies are counted.  This is a quantitative determination 
and can sometimes be qualitative by adding selective agents to the agar 
growth medium.  In your specific experiment, you need to measure by 
observation the difference between food which has fallen onto the floor 
and food that has not.  One fact that must be kept in mind is that the 
over riding variable in this experiment will be the microbiological 
quality of both the food and the floor.  If the food has many 
microorganisms in it naturally then dropping it on a floor with less 
microorganism will not add or remove organisms.

I would suggest dropping a food with no microorganisms such as a freshly 
popped pop corn (which has cooled to room temperature).  I would suggest 
a less than clean (moist if possible) floor.

After dropping a few pop corn pieces on the floor for different time 
intervals place them on the agar surface.  DO not forget to include on 
which did not drop oin the floor (as a control).

Dispose of the agar plates (appropriately) after incubating for 1-5 days 
at room temperature.

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