Re: How long should I keep the petri dishes for my science experiment?

Iris,

Very good question.  Most likely, you will have a number of organisms 
growing on your agar plate inlcuding molds, and fungi.  Initially, the 
bacteria will begin to grow, because they tend to grow faster than molds 
or fungi. However, the longer you keep the plates, the more opportunity 
for the other organisms to begin growing.  You probably started with a 
nutrient rich agar plate -- which means many things will grow on it. So 
the answer depends upon what you want to do with the plate.  If you just 
want to look at differences in bacterial growth you only need the plates 
for a couple of days.  If you are looking to see what else grows, you can 
keep them longer.  Be careful, at some point a couple of things will 
happen, first there will be so much growth that your agar will be 
completely covered with different organisms and second, your agar will 
start to dry out and all growth will stop.

Regardless of what may grow on your plate it is a very good experiment to 
conduct.  It will show you the different types of organisms that reside on 
the human body as well as what is in the air around you.

If you have any other questions, by all means email me back.

I hope the answer was helpful and good luck with your experiment.

Rob Welton