This year as a science fair project, I want to test whether or not certain substances are effective against certain bacteria strains.

 

Is this done by growing bacteria on a petri dish and then put the substances on the dishes?

Where can I obtain the materials needed (petri dishes, agar, place to put the dishes in, strep throat,e. coli, salmonella, etc.) ?

One can either add the substances to the (slightly cooled) agar while before pouring it into the petri dish). Alternatively there are antibiotic discs to test them. Or you can put a sterile filter disk on a plate with bacteria and drop around 10µl on it.

For E. colis you might want to ask nearby universities, however you won't (or at least shouldn't) get Streptococci or Salmonella. Both species are usually human pathogenes and may not be cultivated outside specially equipped laboratories!

 

Edit: forgot to add: most chemicals can be acquired by e.g.Sigma, but you may need an authorization to buy there (depends a bit on local laws).