Brush your teeth with each of the 10 toothbrushes. Allow all of the toothbrushes to sit in a bathroom (near a toilet) for approximately 4 days.
Prepare 20 sterile plates containing nutrient agar.
Put on sterile gloves (do not touch any non-sterile surface when wearing gloves).
Swab a toothbrush with a sterile swab (do not touch anything else with the swab).
Remove the top of the petri dish. Keep the top of the dish in your hand away from any non-sterile surface.
Gently run the swab back and forth in a zigzag pattern on the surface of the agar plate. Do not touch any part of the agar twice.
Put the top of the petri dish back on and label.
Change your gloves.
Repeat steps 3-8 for each of the 10 toothbrushes using a new agar plate each time. You should now have 10 “before” plates. Be sure to number each toothbrush and write the number that corresponds to the sampled toothbrush on each agar plate.