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They're Everywhere: Bacteria

Author(s): Nancy Moreno, PhD, and Barbara Tharp, MS.


Have students work in groups 2-4. 

Materials per Student Group

  • 6 cotton swabs 
  • 3 sterilized petri dishes (see Setup) 
  • Distilled or boiled water (for swabs) 
  • Masking tape for labeling and sealing petri dishes 
  • Nutrient agar (order from a science supply vendor) 
  • Pen or markers


Prepare the petri dishes for students before class begins. Warm the nutrient agar in a pan of boiling water or in a microwave oven until it melts (about 60° C). To avoid condensation in the dishes, let the agar cool slightly before use. 

Open each petri dish, pour in enough agar to cover the bottom and immediately replace the cover. Store the petri dishes upside down, and let the agar solidify before use. 

As an alternative to agar, bacteria can be grown on potato slices. Boil whole potatoes until they are almost soft. Using a clean, dry knife, cut potatoes into 1/4-inch slices and place each slice in a petri dish or a clean re-sealable plastic bag. 

Safety Notes

  • Do not let students test saliva, tears or any other body fluids. 
  • Do not let students open the petri dishes once they have been closed. 
  • Refer to the lesson PDF for proper disposal of materials. 

Funded by the following grant(s)

National Institute of Environmental Health Sciences, NIH

National Institute of Environmental Health Sciences, NIH

My Health My World: National Dissemination
Grant Number: 5R25ES009259
The Environment as a Context for Opportunities in Schools
Grant Number: 5R25ES010698, R25ES06932