Science project

Does Double Dipping Really Spread Germs?

Research Questions:

Does a bowl full of salsa in which people have “double dipped” chips contain more bacteria than a bowl of salsa where each chip has only been dipped once?

“Double dipping,” or dipping twice with the same chip, is widely regarded as a social faux pas because it is believed that this practice transfers saliva to the dip and spreads germs. This experiment will evaluate whether there is any truth to this theory. You will use agar plates to test whether double dipping chips transfers more bacteria to salsa than single dipping.


  • Salsa
  • Chips
  • Sterile swabs
  • Prepared nutrient agar in sterile plates
  • Notebook to record results

Experimental Procedure:

  1. Prepare nine sterile plates containing nutrient agar.
  2. Boil nine glass bowls in water for 10 minutes (to sterilize).
  3. Remove bowls from water and pour in salsa (use salsa from the same container for each bowl).
  4. Place a label in front of each bowl (3 bowls should be for double dipping, 3 bowls should be for single dipping, and chips will not be dipped in the last 3 bowls).
  5. Ask one volunteer to double dip 10 chips in one of the “double dipping” bowls.
  6. Ask the same person to single dip 10 chips in one of the “single dipping” bowls.
  7. Write the name of the volunteer on the labels placed in front of those two bowls.
  8. Repeat process with two more volunteers. Each volunteer has their own “double dipping and “single dipping” bowls.
  9. Dip a sterile swab into a bowl of salsa (do not touch anything else with the swab).
  10. Remove the top of the petri dish. Keep the top of the dish in your hand away from any non-sterile surface.
  11. 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.
  12. Put the top of petri dish back on and label (e.g., double or single dipped and the name of volunteer or “control” (not dipped) salsa).
  13. Repeat procedure for the remaining bowls of salsa. Use a new sterile swab and new agar plate for each bowl.
  14. Place the petri dishes in an environment that is as close to 37ºC as possible. Bacteria will take longer to grow at room temperature. Ensure that all petri dishes are placed in the same location.
  15. Photograph plates at defined intervals of time (e.g., after 24 hours, 36 hours, 48 hours, 60 hours, and 72 hours).
  16. Count bacterial colonies on each plate at each time point. Average the colonies counted among “double dipping” plates and compare to average colonies among “single dipping” plates and “control” plates.

Sample table for recording data:



Number of Bacterial Colonies

24 hours

36 hours

48 hours

60 hours

72 hours


Volunteer 1


Volunteer 2


Volunteer 3


Volunteer 1


Volunteer 2


Volunteer 3


Control Plates

Plate 1


Plate 2


Plate 3



Terms/Concepts: Growing bacteria on agar; Health concerns associated with double dipping; What is a control?


 Lehman, C. “How to Grow Bacteria in Agar.” 

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