Grade Level: Middle School; Type: Biology
Determine if picking up fallen food in five seconds or less prevents the transfer of bacteria from the ground.
- Does picking up fallen food from the ground within 5 seconds prevent the transfer of bacteria?
- Does the type of food that falls to the ground affect how much bacteria is transferred from the ground?
- Does the type of ground on which food falls affect how much bacteria is transferred to the food?
The five-second rule states that food dropped on the ground will be safe to eat and not covered in germs as long as it is picked up within 5 seconds of being dropped. This experiment will evaluate whether there is any truth to this theory. You will use agar plates to test if picking up fallen food from the ground in five seconds prevents the transfer of bacteria. In addition, you will evaluate whether the type of food that falls or the type of ground affects bacterial transfer.
- Food items to be tested (e.g., wet item like lunch meat and dry item like a jelly bean). You will need six of each item tested.
- Sterile swabs
- Sterile gloves
- Prepared nutrient agar in sterile plates
- Notebook to record results
- Prepare 12 sterile plates containing nutrient agar.
- Put on sterile gloves (do not touch any non-sterile surface when wearing gloves).
- Select two types of “ground."
- Drop the first test item (e.g., a piece of lunch meat) on the first type of ground.
- Start the timer.
- Remove the item from the ground after five seconds.
- Swab the item 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 the test with the first item (e.g., a new piece of lunch meat).
- Repeat the test twice with the second item (e.g., two separate jelly beans).
- Change types of ground and perform the test twice with each item.
- Swab items that have not been dropped on any ground. Create 2 “control” agar plates for each item tested.
- 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.
- Photograph plates at defined intervals of time (i.e. after 24 hours, 36 hours, 48 hours, 60 hours, and 72 hours).
- Count the bacterial colonies on each plate at each time point.
Sample graph for displaying results:
Terms/Concepts: Growing bacteria on agar; The 5-second rule; Counting bacterial colonies on agar plates
References: Lehman, C. “How to Grow Bacteria in Agar.”