Science project

What Method of Preparing Salad Eliminates the Most Bacteria?



Safety Issues

This project involves the culture of bacteria to produce bacterial colonies. The project can be conducted under the supervision of a high school science teacher if the agar plates are sealed with tape and are not opened by the student or teacher. Supervision can be provided by a medical laboratory technologist/technician at a hospital laboratory. Disposal of the agar plates involve autoclaving the agar plates at an industrial or research institution.

Material Availability

The material for the project can be ordered from a chemical supply catalog or obtained from a high school laboratory or research institute.

Approximate Time Required to Complete the Project (Hours, days, weeks)

3-6 days


To evaluate the efficiency of lettuce cleaning techniques.

The project goals include assessing the cleanliness of salad preparation techniques by comparing the total number of bacterial colonies produced after lettuce is cleaned.

Materials and Equipment / Ingredients


Agar plates

Head of lettuce


In preparing salad, a variety of lettuce cleaning methods can be used. This experiment assesses the effectiveness of the methods. The methods include rinsing lettuce in cold water, soaking in cold water and blot drying, refrigerating overnight in sugar water, purchasing prewashed lettuce, and use of a lettuce washer. In each case the same head of lettuce is used to minimize differences in initial microbial distribution. To begin each leaf of lettuce is pressed on an agar plate to leave an imprint on the nutrient agar. A lid covers the agar plate and placed in an incubator to determine the initial number of colonies on the lettuce leaf prior to cleaning. The various methods of cleaning are applied to the leaves of lettuce and the imprint of the washed leaves incubated on nutrient agar to facilitate the data comparison before and after washing. 

Research Questions

What food preparation techniques are the most effective at eliminating bacteria?

How can an agar plate be prepared for cultivating bacterial colonies?

How can contamination of food be reduced?

Terms, Concepts and Questions to Start Background Research


Bacterial colonies


Inoculation loop

Experimental Procedure

  1. To prepare nutrient agar plates, measure 25 grams of nutrient agar and dilute in 1 liter of distilled or deionized water in a 2 liter flask.
  2. Boil the mixture at 121 degree Celsius for 20-25 minutes to sterilize the agar solution.
  3. Cool the mixture to 50 degree Celsius and decant 25 – 35 milliliters of the agar broth into agar plates.
  4. Let the agar stand in plates until the agar turns into a gel-like substance.
  5. Take the leaves of lettuce before cleaning and imprint individual agar plates with each leaf.
  6. Record the date of inoculation, the method of cleaning, and temperature of incubator on each agar plate.
  7. Apply a lettuce cleaning technique to individual leaves and imprint the agar plates with the cleaned lettuce leaves.
  8. Repeat steps 5-7 for the various methods of lettuce cleaning.
  9. Following two days of incubation, remove the agar plates from the incubator to identify and count bacterial colonies.
  10. Bacterial colonies can be circular, fibroid, or irregular in shape, size, and color depending on the type of bacteria growing in the Petri dishes. The total number of colonies can be determined by counting the colonies on the agar plate with the use of a microscope to aid the identification of the distributed colonies.
  11. Comparing the number of bacterial colonies produced before and after cleansing can result in determining the percentage of bacteria eliminated by the technique and thereby used to assess the efficiency of the technique.


Bergey’s Manual of Determinative Bacteriology 9th Edition, John G Holt, January 14 1994

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