Summer Research Program for Secondary School Science Teachers

Manhattan Center for Science and Mathematics

Manhattan, NY

Sau Ling Chan

August 1998


How Many Bacteria on Your Food?


Target Audience & Subject: High school biology or microbiology

Type of Activity: Cooperative and inquiry-based learning

Informal discussion

Hands-on activity

Prerequisite: Sterile technique

Gram’s Stain technique [9-12 Content Standard E- Understandings about science and technology]

Objective: Laboratory techniques and critical thinking skills are the center of focus in this lab. At the beginning of lab, students will be given a real-life problem to solve and all of the materials they need to complete the experiment. Students would be assessed based on their ability to incorporate laboratory skills learn from prior labs and the reasoning they used in forming the basic construct of their experiment.

Prior to the beginning of lab, allow 5-10 minutes for students to explore the purpose or reason behind the problem or issue presented to them. Then spend a few minutes to answer any questions that students have regarding the problem at hand.

Motivation: Give each student a copy of the test printed by the Food and Drug Administration

(FDA), "Can your kitchen pass the food safety test?" [Teaching Standard D- Make accessible science media] Then discuss with the students how they did on the test and the purpose behind taking such a test.

Get students into groups of four, ask them to answer the following questions as a group and then perform the lab.

Where can we find bacteria?

How does food poisoning occur?

What is best way to prevent food poisoning? [Teaching Standard B- Orchestrate scientific discourse]

Student Worksheet

Name ___________________________ Date ________________________

Period ________ Lab # ____

Lab: How many bacteria are on your food?

Procedure: I. Making agar plates

Boil 100ml of distilled water.

Dissolve 2.3g of nutrient agar in the water. Using a glass rod, stir the agar continuously until agar powder completely dissolves.

Autoclave agar.

Allow agar to cool until it is comfortable to touch with your hands.

Using sterile techniques, pour a thin layer of agar into sterile petri dish. Allow agar to solidify on the plate.

II. Making bacteria count using agar plates

Using a sterile cotton swab, swab 1 cm2 area from the outer area of a piece of food.

Inoculate the bacteria onto the agar plate following the diagram shown below.

Incubate the plate at 37 C (varies according to bacteria being studied) for 24 hours.

Place the agar over bacteria count apparatus and estimate amount of bacteria present.

Perform a Gram’s stain to determine the morphology of the bacteria.

Summary questions

What did you observe on the agar plate?

How many bacteria did you count?

What was the morphology of the bacteria?

What are the functions of these bacteria?

How do you think changing some of these experimental conditions would affect the growth of bacteria?

Designing an experiment

Design an experiment to test other quantitative factors that can affect the growth of bacteria on this food. For example, pH, temperature, moisture, etc.

[9-12 Content Standard A- Identify questions that guide scientific inquiry/design an experiment]


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