Science and Our Food Supply: Investigating Food Safety from Farm to Table

WHAT’S A “CONTROL DISH”?

In this lab, the purpose of the control dish is to determine whether or not the dish, agar, swab or any 

other associated swab materials are contaminated with bacteria. If after incubation there are colonies 

growing on the control dish, one or more of the listed materials were contaminated. The control dish 

can also be called the “control plate.”

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MIDDLE LEVEL

MODULE 1:

 UNDERSTANDING BACTERIA

BACTERIA EVERYWHERE

PROCEDURE

2

LAB: 

OBSERVE BACTERIA AND RECORD RESULTS

1. 

Have students observe the bacterial growth and record 

the results. Students can use the Bacteria Everywhere 

Data Table to record their results.  

Tip:

 Ask the students to draw their Petri dishes on the 

back of the Data Table and illustrate the colonies that  

are growing. 

2. 

Students can analyze the results based on their  

observations. Ask them: 

• 

What do you see? (Their first observation may be the 

number of bacterial colonies. If so, use a 0–5 scale 

for rating the quantity. Guide students in ranking the 

results.) 

• 

What else do you notice about the colonies? (Their 

next observation may be the size and shape of the 

colonies.) 

• 

Why do they look different? (Different colonies/ 

microorganisms have different characteristics.) 

3. 

Have each team report the following to the class: the 

areas they sampled, the number of colonies they  

observed, and the characteristics (size, shape, and color) 

of the colonies. 

4. 

Ask students: Were there any differences in your results 

compared to the other teams? How did your results  

support or reject your hypothesis? 

5. 

Are the colonies composed of good or harmful  

organisms? (With this lab, students will not be able to 

identify good versus harmful organisms — they would 

need specific agars to grow and identify specific organisms. 

The purpose of the lab is to demonstrate that bacteria are 

everywhere and different surfaces have different levels of 

organisms. Also, stress that not all bacteria are harmful.  

In fact, most bacteria are beneficial to us.) 

6. 

Be sure to properly dispose of the Petri dishes (see  

page 7). 

7. 

Ask students to write a lab report (see the Lab Report 

Outline). 

This lesson involves the following 3 steps in gathering 

information: