Battle of the Bacteria

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Introduction:

In 1928, Alexander Fleming returned from vacation to discover that mold had killed bacteria in one of his Petri dishes. This accidental observation led to the discovery of penicillin. In this hands-on activity, children explore how certain substances can prevent bacterial growth. By safely simulating a Petri dish using gelatin and common materials, students learn how antibiotics work and why Fleming’s discovery was so important to medicine.

What You Will Need:

• Plain gelatin (unflavored) or agar plates

• Cotton swabs

• Plastic or paper plates (if using gelatin)

• Moldy bread or a lemon slice

• Notebook and pencil

• Plastic wrap or ziplock bags for covering plates

Step-by-Step Instructions:

• Prepare gelatin in shallow dishes and let it cool until firm.

• Swab surfaces (e.g., hands, doorknobs) and lightly streak across the gelatin.

• Place a small piece of moldy bread or lemon slice in the center of the plate.

• Cover with plastic wrap and label each plate.

• Place in a warm spot for 2–3 days, undisturbed.

• Observe whether bacteria (cloudy areas) grow around the mold or citrus.

What to Look For:

• Do you see clear areas where nothing grows?

• Did the mold or lemon prevent bacteria from spreading?

• How do different surfaces affect the amount of bacteria that grows?

Fun Fact:

The clear area where bacteria did not grow is called a 'zone of inhibition'—this is how penicillin was first observed in action!

Safety Tip:

Do not open the sealed dish once bacteria has started to grow. Always wash hands thoroughly after handling experiment materials.

Journaling Prompt:

Draw your Petri dish and label the mold or lemon in the center. Write a paragraph about what Alexander Fleming might have thought when he first saw the clear ring around the mold.