Potato Dextrose Agar - Pk 10

Potato Dextrose Agar (PDA) is the ultimate gold standard growth medium used in high school science labs to cultivate, observe, and study fungi, molds, and yeasts.

While general bacteria are usually grown on Nutrient Agar, high school biology courses swap to PDA when they want to focus on the fungal kingdom. Because PDA is packed with potato starch and dextrose (sugar), it provides the perfect high-carbohydrate environment that allows fungal spores to germinate rapidly into massive, fuzzy, and colorful colonies.

1. Environmental Air Quality Testing

This is one of the most classic school science experiments. Students test the invisible microbial load floating in the air.

• The Experiment: Students place PDA plates in different school locations (e.g., the cafeteria, the locker room, the library, or outside) and leave the lids off for 15 to 30 minutes. The plates are then sealed and incubated.
• The Lesson: Over a few days, floating fungal spores land on the agar and grow into colorful, velvety circles. Students count and compare the number of colonies to determine which areas have the highest spore counts or the best ventilation.

2. Evaluating Antifungal Agents & Cleaners

Students test how effectively household items can stop the growth of mold.

• The Experiment: A PDA plate is evenly coated (swabbed) with a specific mold spore. Students then place small paper discs soaked in different treatments—such as tea tree oil, vinegar, commercial bleach, or antifungal creams—onto the agar.
• The Lesson: If a treatment works, a clear ring called a "zone of inhibition" will form around the disc where no mold can grow. Students measure these rings to scientifically determine which household cleaner is the most effective antifungal.

3. The "Molding Food" Biology Lab

Instead of just looking at gross bread in a plastic bag, students look at the microscopic structures causing the rot.

• The Experiment: Students touch a sterile swab to a piece of molding fruit or bread, streak it onto a PDA plate, and let it grow.
• The Lesson: Once the mold grows on the clean, flat surface of the agar, students can use clear tape to lift a sample and view it under a compound microscope. This allows them to clearly identify fungal anatomy like hyphae (root-like structures) and sporangia (the spore-bearing heads).

Why PDA is popular in High School Labs

• Visually Striking Results: Unlike bacteria (which usually look like small, boring white or yellow dots), molds grow into spectacular, giant, fuzzy structures displaying vibrant greens, deep blacks, bright yellows, and pristine whites.
• Naturally Selective: PDA's high sugar content naturally favors fungi over bacteria. Furthermore, teachers can easily buy "Acidified PDA" (which has a lower pH). The acid stops common classroom bacteria from growing entirely, ensuring students only grow the molds they want to study.
• Safety Profile: Most everyday environmental molds collected on PDA plates are relatively low-risk (Biosafety Level 1), making them far safer and less intimidating for teenagers to handle than targeted bacterial strains.

(Note: Just like MacConkey plates, once a PDA plate is open to the air and sealed, students should never reopen the lid to prevent inhaling millions of concentrated fungal spores!)

DANGER: DO NOT USE ALCOHOL TO DISPOSE OF AGAR PLATES

Using isopropyl alcohol or ethanol to destroy or dispose of cultured PDA agar plates is highly dangerous, ineffective, and creates immediate laboratory hazards.

Potato Dextrose Agar (PDA) plates cannot simply be thrown directly into the trash. If a plate contains active molds or yeasts, it is classified as biohazardous laboratory waste and must be sterilized first

• Method 1: The Gold Standard – Autoclaving (Heat Sterilization)
• Method 2: The Practical School Alternative – 10% Bleach Solution

If you do not have an autoclave, a 10% sodium hypochlorite (household bleach) solution will chemically destroy the fungal cells and spores.

1. Gear Up: Put on gloves, safety goggles, and a lab coat. Complete this process in a well-ventilated room.
2. Mix the Solution: Prepare a mixture of 1 part household bleach to 9 parts water.
3. Submerge the Plates: Carefully open the petri dishes and pour the 10% bleach solution directly over the surface of the fuzzy mold or yeast colonies. Ensure the liquid fully covers the agar surface.
4. Soak Time: Allow the plates to sit undisturbed for at least 60 minutes. Fungal spores can be highly resilient, so do not rush this timeline.
5. Drain and Discard: Safely pour the liquid bleach down the lab sink with plenty of cold running water. Place the remaining bleached agar and plastic petri dishes into a heavy-duty trash bag, tie it securely, and place it in the regular wast.