How to Cultivate Bacteria in a Petri Dish: A Comprehensive Guide

Bacteria culture is a fundamental technique in microbiology that allows the growth and observation of bacterial colonies. This method is particularly useful for educational and research purposes. In this guide, we will walk through the step-by-step process of cultivating bacteria in a petri dish, ensuring you have the necessary tools and knowledge to conduct your experiment successfully.

Part 1: Preparing the Petri Dishes

The first step in cultivating bacteria is preparing the culture medium and petri dishes. Agar is a jelly-like substance used to culture bacteria. It is made from red algae and provides an ideal surface for bacterial growth. For this experiment, we recommend using a nutrient agar, which is available in powder form.

Prepare the Agar: Dissolve 1.2 grams of nutrient agar powder in 60 milliliters of hot water. Stir well until the agar is fully dissolved. Multiply these quantities by the number of Petri dishes you plan to use. Boil the Solution: Place the solution in a microwave-safe bowl and heat it until it begins to boil for about 1 to 2 minutes. Ensure the solution does not boil over. Forecast Cooling: Allow the solution to cool for a few minutes before use. Always be cautious as the solution is hot and can cause burns.

Clean and sterilize the Petri dishes before use. Newly purchased Petri dishes should be pre-sterilized but are sealed in packaging. Remove the dish from its packaging and place the top and bottom halves in the hot agar solution. Work in a controlled environment to avoid contamination.

Pour the Agar Solution: Carefully pour the solution into the bottom half of the Petri dish, ensuring it forms a thin layer. Seal the Dish: Quickly replace the top half of the dish to maintain a sterile environment. Set Aside to Harden: Allow the Petri dish to sit for about 30 minutes to 2 hours until it cools and hardens. The surface should resemble set Jell-O.

Store the Petri dishes in the refrigerator if you do not plan to use them immediately. This prevents water evaporation and allows the surface to harden slightly, making it easier to handle without damaging the surface. Store the dishes upside down to prevent condensation from affecting the surface.

Part 2: Growing Bacteria

Cultivating bacteria is a straightforward process that requires introducing the bacteria to the Petri dish. There are two common methods for this: direct contact and sample collection.

Introduce Bacteria: For direct contact, use your fingertip (after washing) to press onto the agar surface. For sample collection, use sterile swabs to swipe surfaces like your mouth, computer keyboard, or remote control. Each sample should be added to a quadrant of the Petri dish. Label and Seal: Replace the lid and seal it with parafilm or Saran Wrap. Label each dish with the source of the bacteria for future reference. Place in a Warm, Dark Place: Store the Petri dishes in a warm, dark place to allow bacteria to grow undisturbed. Ideal temperature ranges between 70 and 98 degrees F (20-37 degrees C). Monitor the dishes for 4-6 days, leaving them upside down to prevent water droplets from disturbing the growth. Record Observations: Use a notebook to record observations and measure bacterial growth, noting the variety of colonies and any unusual activity. Test Antibacterial Agents: For a variation, introduce an antibacterial agent like hand sanitizer or soap into the center of the dish to test its effectiveness against bacteria growth.

Part 3: Safely Disposing of the Bacterium

Proper safety measures are crucial when handling and disposing of Petri dishes containing bacteria. Before disposal, sterilize the bacteria colonies using household bleach.

Take Safety Precautions: Wear protective gloves, goggles, and an apron to prevent skin contact, ingestion, or inhalation of bacteria. Disinfect the Dish: Open the dish, pour a small amount of bleach on the bacteria, and pour the mixture into a sink. Seal the disinfected dish in a plastic bag and discard.

By following these steps, you can cultivate and study bacteria in a petri dish safely and effectively.