The Petri dish is a standard tool in microbiology, consisting of a shallow, circular dish and a loosely fitting lid that creates a controlled environment for culturing cells and microorganisms. Named after its inventor, Julius Richard Petri, this transparent container allows scientists to grow bacteria, fungi, and other microbes on a solid or semi-solid medium, typically a nutrient-rich gel called agar. The dish’s design allows for gas exchange while protecting the growth medium from airborne contaminants, which is necessary for isolating and studying microbial populations. Understanding the proper preparation and handling of these dishes is necessary for safely performing basic science experiments.
Setting Up the Growth Medium
The process begins with preparing the nutrient agar, which serves as the food source and solid surface for microbial growth. Nutrient agar is typically a powder mixed with water, and it must be heated until the powder dissolves completely into a clear liquid. For home or classroom use, a microwave can be used to boil the mixture in short, controlled bursts, ensuring the solution does not boil over. The solution must then be allowed to cool slightly before pouring.
The goal before pouring is to ensure that both the dish and the medium are sterile to prevent unwanted growth that could interfere with the experiment. While commercial Petri dishes often come pre-sterilized, the agar solution itself must be heated to a boiling point for sterilization. Once the agar has cooled slightly but remains liquid, the sterile dishes should be placed on a clean surface in a draft-free environment.
The pouring technique requires speed to minimize exposure to environmental microbes. Lift the lid only enough to pour the liquid agar into the bottom dish, filling it to create a layer approximately three to five millimeters thick. Immediately replace the lid and gently swirl the dish to ensure the medium is evenly distributed across the bottom surface. As the agar cools, it will solidify into a gel, and leaving the lid slightly ajar can help reduce excess condensation.
Collecting and Transferring Samples
Once the agar has fully solidified, the dish is ready to be inoculated with a sample of microorganisms collected from the environment. Proper preparation involves having sterile tools, such as pre-packaged cotton swabs, ready for use. Sterility is maintained by keeping the Petri dish lid on until the moment of sample introduction and only opening it for the briefest time necessary.
Samples can be acquired from various sources, such as swabbing a common surface like a doorknob, collecting a sample from the air, or touching the plate with a finger. When swabbing a surface, the swab should be gently rolled over the area for several seconds to collect a sufficient number of microbes. After collection, the swab is then lightly brushed across the surface of the agar in the Petri dish. Ensure the gel is not gouged or torn during the transfer.
Immediately after the sample has been transferred, the lid must be securely replaced to protect the culture from further contamination. Labeling the dish on the bottom (not the lid) using a permanent marker is necessary following inoculation. This labeling should clearly state the date, the source of the sample, and any other relevant experimental details. This allows for accurate tracking even if the lid shifts or is accidentally separated from the base.
Monitoring and Safe Disposal
The inoculated dishes must be placed in an environment that encourages microbial growth, typically a warm, dark place maintaining a temperature near or slightly above room temperature (70 to 80 degrees Fahrenheit). To prevent condensation from dripping onto the agar surface and disrupting growth, the Petri dishes should be stored upside down. Visible colonies, which are masses of millions of identical microorganisms, will begin to appear within two to three days.
Observation should be conducted daily, recording details like the size, color, shape, and texture of the developing colonies. Keep the dish sealed during the observation period, as opening it exposes the concentrated microbial growth to the air, which can be a health risk. Data should be recorded through detailed notes, sketches, or photographs, always observing the culture through the transparent lid.
Once the experiment is complete, the safe disposal of the cultured dishes is the final step. Because the plates contain concentrated microbial growth, they should never be thrown into the regular trash without decontamination. The safest home method involves placing the sealed dishes into a plastic bag and adding a disinfectant solution, such as a 10% bleach solution (one part bleach to nine parts water). The dishes should soak in this solution for at least 24 hours to kill the microorganisms before the entire sealed bag is disposed of with household waste.