Agar, a gelatinous substance derived from red algae, functions as a solid growth medium in petri dishes. It provides a stable environment for culturing microorganisms like bacteria and fungi. Agar is non-nutritive, allowing specific nutrients to be added to support desired microbial growth. Its ability to solidify and remain stable across a range of temperatures makes it an effective tool for observing microbial development.
Gathering Your Supplies
Making agar plates at home requires specific materials and equipment. You will need bacteriological grade agar powder, which acts as the solidifying agent. Distilled water is essential for preparing the solution, ensuring purity. For a nutrient source, common choices include nutrient broth, peptone, or pre-mixed media like Nutrient Agar for bacteria or Potato Dextrose Agar for fungi.
Equipment includes a heat-resistant container, such as an Erlenmeyer flask or a glass bottle. A stirring rod or magnetic stirrer helps dissolve the agar consistently. Accurate measurements require a measuring cylinder or beaker and a scale for weighing ingredients. A heat source, such as a hot plate or microwave, is needed to dissolve the agar. Aluminum foil for covering and sterile petri dishes are required to contain the prepared medium.
The Agar Preparation Process
Prepare the agar solution by careful measurement and heating. Weigh the agar powder and any chosen nutrient medium according to manufacturer’s instructions or a standard recipe, such as 15 grams of agar per liter of water. Combine the agar and nutrient source with distilled water in your heat-resistant container. Mix these components thoroughly to distribute the dry ingredients evenly before heating.
Gently heat the mixture using a hot plate with constant stirring or through short bursts in a microwave. Continuous stirring is important to prevent the agar from scorching at the bottom. Continue heating until the agar powder is completely dissolved and the solution appears clear. This dissolution is crucial for the agar to solidify properly once cooled.
Sterilization and Pouring Techniques
Sterilizing the agar solution is an important step to prevent contamination. For home use, a pressure cooker acts as an autoclave alternative. Heat the agar solution at 15 pounds per square inch (psi) and 121°C for 15 to 20 minutes to eliminate most bacteria and their spores. Alternatively, for smaller batches, Tyndallization involves boiling the solution for 15-20 minutes daily for three consecutive days, allowing spores to germinate between heating cycles so they can be killed.
Once sterilized, allow the agar solution to cool to a temperature between 45°C and 50°C before pouring; if too hot, it can damage petri dishes or create excessive condensation. Pour the cooled liquid agar into sterile petri dishes, filling them just enough to cover the bottom, typically about 1/8 to 1/4 inch deep or 10-15 ml for standard 90-100mm dishes. Work quickly and in a clean environment to minimize airborne contaminants, potentially near a flame if available, to create an aseptic zone. Close the lids immediately after pouring to maintain sterility.
Proper Storage and Usage Tips
After the agar solidifies, proper storage is important to maintain effectiveness and prevent contamination or dehydration. Store the prepared agar plates inverted in a cool, dry place or in a refrigerator. This inverted position helps prevent condensation from dripping onto the agar surface, which can lead to contamination or interfere with microbial growth. Sealing the plates in bags can further protect them from dehydration and contaminants.
The shelf life of plain agar plates ranges from 30 to 90 days, though nutrient-rich media may have a shorter lifespan. Before using the plates, allow them to warm to room temperature, which can help mitigate condensation issues. Condensation forms due to temperature differences; warming the petri dishes slightly before pouring or letting the plates dry at room temperature after pouring can reduce this. Label each plate on the bottom with essential information: date of preparation, agar type, and any intended microbial cultures.