Agar is a polymer used to grow bacteria. Agar is first supplied as a powder and, when dissolved in boiling water and cooled, solidifies to a firm surface for bacterial growth. There are many different types of agar that can be used to grow your bacteria of interest, depending on the species and your research needs. Nutrient agar is popular because it can grow a variety of types of bacteria and fungi, and contains many nutrients needed for robust bacterial growth.
Components of Nutrient Agar
Nutrient agar contains 0.5 percent peptone, which is an enzymatic digest of animal protein. Peptone is the principal source of organic nitrogen for the growing bacteria. Nutrient agar also contains 0.3 percent beef extract, which contains water-soluble substances which aid in bacterial growth, such as vitamins, carbohydrates, organic nitrogen compounds and salts. The last component of nutrient agar is 1.5 percent agar, which is the solidifying agent.
How to Make Nutrient Agar
Dissolve the previously listed components in the desired volume of water. To make 1 liter of nutrient agar, add 5 grams of peptone, 3 grams of beef extract, and 15 grams of agar and ensure the final volume of this mixture is 1 liter by adding distilled water. Heat this mixture while stirring to fully dissolve all components. Autoclave the dissolved mixture at 121 degrees Celsius for 15 minutes on the liquid cycle.
How to Pour Nutrient Agar Plates
Once the nutrient agar has been autoclaved, allow it to cool but not solidify. Add antibiotics at this point if desired. Take off the lids and spread the appropriate number of Petri dishes onto a sterile surface. In general, 1 liter of agar will pour about 50 large Petri dishes -- 100-millimeter diameter -- or 100 small Petri dishes -- 60-millimeter diameter. Ignite a flame nearby to keep the area around the plates sterile as well. When opening the bottle of agar, pass the opening through the flame in order to sterilize it. Pour nutrient agar into each plate and leave plates on the sterile surface until the agar has solidified. Replace the lid of each Petri dish and store the plates in a refrigerator.
Recommendations for Pouring and Storing Plates
Fill each plate one-half to two-thirds full. Try to eliminate all air bubbles, as these will make it difficult to spread bacteria over the plate once solidified. Bubbles can be popped using the flame. Store agar plates upside down in the refrigerator. This will prevent condensation on the lid of the Petri dish from dripping onto the solidified agar. If storing plates long-term, check for bacterial growth or drying before using them. Plates containing antibiotics should be used within 2 weeks of preparation.