Agar is a gelatinous substance derived from the cell walls of certain red seaweeds, used extensively in the study and cultivation of fungi. This material serves as a stable, solid surface for growing the vegetative part of the mushroom, called mycelium. Agar is a complex carbohydrate that provides the necessary structure without being consumed by the fungus. Its controlled, transparent environment allows cultivators to observe and manage the microscopic beginnings of their mushroom cultures.
The Essential Function of Agar in Mycology
The primary purpose of using agar is to manage and manipulate fungal genetics in a sterile setting. Agar plates provide a solid platform for isolating specific, vigorous strains from a mixed population of spores. When spores germinate, they create multiple, individual colonies, allowing a cultivator to visually select the healthiest mycelium for transfer to a fresh plate, ensuring genetic purity.
Agar is also the foundational tool for cloning existing mushrooms. This is done by transferring a small piece of tissue from a desired specimen directly onto the nutrient surface. This tissue transfer bypasses the genetic variation inherent in spores, resulting in a culture with the exact same characteristics as the source mushroom. Agar cultures are also used for the long-term maintenance and storage of valuable strains.
Formulating the Nutrient Medium
The added nutrients fuel the growth of the mushroom mycelium. Agar powder is mixed with water and a food source to create a complete growth medium. Common formulations include Potato Dextrose Agar (PDA) and Malt Extract Agar (MEA), which are widely used across many mushroom species.
These nutrient bases supply the mycelium with essential carbon sources, typically starches or simple sugars like dextrose. For example, MEA uses malt extract, providing carbohydrates and minerals, while PDA utilizes potato infusion and dextrose.
Preparing and Inoculating Culture Plates
The practical process begins with mixing the dry ingredients—agar powder, water, and the nutrient source—in a heat-resistant container. This mixture must then be sterilized, typically in a pressure cooker or autoclave at 15 pounds per square inch (PSI) for about 30 minutes, to eliminate all competing microorganisms and spores. After sterilization, the liquid agar is allowed to cool before pouring.
Pouring the agar into sterile petri dishes must be conducted in an aseptic environment, such as a laminar flow hood or a still-air box, to prevent airborne contaminants from settling onto the exposed medium. Once the agar has solidified, the plate is ready for inoculation, which is the transfer of the fungal culture. This involves using a sterile tool, like a scalpel, to transfer a small piece of mushroom tissue, a drop of spore solution, or a segment of an existing culture onto the center of the fresh agar surface. Aseptic technique is paramount throughout this process.
Differentiating Mycelial Growth from Contamination
Once inoculated, the agar plate serves as a quality control checkpoint, allowing the cultivator to visually assess the health and purity of the culture. Healthy mushroom mycelium often appears as a white, cottony growth (tomentose) or as distinct, rope-like strands (rhizomorphic) that fan out across the agar surface. The mycelium’s growth should be uniform and filamentous, radiating from the point of inoculation.
Contamination, which refers to any unwanted microbial growth, presents distinct visual cues that differentiate it from the desired culture. Bacterial contamination typically appears as wet, slimy, or dense, circular colonies that lack the filamentous structure of fungi. Common molds often start as white patches that are denser or fuzzier than mycelium, but they quickly produce spores that turn the colony green, black, or blue. The presence of these contaminants signals that the plate must be discarded or that a small, clean section of the mycelium must be rapidly transferred to a new plate to save the culture.