Mycelium is the vegetative body of a fungus, a network of thread-like cells (hyphae) that seek out and digest nutrients. Cultivating this network on a solid nutrient base, known as agar, allows for the observation, isolation, and maintenance of pure fungal cultures. Agar, a gelatinous substance derived from seaweed, is mixed with food sources like potato sugars or malt extract to create a rich, stable growth medium in a petri dish. This controlled environment makes it possible to purify a strain, separate it from contaminants, and preserve specific genetics for future cultivation efforts.
Preparing and Inoculating Agar Plates
The agar medium must first be sterilized and poured into petri dishes, often using common nutrient recipes like Potato Dextrose Agar (PDA) or Malt Extract Agar (MEA). These nutrient-rich formulations provide the necessary carbohydrates and proteins that fuel the initial expansion of the fungal colony. Once the plates have cooled and solidified, inoculation (the introduction of fungal material) must occur under highly sterile conditions, typically within a Still Air Box or in front of a laminar flow hood.
Inoculation methods depend on the starting material. An agar-to-agar transfer uses a sterile scalpel to move a small wedge of established mycelium from an existing culture to a new plate. Alternatively, a sterile syringe containing microscopic spores can deposit drops onto the surface, or a tiny piece of tissue from a fresh mushroom can be placed directly onto the agar. The speed of visible growth varies greatly based on the source material used.
Typical Mycelial Growth Timelines
The time required for visible growth depends heavily on the source material. Starting from spores involves a significant lag phase, as spores must first germinate and merge to form the mycelial network. Visible mycelium from a spore syringe may not appear for seven to fourteen days, sometimes up to three weeks. In contrast, a transfer from an established, healthy culture often shows visible growth within two to five days, as the network is already organized and active.
Once established, the colonization rate varies dramatically by species, with full colonization generally taking one to three weeks. Fast-growing species, such as Oyster mushrooms (Pleurotus species), can cover a standard 90mm plate in five to ten days. Slower species, like Shiitake (Lentinula edodes), may require two to three weeks. Visually, growth begins as small, white specks that radiate outward in a circular pattern, eventually forming a dense, white, filamentous mat across the surface.
Key Variables Influencing Growth Speed
Several environmental and genetic factors contribute to the speed at which mycelium spreads across the agar medium. Temperature control is crucial, as most common mushroom species thrive within a mesophilic range. The optimal temperature for colonization typically falls between 70 and 75 degrees Fahrenheit (21 to 24 degrees Celsius). Temperatures outside this band significantly slow down growth and can increase the risk of contamination.
The specific formulation of the agar medium also affects colonization speed, primarily due to nutrient concentration. Carbohydrate-rich media, such as PDA and MEA, accelerate growth compared to low-nutrient options like plain water agar. The inherent vigor and genetics of the fungal strain also play a large role; a healthy, established culture grows faster and more aggressively than a weak or newly isolated one. Even the physical stiffness of the agar can influence speed, with some species growing faster on a more rigid medium because the hyphae spread horizontally across the surface.
Identifying and Addressing Contamination
Contamination by foreign organisms is the most common reason for delayed or halted mycelial growth. Bacterial contamination typically appears as shiny, slick, or slimy spots that are often yellowish or creamy white, sometimes pooling around the inoculation point. These colonies often appear as tiny, oily spots and lack the filamentous structure of mycelium. Molds like Trichoderma often start as a fast-growing white patch, easily mistaken for mycelium, before turning a characteristic bright or dull green once sporulation begins.
Maintaining a clean workspace, such as utilizing a flow hood or a Still Air Box, is the primary preventative action against contamination. If a plate shows contamination, the best course of action is usually to discard it to prevent the spread of spores, especially green molds. If contamination is localized, however, a small, clean piece of mycelium can be carefully isolated from the healthy growth’s leading edge and transferred to a new, sterile plate. This process allows for strain purification and continued cultivation without competing microorganisms.