Mold is a type of fungus that appears as a fuzzy or discolored growth. A petri dish containing a nutrient-rich gel called agar provides an isolated, controlled environment for its study. Observing mold growth allows scientists to examine its structure, color, and texture, which are characteristics used for identifying different species. Culturing mold on this solid medium isolates and multiplies the fungal cells until they form a visible colony, making the fungus observable to the naked eye.
How Mold Colonies Form
Mold growth in a petri dish begins with a single, often invisible, spore landing on the agar surface. Once the spore senses favorable conditions, it germinates and begins to grow thread-like filaments known as hyphae. These hyphae extend outward and downward into the agar, forming a dense, branching network that absorbs nutrients.
This interconnected mass of hyphae is called the mycelium, representing the entire fungal body, though only a fraction is visible on the surface. As the mycelium expands, it forms a macroscopic growth referred to as a mold colony. The colony typically grows in a circular pattern, radiating outward from the initial point of spore germination as the hyphae continuously penetrate the fresh nutrient medium.
As the colony matures, some hyphae grow upward, away from the agar surface, to form specialized structures called aerial mycelium. These aerial structures are responsible for producing and releasing spores, which is the reproductive stage of the fungus. The appearance of these spore-producing structures marks the point when the colony’s color and texture become established for identification.
Decoding Visual Characteristics: Texture and Color
Mold appearance is described using terms related to the density and structure of the aerial mycelium and spores. A fuzzy or cottony texture, for instance, is caused by long, loose, and irregularly arranged aerial hyphae that give the colony a high, fluffy profile. In contrast, a velvety texture suggests a shorter, denser layer of hyphae, making the surface look like a fine, closely trimmed pile.
A powdery or granular appearance indicates a high concentration of mature spores on the surface, which are easily dislodged and give the colony a dry, dusty look. The color of the mold colony comes from the vast numbers of spores produced on the aerial hyphae. Various genera produce spores with distinct pigments, resulting in colors like green, blue, yellow, black, or even pink and white.
The reverse side of the petri dish, viewed through the agar, can display a color called reverse pigmentation. This color often results from pigments secreted by the mold into the agar medium itself, and it can be a different color than the surface growth. Observing the surface texture, color, and any reverse pigmentation provides a composite visual profile used to distinguish between different types of molds.
Common Examples of Mold Morphology
The structure and color of a mold colony often allow for preliminary identification of its genus. Penicillium species typically produce colonies that are velvety or powdery in texture, displaying a characteristic blue-green to grayish-green color. These colonies often have a distinct white border around the edge, where the hyphae are still actively growing but have not yet produced pigmented spores.
Aspergillus is another common genus with highly varied appearances, though many species exhibit a dense, velvety, or cottony texture. Their colors are diverse, ranging from deep black (such as Aspergillus niger) to shades of yellow, brown, or vibrant green. The structure of their spore-producing heads gives them a dense, sometimes columnar look.
The genus Rhizopus, often called bread mold, is known for its rapid growth and highly fuzzy or cottony appearance due to long, abundant aerial hyphae. Rhizopus colonies quickly fill the petri dish with an elevated, wispy growth. They often display a grayish-brown color when mature due to the formation of black sporangia, which are sacs containing the spores.
Distinguishing between these genera relies on noting the combination of growth rate, surface texture, and color displayed by the spore masses.
Safe Handling of Cultured Mold
Because a petri dish culture contains a high density of mold, it must be handled with caution. The lid should never be opened once mold growth is visible, as this releases concentrated spores into the air, which can be inhaled. It is important to keep the lid securely sealed, often with tape, to contain the biological material.
For disposal, the cultured dish is classified as biohazardous waste and should not be thrown into regular trash. In a laboratory setting, the entire dish is sterilized, typically by autoclaving, to kill all living organisms before final disposal. For home or educational experiments, a common method is to place the sealed dish in a plastic bag, add a disinfectant like bleach, seal it tightly, and dispose of the package according to local biohazard guidelines.