Penicillium is a widespread genus of mold, recognizable by its fuzzy, often colorful colonies found on spoiled food, damp materials, and in specialized cheeses. While shades of blue-green are most common, the range is broad, extending to yellows, grays, and pinkish-browns. This color variation is determined by the mold’s intrinsic biology—its genetics and stage of development—and the external environment in which it grows.
The Physical Source of Penicillium Color
The color observed in Penicillium mold is not held within the main body of the organism, which is a network of thread-like filaments called mycelium. Instead, the visible color comes from the asexual spores, known as conidia. These spores are the mold’s reproductive units, produced in massive numbers, giving the colony its characteristic powdery or velvety texture.
Conidia are formed in dry chains on specialized structures called conidiophores, which resemble a tiny brush or broom, giving the genus its name from the Latin word penicillus. The color is derived from pigmented compounds, such as melanins and polyketides, deposited directly into the walls of these spores. When these pigmented spores are produced, their mass effect becomes the dominant visual characteristic of the mold colony.
How Species and Maturity Influence Appearance
Color differences in Penicillium are due to the genetic programming of the species. Different species are encoded to produce distinct pigment molecules, resulting in a wide array of colony colors. For example, Penicillium roqueforti, used for blue cheese, produces a characteristic dull or grayish-green color. Penicillium chrysogenum, famous for producing penicillin, typically displays a blue-green color, often with a yellowish tint.
The maturity of the colony is a second factor influencing appearance. When the mold first begins to grow, it consists only of the colorless, vegetative mycelium, making the colony appear white or pale. The true color only appears and intensifies as the mold matures and produces vast quantities of pigmented conidia. This means a young colony may look white, while an older colony of the same species displays its genetically determined color.
Environmental and Substrate Factors Causing Variation
Beyond genetics and maturity, the external conditions where the mold grows play a substantial role in pigment production, leading to color shifts even within the same species.
Substrate and Nutrients
The material the mold feeds on, known as the substrate, directly impacts the color because available nutrients are precursors for the pigment compounds. If key nutrients are limited, the mold might not synthesize its full complement of pigments, resulting in a paler or less vibrant colony. For instance, a low concentration of phosphorus was shown to induce a yellow discoloration in Penicillium caseifulvum, illustrating how a specific nutrient deficiency can alter the intended color.
pH Levels
The chemical environment, particularly the pH of the substrate, is another influential factor. The acidity or alkalinity of the growth medium can chemically alter the structure of the pigment molecules, causing a visible color change. A species might produce a pigment that appears blue under neutral conditions, but if grown on a highly acidic substrate, the pigment’s chemical structure can shift, causing the color to appear green or yellow instead. This pH-dependent color change explains why the same mold species can look different on various foods or materials.