Mold is a common term for various types of fungi, which reproduce by releasing spores into the air. Cultivating mold in a petri dish provides a contained environment to observe the growth, structure, and diversity of these organisms. This experiment is a practical way to explore mycology, allowing for the controlled study of how fungi thrive on a nutrient source under specific conditions. The process requires careful preparation to ensure the mold grows successfully and safely. Successfully growing mold starts with creating the perfect nutrient-rich base for the spores to germinate and colonize.
Preparing the Growth Medium
The foundation of any successful mold culture is the growth medium, which provides the necessary water, carbon, and nitrogen sources for the fungi to develop. For a home science experiment, a simplified nutrient base can be made using common kitchen ingredients like unflavored gelatin, beef bouillon, and sugar. This mixture is an accessible alternative to complex laboratory-grade agar, acting as the solidifying agent to hold the nutrients in place.
To prepare the medium, dissolve a bouillon cube, a teaspoon of sugar, and a packet of gelatin in hot water (typically around 250 milliliters). The sugar serves as a carbon source, while the beef bouillon provides the proteins and nitrogen compounds fungi metabolize for growth. The mixture must be gently heated and stirred until all components, particularly the gelatin, are completely dissolved, creating a homogenous liquid.
Once dissolved, the medium must be cooled slightly to a temperature that is warm to the touch (typically around 110–120°F). This temperature prevents the plastic petri dishes from melting but keeps the medium liquid enough to pour easily. Pouring should be done in a clean, still-air environment to minimize the chance of introducing unwanted environmental contaminants.
Carefully pour a thin layer of the liquid medium into the bottom of each petri dish, covering the base to a depth of about 1/8 to 1/4 of an inch. Immediately replace the lid to prevent airborne spores from landing prematurely. Allow the dishes to cool undisturbed on a flat surface for several hours until the gelatin has solidified into a firm, jelly-like consistency.
Inoculation and Sealing the Dish
Once the nutrient medium has solidified, the process of inoculation, or introducing the mold spores, can begin. One simple method is to briefly open the petri dish to the air in an area where mold spores are abundant, such as near a window or in a kitchen. This technique, known as passive inoculation, allows naturally occurring airborne spores to settle directly onto the nutrient surface.
For a more targeted approach, use a sterile cotton swab to sample a specific surface, such as stale bread, a damp area, or an air filter. Gently rub the swab in a zigzag pattern across the surface of the solidified medium to transfer the collected spores. The goal is to lightly deposit the material without tearing the delicate gelatin surface.
After inoculation, immediately and securely place the lid back onto the petri dish. The next step is to seal the dish completely with a thin strip of clear tape, wrapping it around the perimeter where the top and bottom meet. This sealing process serves two purposes: it prevents further contamination from the external environment, and it ensures that the spores remain safely contained within the dish, preventing their release into the air once the mold grows.
Incubation Conditions and Observation
After the dishes are sealed, place them in an environment that encourages fungal growth. Mold prefers warm, dark, and humid conditions, similar to its natural habitat. A room temperature range between 70°F and 85°F (21°C to 29°C) is optimal for the rapid growth of many common mold species.
The sealed dishes should be placed in a secluded area away from direct sunlight, which can inhibit growth, and stored upside down. Storing the dishes inverted prevents condensation that forms on the lid from dripping onto the growth medium. This dripping could cause the mold colonies to spread and merge. Visible mold growth should appear within three to seven days, depending on the spore concentration and ambient temperature.
As the mold develops, look for distinct colonies, noting the variety of colors and textures that emerge (e.g., fuzzy white, powdery green, or velvety black patches). Observations should be recorded daily, documenting the size, shape, and appearance of each colony. Never open the sealed dish during observation. This visual record allows for an analysis of how different environmental fungi compete and thrive on the nutrient source.
Safe Handling and Disposal
Safety protocols must be followed strictly, as the mold cultivated is a concentrated culture of microorganisms whose identity and potential health effects are unknown. Once visible growth is established, the sealed petri dish must never be opened, as this would release a high concentration of spores into the air. All observations and measurements must be performed while the dish remains completely sealed.
Upon completion of the experiment, the entire culture must be sterilized before disposal to ensure the microorganisms are deactivated. A simple method for home experiments is to flood the sealed dish with a household sterilizing agent. Place the dishes into a resealable plastic bag, and add a small amount of a 10% bleach solution (one part bleach to nine parts water) to the bag.
The sealed bag containing the dishes and the bleach solution should be left for at least 24 hours to ensure the bleach penetrates and kills all fungal growth. After this sterilization period, the entire sealed bag can be safely double-bagged and placed into the household trash. Thorough handwashing with soap and water should follow any contact with the petri dishes, even when sealed, to maintain hygiene.