Molds are often made up of microscopic filaments called hyphae, which together form a web-like growth system that supports the mold, called a mycelium. The appearance of these hyphae can sometimes give clues to identify molds (septate - individual cells are visible due to divisions in the hyphae vs nonseptate - long continuous cells with no apparent divisions).
Examples of septate and nonseptate hyphae
Idenfitication of molds requires observation of these hyphae, of their reproductive asexual spores, and of their spore-forming structures. Asexual spore-forming structures generally fall under two types: conidia - where spores are produced and freely released, or sporangia - where spores are contained in a sac-like head. There is a large amount of variety in how these spores and structures appear under a microscope, and thus is ideal for helping distinguish species. When molds appear on food or decaying matter, the 'fuzzy' appearance is a large collection of these structures.
microscopic and macroscopic presentation of fungal asexual spores of a sporangium
As noted previously, some molds can also be dimorphic, and have both yeast and filamentous-like appearance.
Some molds have medical importance not just because of their capacity for disease, but their ability to produce compounds used in treatment of disease. For example, the mold Penicillium is responsible for the production of the antibiotic penicillin. Many other molds are known to also product anti-bacterial compounds.
penicillium culture inhibiting bacterial growth
When needing to study a mold, it is common to use methods that will not distort the shape of the hyphae or spore-forming structures, as well as clearly distinguish fungal structures from other debris. A 'contact' slide method is commonly used.
Contact slide method with tape
To properly visualize, mounting dye like lactophenol cotton blue is used. The ethanol in the mixture helps preserve fungal structures, while cotton blue specifically stains chitin found in fungal cell walls. Aspergillus stained with lactophenol cotton blue
Sample Solution