Molds (or moulds, see spelling differences) include all species of microscopic fungi that grow in the form of multicellular filaments, called hyphae. In contrast, microscopic fungi that grow as single cells are called yeasts. Molds do not form a specific taxonomic or phylogenetic grouping, but can be found in the divisions Zygomycota, Deuteromycota and Ascomycota. Although some molds cause disease or food spoilage, others are useful for their role in biodegradation or in the production of various foods, beverages, antibiotics and enzymes.
There are thousands of known species of molds, which include opportunistic pathogens, exclusive saprotrophs, aquatic species and thermophiles. Like all fungi, molds derive energy not through photosynthesis but from the organic matter on which they live. Typically, molds secrete hydrolytic enzymes from predominantly the hyphal tips. These enzymes degrade complex biopolymers such as starch, cellulose and lignin into simpler substances which can enter the hyphae. In this way, molds play a major role in causing decomposition of organic material, enabling the recycling of nutrients throughout ecosystems. Many molds also secrete mycotoxins which, together with hydrolytic enzymes, inhibit the growth of competing microorganisms.
Molds reproduce through small spores. Mold spores can be asexual (the products of mitosis) or sexual (the products of meiosis), and many species can produce both types. They may contain a single nucleus or many. Some can remain airborne indefinitely, and many are able to survive extremes of temperature and pressure.
Although molds grow on dead organic matter everywhere in nature, their presence is only visible to the unaided eye when mold colonies grow. A mold colony does not comprise discrete organisms, but an interconnected network of hyphae called a mycelium. Nutrients and in some cases organelles may be transported throughout the mycelium. In artificial environments, humidity and temperature are often stable enough to foster the growth of mold colonies, commonly seen as a downy or furry coating growing on food or surfaces. Thus buildings, being stable environments, enable mold proliferation.
Some molds can begin growing at temperatures as low as 2°C. When conditions do not enable growth, molds can remain alive in a dormant state, within a large range of temperatures before they die. This explains how molds can survive harsh conditions such as containers in refrigerators or inside building structure cavities.
Xerophilic molds use the humidity in the air as their only water source; other molds need more moisture.