Molds (or moulds, see spelling differences) include all species of microscopic fungi that grow in the form of multicellular filaments, called hyphae.[1] In contrast, microscopic fungi that grow as single cells are called yeasts. A connected network of these tubular branching hyphae has multiple, genetically identical nuclei and is considered a single organism, referred to as a colony or in more technical terms a mycelium.
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.
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There are thousands of known species of molds, which include opportunistic pathogens, saprotrophs, aquatic species, calders and thermophiles.[2] Like all fungi, molds derive energy not through photosynthesis but from the organic matter, inside of which they live. Typically, molds secrete hydrolytic enzymes, mainly from the hyphal tips. These enzymes degrade complex biopolymers such as starch, cellulose and lignin into simpler substances which can be absorbed by 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,[2] which may contain a single nucleus or be multinucleate. Mold spores can be asexual (the products of mitosis) or sexual (the products of meiosis); many species can produce both types. 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 like buildings, 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 other surfaces.
Some molds can begin growing at temperatures as low as 2°C. When conditions do not enable growth, molds may remain alive in a dormant state, within a large range of temperatures before they die. The many different mold species vary enormously in their tolerance to temperature and humidity extremes. Certain molds can survive harsh conditions such as the snow-covered soils of Antarctica, refrigeration, highly acidic solvents, and even petroleum products such as jet fuel.
Xerophilic molds use the humidity in the air as their only water source; other molds need more moisture.
Cultured molds are used in the production of foods, including:
The koji molds are a group of Aspergillus species, notably Aspergillus oryzae, that have been cultured in eastern Asia for many centuries. They are used to ferment a soybean and wheat mixture to make soybean paste and soy sauce. They are also used to break down the starch in rice (saccharification) in the production of sake and other distilled spirits.
Alexander Fleming's famous discovery of the antibiotic penicillin involved the mold Penicillium chrysogenum.
Several cholesterol-lowering drugs (such as Lovastatin, from Aspergillus terreus) are derived from molds.
The immunosuppressant drug cyclosporine, used to suppress the rejection of transplanted organs, is derived from the mold Tolypocladium inflatum.
Other molds are cultivated for their ability to produce useful substances. Aspergillus niger is used in the production of citric acid, gluconic acid and many other compounds and enzymes. The mold Aspergillus nidulans is an important model organism. Ashbya gossypii is used in industrial production of riboflavin and is further studied as a model organism.
Molds are ubiquitous in nature, and mold spores are a common component of household and workplace dust. However, when mold spores are present in large quantities, they can present a health hazard to humans, potentially causing allergic reactions and respiratory problems.
Some molds also produce mycotoxins that can pose serious health risks to humans and animals. Exposure to high levels of mycotoxins can lead to neurological problems and in some cases death. Prolonged exposure, e.g. daily workplace exposure, can be particularly harmful. The term toxic mold refers to molds that produce mycotoxins, such as Stachybotrys chartarum, and not to all molds in general.
Mold growth in buildings can lead to a variety of health issues. Various practices can be followed to mitigate mold issues in buildings, the most important of which is to reduce moisture levels that can facilitate mold growth. Removal of affected materials after the source of moisture has been reduced and/or eliminated may be necessary for remediation.