Molds include all species of microscopic fungi that grow in the form of multicellular filaments, called hyphae.
Microscopic fungi that grow as single cells are called yeasts.
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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.
There are thousands of known species of molds, which include opportunistic pathogens, saprotrophs, aquatic species, and thermophiles. Like all fungi, molds derive energy not through photosynthesis but from the organic matter in 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, 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.
Many molds can begin growing at 4 °C (39 °F), the temperature within a typical refrigerator, or less. When conditions do not enable growth, molds may remain alive in a dormant state depending on the species, 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: cheese (Penicillium spp.), tempeh (Rhizopus oligosporus), oncom (Neurospora sitophila), Quorn (Fusarium venenatum), bread, sausages, soy sauce.
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.
Molds are ubiquitous in the biosphere, and mold spores are a common component of household and workplace dust. However, when mold spores are present in unspecified quantities, they can present a health hazard to humans, potentially causing allergic reactions and when present in large quantities, other types of respiratory problems. Mold allergies have always been a serious problem for many people. Research in the field of environmental health has yielded tests such as the MELISA assay, which can identify whether a person is allergic to specific epitopes associated with any particular mold. If a person is found to be allergic, the remedies listed below are often helpful in reducing allergic reaction.
Some molds also produce mycotoxins that can pose serious health risks to humans and animals. Some studies claim that exposure to high levels of mycotoxins can lead to neurological problems and in some cases death. Prolonged exposure, e.g. daily workplace exposure, may be particularly harmful. Research on the health effects of mold has not been conclusive. The term toxic mold refers to molds that produce mycotoxins, such as Stachybotrys chartarum, and not to all molds in general.
Mold assessment and mold remediation are techniques used in occupational health: mold assessment is the process of identifying the location and extent of the mold hazard in a structure, and mold remediation is the process of removal and/or cleanup of mold from an indoor environment.
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.
Rhizopus stolonifer (black bread mold) is a widely distributed thread-like Mucoralean mold. Commonly found on bread surfaces, it takes food and nutrients from the bread and causes damage to the surface where it lives.
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