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The colloquial term mold (or mould; see spelling differences) is applied to a large and taxonomically diverse number of fungal species where their growth results in a moldy appearance of objects, especially food. The objects become discolored by a layer of fungal growth. Molds are fungi that grow in the form of multicellular filaments called hyphae. A connected network of these tubular branching hyphae, called a mycelium, is considered a single organism. The hyphae are generally transparent, so the mycelium appears like very fine, fluffy white threads over the surface. Cross-walls (septa) may delimit connected compartments along the hyphae, each containing one or multiple, genetically identical nuclei. The dusty texture of many molds is caused by profuse numbers of asexual spores conidia formed by differentiation at the ends of hyphae. The mode of formation and shape of these spores is traditionally used to classify the mold fungi. Many of these spores are colored, making the fungus much more obvious to the human eye at this stage in its life-cycle. In contrast, fungi that can adopt a single celled growth habit are called yeasts.
Molds are considered to be microbes and do not form a specific taxonomic or phylogenetic grouping, but can be found in the divisions Zygomycota and Ascomycota. In the past, most molds were classified within the Deuteromycota. Molds cause biodegradation of natural materials, that can be unwanted when it becomes food spoilage or damage to property. They also play important roles in biotechnology and food science in the production of various foods, beverages, antibiotics, pharmaceuticals and enzymes. Some diseases of animals and humans can be caused by molds, usually as a result of allergic sensitivity to their spores or caused by toxic compounds produced as molds grow.
There are thousands of known species of molds which have diverse life-styles including saprotrophs, mesophiles, psychrophiles and thermophiles and a very few opportunistic pathogens. They all require moisture for growth and there are some aquatic species. Like all fungi, molds derive energy not through photosynthesis but from the organic matter on which they live, utilising heterotrophy. Typically, molds secrete hydrolytic enzymes, 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 synthesise mycotoxins and siderophores which, together with lytic enzymes, inhibit the growth of competing microorganisms.
Molds reproduce through producing very large numbers of 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. Mold spores may remain airborne indefinitely, may cling to clothing or fur or may be able to survive extremes of temperature and pressure.
Although molds can grow on dead organic matter everywhere in nature, their presence is visible to the unaided eye only when mold colonies grow. A mold colony does not consist of discrete organisms but of an interconnected network of hyphae called a mycelium. All growth occurs at hyphal tips, with cytoplasm and organelles flowing forwards as the hyphae advance over or through new food sources. Nutrients are absorbed at the hyphal tip. In artificial environments such as 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.
Few molds can begin growing at 4 °C (39 °F), the temperature within a typical refrigerator, or less. When conditions do not enable growth to take place, 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, anti-bacterial soap 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.
Common molds 
Food production 
The Kōji (麹) molds are a group of Aspergillus species, notably Aspergillus oryzae, and secondarily A. sojae, 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. Koji molds break down the starch in rice, barley, sweet potatoes, etc., a process called saccharification, in the production of sake, shōchū and other distilled spirits. Koji molds are also used in the preparation of Katsuobushi.
Red rice yeast is a product of the mold Monascus purpureus grown on rice, and is common in Asian diets. The yeast contains several compounds collectively known as monacolins, which are known to inhibit cholesterol synthesis. A study has shown that red rice yeast used as a dietary supplement, combined with fish oil and healthy lifestyle changes, may help reduce "bad" cholesterol as effectively as certain commercial statin drugs.
Other molds that have been used in food production include:
- Fusarium venenatum – quorn
- Geotrichum candidum – cheese
- Neurospora sitophila – oncom
- Penicillium spp. – cheese
- Rhizomucor miehei – rennet for making vegetarian and other cheese
- Rhizopus oligosporus – tempeh
- Ustilago maydis – filling in tortilla-based foods
Pharmaceuticals from molds 
Alexander Fleming's famous discovery of the antibiotic penicillin involved the mold Penicillium, although the species identity is disputed (Penicillium notatum, Penicillium chrysogenum or Penicillium rubens).
Howard Florey, Ernst Chain, Norman Heatley, Edward Abraham and teams of scientists in the UK and USA developed industrial-scale production of penicillin between 1941–45 and arguably started the use of antibiotics in medicine.
Health effects 
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. Mycotoxins include aflatoxins, ochratoxins, fumonisins, trichothecenes, citrinin, and patulin, among others.
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 home 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. These toxic properties may also be used to the benefit of humans e.g. penicillin from Penicillium and so on. In low doses these toxins that could otherwise be deadly can be controlled to our benefit to fight off infection.
Mold in the home can usually be found in damp, dark or steamy areas e.g. bathroom or kitchen, cluttered storage areas, recently flooded areas, basement areas, plumbing spaces, areas with poor ventilation and outdoors in humid environments. Symptoms caused by mold allergy are watery, itchy eyes, a chronic cough, headaches or migraines, difficulty breathing, rashes, tiredness, sinus problems, nasal blockage and frequent sneezing.
Growth in buildings and homes 
Mold growth in buildings can lead to a variety of health problems. 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.
See also 
- Morgan, Mike. "Moulds". Microscopy UK. Retrieved 26 June 2012.
- Moore D; Robson GD; Trinci APJ (editors). (2011). 21st Century Guidebook to Fungi (1st ed.). Cambridge University Press. ISBN 978-0521186957.
- Madigan M; Martinko J (editors). (2005). Brock Biology of Microorganisms (11th ed.). Prentice Hall. ISBN 0-13-144329-1. OCLC 57001814.
- Chiba University, Japan. "Fungus and Actinomycetes Gallery". Chiba University Medical Mycology Research Center. Retrieved 26 June 2012.
- Hibbett DS, Binder M, Bischoff JF, Blackwell M, Cannon PF, Eriksson OE, et al. (2007). "A higher level phylogenetic classification of the Fungi" (PDF). Mycological Research 111 (5): 509–547. doi:10.1016/j.mycres.2007.03.004. PMID 17572334.
- Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 633–8. ISBN 0-8385-8529-9.
- "Red yeast rice (Monascus purpureus)". Mayo Clinic. 2009-09-01. Retrieved 2010-02-01.
- "Study: Red Rice Yeast Helps Cut Bad Cholesterol". National Public Radio. 2008-07-01. Retrieved 2010-02-01.
- L. H. Stahnke, L. O. Sunesen; Stahnke, L.H (2003-11). "Mould starter cultures for dry sausages—selection, application and effects". Meat Science 65 (3): 935–948. doi:10.1016/S0309-1740(02)00281-4. PMID 22063673. Retrieved 2008-06-06.
- "The Nobel Prize website". Retrieved 27 June 2012.
- Wikipedia. "Statins". Wikipedia. Retrieved 13 June 2012.
- ACS Chemistry for Life. "The discovery of penicillin". Retrieved 27 June 2012.
- Money, Nicholas (2004). Carpet Monsters and Killer Spores: A Natural History of Toxic Mold. Oxford, UK: Oxford University Press. p. 178. ISBN 0-19-517227-2.
- Indoor Environmental Quality: Dampness and Mold in Buildings. National Institute for Occupational Safety and Health. August 1, 2008.
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