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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 the same DNA 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.

Mold covering a decaying peach over a period of six days. The frames were taken approximately 12 hours apart. There are 12 frames of changes.

Moldy nectarines that were in a refrigerator. The nectarine with black mold is also affecting the nectarine underneath.

There are thousands of known species of molds, which include opportunistic pathogens, exclusive saprotrophs, aquatic species and thermophiles.^[2] 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,^[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 surfaces.

Some mold 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.

Stilton cheese contains edible mold.

Cultured molds are used in the production of foods including cheese (Penicillium spp), tempeh (Rhizopus oligosporus), Quorn (Fusarium venenatum), the black tea pu-erh and some sausages.

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 notatum.

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 molds Neurospora crassa and Aspergillus nidulans are commonly used model organisms.

Main article: Mold health issues

Mold spores can be allergenic, causing irritations of eye, nose, throat, and lungs.

Molds excrete liquids or gases as defecatory matter; not all can be detected by smell. Some molds generate toxic liquid or gaseous compounds, called mycotoxins. Molds that produce mycotoxins are sometimes referred to as toxic molds. Of these molds, some only produce mycotoxins under specific growing conditions. Mycotoxins are harmful or lethal to humans and animals when exposure is high enough.

Dermatophytes are the parasitic fungi that cause skin infections such as Athlete's foot and Jock Itch. Most dermataphyte fungi take the form of a mold, as opposed to a yeast, with appearance (when cultured) that is similar to other molds.

Infection by opportunistic pathogen molds such as Penicillium marneffei and Aspergillus fumigatus is a common cause of death among immunocompromised people, including AIDS victims.

Main articles: Mold growth, assessment, and remediation and Indoor air quality

Mold growth in buildings can lead to variety of health issues. Various practices can be followed to mitigate mold growth in buildings.

Wikimedia Commons has media related to:

Mold

• Mildew
• Mold mite
• Slime mould
• Stachybotrys
• Oomycete
• Bioaerosol
• Indoor air quality

1. ^ Madigan M; Martinko J (editors). (2005). Brock Biology of Microorganisms, 11th ed., Prentice Hall. ISBN 0131443291. 
2. ^ ^{a b} Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology, 4th ed., McGraw Hill, pp. 633–8. ISBN 0838585299. 

• Environmental Protection Agency - Mold Homepage