Mold

For other uses, see [[:the fungi known as molds. Slime molds and water molds are not fungi and are discussed in separate articles]].

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.

Biology

Mold covering a decaying peach over a period of six days. The frames were taken approximately 12 hours apart. There are 10 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. 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.

Uses

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.

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. Alexander Fleming's famous discovery of the antibiotic penicillin involved the mold Penicillium notatum.

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

Health effects

Light micrograph of the hyphae and spores of the human pathogen Aspergillus fumigatus

Some mycotoxins produced by molds are harmful to all people. Other mycotoxins cause immune system responses that vary considerably, depending on the individual. The duration of exposure is a key factor in triggering immune system response. Farm animals often die or suffer from mycotoxin poisoning. Mycotoxins resist decomposition from cooking, and remain in the food chain.

Mold spores can be allergenic. When inhaled, mold spores may germinate, attaching to cells along the respiratory tract and causing further problems in those with weak immune systems. One example is Stachybotrys chartarum which has been associated with sick building syndrome.

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

Growth in buildings

Although this home suffered only minor exterior damage from Hurricane Katrina, small leaks and inadequate air flow permitted this mold infestation.

Mold growing on carpet 2006-03-13.

Same mold as above on carpet, 4 months later - 2006-07-09.

Mold on a liquid surface

Mold colonies are commonly seen in damp areas such as basements and bathrooms, and can also grow inside building structures. Growth is fostered by moisture, especially following floods and water leaks. Mold may produce an identifiable smell. However, the main problem with the presence of mold in buildings is the inhalation of mycotoxins. After a flood or major leak, mycotoxin levels are higher in the building even after it has dried out (source: CMHC).

Food sources for molds in buildings include cellulose-based materials, such as wood, cardboard, and the paper facing on both sides of drywall, and all other kinds of organic matter, such as soap and dust. Carpet contains dust and other organic matter.

Indoor Mold needs four things to grow:

(1) Spores: Both our indoor and outdoor environment have mold spores present. There is no such thing as a mold free environment.
(2) Nutrients: The Spores need food, and in the indoor environment these are normally cellulose materials (wood, dust, fabrics, cardboard, paper, etc.)
(3) Moisture Moisture is required to begin the decaying process of the cellulose material by the mold.
(4) Time: Mold growth typically begins between 24 hours and 10 days from the provision of the growing conditions. There is no way to date mold.

If a house has excessive mold, there is usually too much moisture in the house, possibly from the basement, a leaking roof, or leaks from plumbing pipes behind the walls. Insufficient ventilation can also enable moisture build-up. The more people in a space, the more humidity builds up. This is from normal breathing and perspiring. Mold colonies often form where ventilation is poorest, and on perimeter walls, because they are coolest, thus closest to the dew point. If there are mold problems in a house only during certain times of the year, then it is probably either too air-tight, or too drafty. Mold problems occur in airtight homes more frequently in the warmer months (when humidity reaches high levels inside the house, and moisture is trapped), and occur in drafty homes more frequently in the colder months (when warm air escapes from the living area into unconditioned space, and condenses). If a house is humidified artificially during the winter, this can create conditions favorable to mold. Moving air may prevent mold from growing since it has the same desiccating effect as lowering humidity.

There are many ways to prevent mold growth; see ventilation issues in houses.

In extreme cases of mold growth in buildings, it may be easier to condemn the building rather than clean the mold to safe levels; see Deutsche Bank Building.

See also

• Indoor air quality
• Mildew
• Mold mite
• Slime mould
• Toxic mold
• Water Mould