Mold

Microscopic photo of the hyphae and spores of the mold Aspergillus fumigatus

Moldy bread

Moldy nectarines

Molds (American English) or moulds (British English) are microscopic, multicellular fungi. They are generally composed of hyphae (filamentous structures) that are usually separated from each other by divisions called septa. They are genetically similar to yeasts (single-celled fungi).

Fungi (singular fungus), which also include mushrooms, mildews, and yeast, make up one of the kingdoms in biological classification and are eukaryotic organisms that digest their food externally and absorb the nutrient molecules into their cells. Fungi differ from bacteria in having a cell nucleus; from plants in not making their own food through photosynthesis; and from animals in not being mobile and in absorbing nutrition externally rather than internally.

Molds do not form a specific taxonomic grouping. They can be found in the divisions Zygomycota, Deuteromycota, and Ascomycota. Xerophilic molds use the humidity in the air as their only water source; other molds need more moisture. The molds Neurospora crassa and Aspergillus nidulans are commonly used model organisms.

Slime molds and water molds are not fungi, and thus not true molds. Slime molds (or slime moulds) are peculiar protists that normally take the form of amoebae, but under certain conditions develop fruiting bodies that release spores, superficially similar to the sporangia of fungi. Water molds (or water moulds) or oomycetes are a group of filamentous, unicellular protists, physically resembling fungi.

Growth

There are thousands of known species of molds. Their primary energy source is organic matter, which is broken down by enzymes released from the mycelia (the mass of hyphae) into simpler compounds. By decomposing organic matter, molds play a big part in material biodegradation, enabling decay and rot necessary in all ecosystems. The enzymes and mycotoxins can also inhibit the growth of other molds and microorganisms. Some mycotoxins are considered to be harmful to health. Adequate humidity and temperature are needed for optimal growth. Molds do not use photosynthesis to receive energy.

Molds reproduce through small spores, which can remain airborne indefinitely. Spores are able to survive extreme temperatures and pressure. 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. Moving air may prevent mold from growing since it has the same desiccating effect as lowering humidity.

Although molds grow on organic matter everywhere in nature, their presence is only visible to the unaided eye where conditions allow mold colonies to grow. In man-made 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.

Stilton cheese contains edible mold.

Although the presence of mold may indicate unwanted decomposition, some molds are cultivated deliberately for their byproduct compounds. Examples include making certain types of moldy cheese, and for the production of antibiotics derived from their natural defenses against bacteria. Alexander Fleming's famous discovery of the antibiotic penicillin came from the mold Penicillium notatum.

Growth in buildings

Mold growing on carpet (March 13, 2006).

Same mold as above on carpet, 4 months later (July 9, 2006)).

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

File:Mouldonliquidsurface.jpg

Mold on a liquid surface

Mold colonies are commonly seen in damp areas, such as basements and bathrooms. Mold colonies also grow inside building structures. Growth is fostered by the moisture in a building's structure, especially following floods and water leaks. Sometimes its presence is seen on wall surfaces; it may produce an identifiable smell. 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). All organic matter is a food source for mold; soap, paper, wood, dust are just a few examples.

• 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 cause moisture build-up from normal human activities such as taking showers and baths and cooking. 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. There are many ways to prevent mold growth; see ventilation issues in houses.

In extreme cases of mold growth in buildings, it is easier to condemn the building rather than clean the mold to human-safe levels. See Deutsche Bank Building.

Uses

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. Soy sauce and miso production uses the mould Aspergillus oryzae and other micro-organisms to ferment a soybean and wheat mixture.

Health effects

Some mycotoxins produced by molds are known to be harmful to health in 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.

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.

See also

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

References

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