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How Mold Grows
by William B. Rose, in his book Water In Buildings: An Architect’s Guide to Moisture and Mold, pp. 236-237

Mold spores are the principal means of mold reproduction, and most mold spores are transported in the air, in order to find new sites for colonization. Spores may carry toxins capable of disposing of previous colonizing mold organisms of other mold species. Penicillium mold, for example, may poison bacteria in the neighborhood with penicillin. Stachybotrys spores may…contain toxins such as tricothecenes to wipe out the indigenous local bacteria or simpler fungi.

All organisms digest food; mold does its digestion outside the organism itself. It excretes enzymes capable of breaking down long-chain hydrocarbons. Those enzymes need to travel by diffusion in a thin water film to the food molecules, which they break down into simpler sugars and starches. These water-soluble simpler compounds then diffuse outward from the site where they were created, and some of them migrate back to the hungry mold organism. Mold relies to a great extent on having a thin film of water several molecules thick that can transport the enzymes outward, but not too far, and transport the broken-down compounds back to the organism. The film, of course, is nothing like a flat membrane because the surface of materials at the microscopic level is mountainous and cavernous. The mold organism grows hyphae, which are thin strands the tips of which are capable of conducting the same reactions where they touch the upholstery (or other cellulose-based surfaces) on which the organism resides. The strands of hyphae form a mat called a mycelium. The mycelium creates a buffer that helps to regulate the wetness of the surface. It retards evaporation of the surface, helping to guarantee a stable moisture film necessary for digestion. Mold needs an edible substrate, air, and a multi-molecular film at the surface.

The conditions for initial mold growth on a surface may be quite different from the conditions under which growth continues,, or under which growth recurs on affected but scrubbed surfaces. Generally, initial growth requires higher levels of moisture than is required for continued growth. Mold continues to grow on surfaces:

►Where the moisture content of the substrate provides a dependable water source.
►Where the air humidity is high. This ensures that the water film is thick enough to permit diffusion of enzymes outward and simple hydrocarbons back to the organism.
►Where there is no water flow on the surface.
►Where sufficient food can be found.
►Where there are no significant inhibiting chemicals or treatments.

Different (mold) organisms require different thicknesses of films. The phylloplane (leaf-inhabiting) fungi, such as Penicillium and Cladosporium, get by with rather thin films. Thehy can thrive where the relative humidity in the air is 80% or greater. Stachybotrys requires more water and a thicker film; it requires a surface relative humidity around 95% or greater.

 
[How Mold Grows]

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