What is Mold and How Does It Grow?

Mold is part of a group of organisms that are called fungi (plural for fungus), which are structurally more complex than are bacteria and viruses. There are more than 200,000 species of mold, some of which are large (e.g. toad stools, mushrooms, shelf brackets, puffballs, stinkhorns, etc.), and others which are smaller, often fuzzy/furry and are commonly seen on spoiled food and shower walls (e.g. Penicillium, Alternaria, Cladosporium, etc.).

Mold has thrived outdoors for eons of time, and serves a very useful function in nature. By breaking down dead plant materials into chemicals that can be recycled in nature, we are spared from having to live in a world where plant detritus would accumulate forever, burying us up to our necks or more!

Nearly all of the molds eat either living (pathogenic) or dead (saprobic) plants as the most common source of nutrients, but a few are also capable of infecting humans. As human pathogens, mold can infect the skin (e.g. athlete’s foot and ring worm) as well as invade systemically, causing lung, upper respiratory tract, eye infections and much more. Generally those at risk are either very old, very young, those who have had chemotherapy or who have taken immunosuppressive drugs, those with an immune system disease or those who have become immunocompromised by AIDS.

Along with house dust, pollen grains, dust mite droppings, and cat dander, mold spores are notorious for causing allergies and asthma, as they all become airborne and are readily inhaled. These allergens (antigens) cause the body to produce antibodies (immunoglobulin, class E), which try to eliminate the antigens from our respiratory system. But, in doing so, this process often causes the release of histamine, serotonin, etc., which trigger the symptoms commonly associated with hay fever.

Fungi (of which mold is a member) are complex organisms, having several forms of the same organism as well as both sexual and asexual life cycles. For the sake of simplicity, however, they can be likened to things we can relate to more easily – plants.

Mold spores can germinate to form mold colonies within approximately 24 hours when favorable growth conditions are encountered. Like plant roots which grow deeply into the soil, mold will also grow deeply into whatever it is living on (typically things derived from plants) for nutrition. The “roots” of molds are called hyphae, and penetrate throughout the substrate (food). Molds have no digestive organs – no mouth or gastrointestinal tract – consequently, the digestion takes place outside of the mold. Enzymes are secreted at the tips of the hyphae which break down large chemical substances into smaller ones that can be absorbed into the cell by diffusion or osmosis. Before this absorption can take place, however, these smaller chemicals must be dissolved in water – an abundance of water. Thus, the absence of water will cause mold to starve to death. Moreover, because of its absolute need for water, mold growth inside homes can be easily prevented by assuring that the structural materials are kept dry by preventing water spills and leaks.

Mold colonies thrive both outdoors and indoors, requiring not only abundant moisture and nutrients, but also need oxygen and the absence of both toxic substances and competition from other organisms in the environment. Under optimal growth conditions some molds will grow to maturity indoors in as little as five days, producing more mold spores, which function like seeds. Billions to trillions of spores are generated even in a relatively small infested area. However, unlike the mold hyphae, which are easy to kill and are buried deeply within the substrate (food), the spores are extremely hardy and are always produced at the surface (at the interface of the substrate and the air). Spores are self-contained units, which have sufficient nutrients and water inside, enabling them to remain viable for many decades (perhaps a hundred years or more) because they metabolize (rate of respiration) extremely slowly. They are difficult to kill because they have tough cell walls and have little response to poisons, being relatively dormant. Thus, spores remain viable even in the absence of environmental moisture, just waiting for favorable conditions to germinate and to produce a new mold colony. Moreover, mold spores are biologically designed to be easily disseminated (scattered), normally becoming airborne and traveling great distances before landing. As a consequence, it is possible for huge populations of spores to contaminate the entire residence well away from the original site of infestation. Thus, it is the spores, rather than the mold itself, which are the primary cause of respiratory tract diseases.