Indoor "Black Mold" Fungus Has Unexpected Relatives

Until now, scientists didn't know how to classify Stachybotrys chartarum, the black mold that can grow in humid, indoor environments and is often associated with "building sickness." But an Agricultural Research Service scientist recently found that the toxin- producing fungus comprises a brand-new family within the order Hypocreales.

ARS mycologist Lisa A. Castlebury discovered that Stachybotrys' closest relatives are actually members of the genus Myrothecium, fungi that cause serious diseases in crop plants and invasive weeds. To determine this relationship, Castlebury and her colleagues at the ARS Systematic Botany and Mycology Laboratory in Beltsville, Md., sequenced and analyzed five genes of Stachybotrys species and similar fungi.

Stachybotrys thrives on wet, cellulose-rich materials and can become airborne. It also produces metabolic byproducts known as mycotoxins. A class of these, known as macrocyclic trichocenes, are especially potent and have been linked to serious illness in humans and livestock.

Up to now, when checking for fungi that produce harmful mycotoxins, environmental engineers and others have generally tested houses and buildings for the presence of Stachybotrys alone. But, according to the new finding, a possible source of toxic macrocyclic trichocenes are also fungi of the genus Myrothecium.

Despite the concern surrounding the presence of toxin-producing fungi in homes, Stachybotrys and its relatives are relatively harmless when found in nature. It's when they occur inside of artificially airtight spaces, with an abundance of moist cellulosic materials, that the accumulating toxic chemicals can reach high concentrations.

In a natural setting, moist, densely cellulosic material is scattered, and mycotoxins produced by the fungi simply diffuse into the air.

ARS is the U.S. Department of Agriculture's chief in-house scientific research agency.