The Fungus

Stachybotrys chartarum (Ehrenb. ex Link) Hughes (synonyms= S. atra, S. alternans) was first described as S. atra by Corda in 1837 (5) from wallpaper collected in a home in Prague. It is a member of the Deuteromycetes, order Moniliales, family Dematiaceae, and is common on plant debris and in soil. The taxonomic treatment of the genus by Jong and Davis (38) is a good reference on identification while Hintikka (27) provides general information on biology. The fungus grows well on common mycological media such as potato dextrose, V-8 or cornmeal agar, and sporulates profusely forming dark masses of conidia. The fungus is relatively easy to identify because of the unique phialides of the genus and conidial morphology of the species. Conidiophores are determinate, macronematous, solitary or in groups, erect, irregularly branched or simple, septate, dark olivaceous, and often rough walled on the upper part. The phialides are large, 9-14 µm in length, in whorls, ellipsoid, olivaceous, and often with conspicuous collarettes. Conidia are ellipsoidal, unicellular, 7 to 12 by 4 to 6 µm, dark brown to black and often showing a ridged topography when mature. The ridged nature is readily apparent with scanning electron microscopy, but can also be observed with an oil immersion lens at 1000x. On lower power the spores appear verrucose. Young spores and some mature spores may be smooth. The phialides produce conidia singly and successively into a slime droplet that covers the phialides. Eventually the slime dries and the conidia are covered with the slime residue and remain on the conidiophore as a mass or ball of spores . The spores are therefore not readily disseminated in the air compared to other fungi such as Aspergillus. However, when the fungus and substrate dries and is disturbed by mechanical means or air movement, conidia can become bioaerosols. A genus similar to Stachybotrys, but with spores in chains is Memnoniella (38); it also has species that produces trichothecenes (35). Haugland et al. (21) have proposed relegation of Memnoniella to synonymy with Stachybotrys based on morphological characteristics and comparative sequence analysis of the nuclear ribosomal RNA operon.

S. chartarum growing on natural or man made substrates can often be identified by a person familiar with its growth pattern. However, there are some very dark dematiaceous Hyphomycetes which look similar, therefore microscopic examination of the fungus is needed to confirm identification. When the fungus is actively growing, the characteristic phialides and conidia are easy to observe, but when dry, the phialides collapse, are more difficult to observe, and emphasis must be placed on morphology of conidia. Although the traditional method of identification is based on morphology of the sporulating structures, PCR primers specific for S. chartarum are reported and may now be used in commercial microbiological laboratories to identify this fungus (7,20,58). A PCR product analysis using a fluorogenic probe has also been developed to quantify conidia of S. chartarum and can be used in the analysis of samples from mold contaminated indoor environments (22,52,58).

The fungus is strongly cellulolytic and will grow under conditions of low nitrogen. A simple way to grow the fungus is to streak some conidia onto wet Whatman filter paper in a petri dish and within a week spores are produced. If spores are placed on a small ridge made in the paper, the conidiophores will grow at an angle and allow a side view of conidial formation with a stereoscope. This is a convenient method to determine if spores are in chains to distinguish Stachybotrys from Memnoniella. Also, the filter paper method will allow isolation of S. chartarum away from many other fast-growing, but non cellulolytic fungi that would out-compete S. chartarum on rich media.