The term dematiaceous applies to fungi that are dark in color. Many fungi causing human or animal infections belonging to this group are pale brown, dark brown or black. Clinical specimens obtained from the patients having fungal infections are processed for culture as well as examined under the microscope after staining appropriately by fungal stains. The fungal elements present in the direct smear of the clinical specimen are also examined for the presence of dark pigment in the fungal cell wall. Dark pigmented cell wall classifies fungus in dematiaceous group. In the absence of dark pigment in the cell wall of fungal elements, the structure of the fungal elements may provide some clues that a dark fungus may be involved. In this case, the direct smear prepared from the clinical specimen is stained by a special stain called Fontana Masson (FM) to observe melanin pigment in the fungal cell walls. Many fungi from this group are responsible for causing opportunistic infections (local or systemic) in a variety of patients especially the immunocompromised hosts.

There are unique terms Chromoblastomycosis and Phaeohyphomycosis used for the dark pigmented fungi, indicating the site specificity of certain fungi causing infections in patients infected by dark pigmented fungi.

Dematiceous Fungi Diagnostic Testing Flow Charts
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Fungi producing olive-grey, brown or black (melanin) pigment in the cell wall of fungal hyphae or conidia are classified as dematiaceous (dark) fungi. Most fungi in dematiaceous group produce grey to dark color from the very beginning as they first appear on culture media. However, some dark fungi may start off as white and pale at first, turning darker upon further incubation.

Fungi producing conidia enclosed within brown to dark colored round bodies such as pycnidia or producing sexual spores in ascocarp are not included among dematiaceous group.

In clinical microbiology laboratory, simple tests such as Gram stain and wet preparation may be very useful to suspect dark fungi during direct microscopic examination of the clinical specimen. Many times the shape of the fungal elements showing pale to dark boundary of the cell wall may indicate the presence of a dark fungus in the clinical specimen. It is important to demonstrate melanin pigment in suspected fungi and must be confirmed by a specific melanin stain such as Fontana Masson (FM). Early detection of phaeohyphomycotic agent present in the clinical specimen may provide relevant information to the clinician to select appropriate antifungal therapy for the patient.

Not all fungi showing pale darker cell wall in Wet Prep or Gram stain are true dematiaceous. Figs. 5 & 6 show structures that mimic darker cell wall and Fig. 7 is GMS x1000 showing darkness of septate hyphae (fungal elements). This darkness does not confirm the presence of melanin pigment since the end point of GMS is to stain all fungi as dark. This fact is proven in this example that Fig. 8 FM stain x1000 is negative for melanin pigment. The fungus in question is not dark in nature. This fungus was identified as Aspergillus terreus that belongs to hyaline group.

Some dark fungi do not display dark cell wall in Gram stained smear. Such fungi may be suspected as dark by morphology and confirmed by melanin stain. Fig. 9 Gram x1000 & Fig. 10 FF x400 do not show pigmented cell wall. However, morphology of the filamentous form appears to be different and not fitting into pseudohyphae or hyaline septate hyphae categories. Fig. 11 stained by FM x1000 showing brown pigmentation in the cell wall.