The dematiaceous fungal genus Bipolaris and its role in human disease

Abstract

Introduction Bipolaris is one of several genera of dematiaceous (darkly pigmented) fungi causing phaeohyphomycosis in animals as well as in humans (1-3). Species in this genus have only recently been recognized as having major clinical roles in phaeohyphomycosis and allergic bronchopulmonary disease in humans. Contributing to this delay in their recognition, even when they were isolated in culture, was the original misclassification of members of this genus as either Helminthosporium or Drechslera. McGinnis and colleagues demonstrated that isolates previously reported in the literature us these two latter genera were actually Bipolaris or its relative Exserohilum. A second delaying factor was the histopathologic practice of basing fungal identification on microscopic findings alone. Hyphae seen on histologic sections that did not resemble the coenocytic (nonseptate) hyphae of the zygomycetes and that were not darkly pigmented were commonly reported as aspergiUus without confirmarion by culture. Thus, it is only within the past decade that the spectrum and extent of the association of certain species of Bipolaris with human disease have become appreciated. Mycology Originally, isolates of the genera Drechslera, Exserohilum, and Bipolaris were lumped in the genus Helminthosporium, which was traditionally considered an environmental contaminant (1). However, isolates within this group can be separated by the characteristic development of their conidiophores (spore-bearing hyphae) and by morphology of their conidia (asexual spores). Initially, those isolates with conidiophores ceasing to grow after terminal conidial formation (determinate conidiophores) were retained in the genus Helminthosporium, while isolates whose conidiophores continued increasing in length through formation of new growing points under each terminal conidium (sympodial growth, indeterminate development) were all segregated in the genus Drechslera (1,4). Because of their sympodial manner of growth, the conidiophores in this group of fungi are frequently characterized as having a geniculate (zig-zag or bent-knee) appearance (Fig. la and b). It was later recognized that these species of Drechslera could be further separated into two additional genera, Bipolaris and Exserohilum, based on the morphology of their conidia (shape, size, position, and nature of septal divisions, and position and size of the hilum, which is the scar on the conidium that marks the latter's attachment point to the conidiophore) (1,4). Early growth of Curvularia spp. may also resemble that of Bipolaris app. Although slide cultures may be performed, differential morphology of these genera can be readily observed using adhesive tape preparations or tease mounts. If necessary, conidial germination may be observed by direct examination of slide cultures before removing the cover slip. Alternatively, an aqueous suspension of conidia may be incubated at ambient temperature for 8 to 24 h and aliquots observed microscopically at intervals for germination (1). The genus Bipolaris is characterized by egg-shaped or ellipsoidal-to-fusoid, multicelled conidia with a la'uncate hilum. The cells within the conidia are divided by distosepta (pseudosepta), of which only the polar cells germinate axially from one or both ends of the conidia and typically from the hilar end