The rapidly growing dematiaceous molds are separated by those with conidia with transverse septae and those with conidia with transverse and longitudinal septae.
Bipolaris spp. are characterized by oval, transversely septate conidia that arise from bent (geniculate) conidiophores. The name Bipolaris is derived from the fact that these organisms produce germ tubes on both ends of their conidia in saline mounts that are incubated for 12-24 hours. Each conidium contains 3-5 septations.
Curvularia spp. have transversely septated conidia that curve distinctly when they mature.
Alternaria spp. produce chains of transverse and longitudinally septate (often described as muriform which resembles a brick wall) conidia that have alternating blunt and pointed ends.
The top right image shows a Curvularia hawaiiensis, formerly referred to as Bipolaris hawaiiensis lactophenol cotton blue preparation showing conidia that are septate, multicellular, and cigar-shaped. The bottom right image is a Gomori Methenamine-Silver stain (GMS) of Bipolaris spp. demonstrating numerous dark-walled, filamentous septate hyphae.
Images courtesy of the CDC.