Aspergillus spp. are hyaline septate molds found in soil, decaying organic matter or as plant pathogens. We all regularly breathe in the conidia of Aspergillus spp., but those with weakened immune systems and lung disease are at risk for disease. Of the hundreds of recognized species, only about a dozen are human pathogens. Aspergillus fumigatus, Aspergillus flavus and Aspergillus niger account for the majority of cases of human aspergillosis.
The typical hyphae of the Aspergillus spp. are 3 to 6 µm in width, septate, and dichotomously branched at 45 degree angles. The invasive hyphae are often oriented in parallel arrays as grow throughout tissues. Much of the tissue damage that occurs in invasive aspergillosis results from infarction consequent to vascular invasion and thrombosis.
Fruiting heads (vesicles covered with phialides and conidia) may be seen in air filled tissue pockets.
The top right image is a diagram of Aspergillus. The bottom right image is an H&E stain demonstrating acute-angle branching at 45 degrees.
Images courtesy of the CDC.