[6] Microscopically, the fungus is characterized by elongated conidial heads with bent Hülle cells scattered throughout the pigmented mycelium.

One of the first metabolites to be isolated and characterized from this fungus, ustic acid (C11H7O7), reacts with Iron (III) chloride to form a deep purple-colored compound.

[8] Aspergillus ustus has been found on the surfaces of walls of caves and in indoor air of buildings[8] including hospitals,[3][9] soils and bat dung.

[3] It only rarely found to cause serious infection (e.g., endocarditis, pneumonia, disseminated disease), typically as an opportunist in severely immunocompromised people, often secondary to immunosuppressive chemotherapy following hematopoietic stem cells transplant.

However, the true incidence of A. ustus infection is very difficult to estimate based on challenges and inconsistencies with identifying this agent in the clinical setting, and the patchy nature of reporting.

[3] Although disseminated infection carries a high mortality rate, the response of skin to treatment varies depending on the degree of drug susceptibility exhibited by the particular strain.