Colony morphology of Aspergillus terreus on Corn Meal Agar (CMA) showing a characteristic cinnamon-brown to sandy-beige pigmentation with a velvety, granular texture and a light-colored peripheral growth zone
Introduction & Taxonomy
Table of Contents
Aspergillus terreus is a filamentous fungus increasingly recognized as a significant emerging opportunistic pathogen. It is uniquely distinguished from other common species by its intrinsic resistance to amphotericin B, its characteristic cinnamon-brown colony color, and the production of specialized spores called aleurioconidia.
Fig. Colony morphology of Aspergillus terreus on Corn Meal Agar (CMA) showing a characteristic cinnamon-brown to sandy-beige pigmentation with a velvety, granular texture and a light-colored peripheral growth zone
Classification: Belongs to the phylum Ascomycota, section Terrei.
Habitat: Ubiquitous saprobe found worldwide in soil, decomposing vegetation, compost, and stored grains, primarily in warmer tropical and subtropical regions.
Economic Importance: Used industrially to produce lovastatin (a cholesterol-lowering drug), itaconic acid, and various enzymes.
Morphology
Macroscopic: Colonies on Sabouraud agar are typically velvety and cinnamon-brown to sandy-beige. The reverse side often appears yellow to brown, sometimes with yellow soluble pigments.
Microscopic:
Conidial Heads: Compact, biseriate, and densely columnar.
Conidiophores: Long, smooth-walled, and hyaline (clear).
Conidia: Small (approx. 2 µm), globose, and smooth-walled.
Fig. Photomicrograph of Aspergillus terreus in a Lactophenol Cotton Blue (LPCB) tease mount showing the characteristic biseriate conidial heads with compact, columnar chains of globose conidia supported by smooth-walled conidiophore (Mag. 400X)
Aleurioconidia (Accessory Conidia): A unique feature produced directly on the vegetative hyphae. These are larger (6–7 µm) than standard spores and can induce elevated inflammatory responses.
Pathogenicity
Spectrum of Disease: Causes a range of infections from superficial (e.g., onychomycosis, otomycosis) to life-threatening invasive aspergillosis (IA).
Dissemination: It has a higher propensity for hematogenous dissemination (spreading through the blood) to secondary organs like the brain, kidneys, and liver, especially in neutropenic patients.
Virulence Factors: Includes the production of mycotoxins (e.g., terretonin, geodin, citrinin) and terrelysin, a hemolysin that lyses red blood cells to scavenge iron.
Lab Diagnosis
Culture: Dependable identification relies on culture morphology (cinnamon colonies) and microscopic inspection (columnar heads).
Microscopy: Direct examination using KOH or Calcofluor White reveals septate hyphae branching at 45-degree angles.
Fig. Aspergillus terreus in LPCB tease mount showing columnar conidial heads, biseriate vesicles, and smooth-walled conidiophores at a high magnification of 40OX with extra 4X optical zoom
Biomarkers: Galactomannan (GM) antigen and (1→3)-β-D-glucan detection in serum or bronchoalveolar lavage (BAL) fluid are used, though GM sensitivity can vary.
Molecular: PCR and Next-Generation Sequencing (NGS) provide rapid, species-level identification directly from clinical specimens.
Treatment & Prevention
Drug of Choice: Voriconazole or Isavuconazole is typically preferred due to the organism’s intrinsic resistance to amphotericin B.
Resistance: Amphotericin B failure is a hallmark of this species, leading to higher mortality rates (often >80% in invasive cases) compared to other Aspergillus species.
Prevention:
Environmental: Use of HEPA filtration in hospital wards and avoiding construction-related dust exposure.
Reservoir Control: Eliminating indoor reservoirs, such as potted plants, from the rooms of immunocompromised patients.
Keynotes
“Cinnamon” Fungus: Known for its sandy-brown colony color.
Intrinsic Polyene Resistance: It is naturally resistant to amphotericin B.
Wait-and-Watch Role: Conidia can persist long-term within alveolar macrophages, potentially germinating when the host’s immune system further weakens.
Environmental Versatility: Capable of decomposing complex materials, including polypropylene plastic.