Aspergillus and Actinomyces: Distinct Taxonomic Families
No, Aspergillus is not a member of the Actinomyces family—they belong to entirely different kingdoms of life. Aspergillus is a fungus, while Actinomyces is a bacterium, making this a fundamental taxonomic distinction that cannot be confused 1.
Taxonomic Classification
Aspergillus (Fungal Kingdom)
- Aspergillus belongs to the fungal kingdom Ascomycota and is classified within the order Eurotiales, family Aspergillaceae 2.
- The genus comprises 446 accepted species distributed across six subgenera and 27 sections 2.
- Phylogenetic analyses using multilocus gene sequencing (RPB1, RPB2, Tsr1, Cct8, BenA, CaM) have confirmed Aspergillus as a monophyletic genus, clearly separated from other fungal genera like Penicillium 3.
- Aspergillus species are characterized histopathologically as acute-angle branching, septated nonpigmented hyphae measuring 2–4 µm in width 4.
Actinomyces (Bacterial Kingdom)
- Actinomyces belongs to the bacterial order Actinomycetales, which includes phylogenetically diverse but morphologically similar gram-positive bacteria 1.
- The order Actinomycetales includes both aerobic genera (Nocardia, Streptomyces, Mycobacteria) and anaerobic genera of medical importance (Actinomyces, Arachnia, Rothia, Bifidobacterium) 1.
- Actinomyces species are catalase-negative, facultatively anaerobic, gram-positive, rod-shaped bacteria 5.
Critical Clinical Distinction
Why This Matters in Practice
- The distinction is essential because these organisms require completely different antimicrobial therapies: Aspergillus infections require antifungal agents (amphotericin B, voriconazole, itraconazole), while Actinomyces infections require prolonged courses of antibacterial therapy 4, 1.
- Both organisms are called "great masqueraders" because diagnosis is often delayed, but for different reasons related to their distinct pathophysiology 1.
- Aspergillus infections are characterized by vascular invasion and tissue infarction, while Actinomyces infections are characterized by chronic progression, abscess formation with fistulous tracts and draining sinuses 4, 1.
Common Diagnostic Pitfall
The confusion may arise because both organisms can exhibit filamentous branching structures on microscopy 1. However:
- Actinomyces bacteria exhibit filamentous branching that fragments into bacillary or coccoid forms 1.
- Aspergillus fungi show true septated hyphae that do not fragment and are best detected by Gomori methenamine silver and Periodic acid-Schiff stains 4.
Tissue biopsy with both bacterial and fungal cultures is mandatory when either organism is suspected to ensure correct identification and appropriate therapy 6.