Fungal Structure & Classification
Eukaryotic, chitin-walled organisms existing as yeasts, molds, or dimorphic forms — and why antifungal therapy is challenging.
Interactive Diagram
Tap a teal label to learn about each structure.
Fungi Are Eukaryotes
Unlike bacteria, fungi are eukaryotes with a true nucleus, mitochondria, 80S ribosomes, and a cell wall of chitin and glucan. Because they share cellular machinery with humans, selective toxicity is hard — most antifungal targets exploit the one major difference: fungal membranes use ergosterol rather than cholesterol. This is why amphotericin B (binds ergosterol) is nephrotoxic and azoles (block ergosterol synthesis) have drug interactions via hepatic CYP enzymes.
Yeasts vs Molds vs Dimorphic
Yeasts are single-celled, reproducing by budding (Saccharomyces, Candida). Molds are multicellular filaments (hyphae) forming a mycelium and reproduce by spores/conidia (Aspergillus, dermatophytes). Dimorphic fungi switch form with temperature: mold in the environment (25 °C) and yeast in tissue (37 °C). This group includes Histoplasma, Blastomyces, Coccidioides, Paracoccidioides, and Sporothrix — important clues to diagnosis because culture must be done at both temperatures.
Clinically Useful Classification
Fungi are grouped by disease pattern: superficial (Malassezia, tinea versicolor), cutaneous (dermatophytes — Trichophyton, Microsporum, Epidermophyton — digest keratin), subcutaneous (Sporothrix after thorn injury), and systemic/endemic (Histoplasma in Ohio/Mississippi valleys, Coccidioides in the US Southwest 'valley fever'). Opportunistic fungi (Candida, Cryptococcus, Aspergillus, Mucorales, Pneumocystis) cause disease mainly in immunocompromised hosts.
Diagnostic Clues
KOH preparation dissolves host cells but leaves chitin walls visible. Calcofluor white fluoresces fungal walls. India ink shows the capsule of Cryptococcus in CSF. Germ tube test identifies Candida albicans (forms germ tubes in serum within hours). Periodic acid–Schiff (PAS) and Gomori methenamine silver (GMS) stains highlight fungi in tissue. Serology (antigen tests for Histoplasma and Cryptococcus) and beta-D-glucan (broad fungal marker, negative in Cryptococcus and Mucorales) round out the workup.
Antifungal Targets
Polyenes (amphotericin B, nystatin) bind ergosterol and form pores. Azoles (fluconazole, voriconazole) inhibit lanosterol 14α-demethylase, depleting ergosterol. Allylamines (terbinafine) inhibit squalene epoxidase. Echinocandins (caspapfugin, micafungin) block β-1,3-glucan synthesis — unique to fungi. Flucytosine is converted to 5-fluorouracil in fungi. Resistance is rising (Candida auris, azole-resistant Aspergillus), making species identification critical.
