Introduction
Table of Contents
Microscopy remains one of the cornerstones of clinical mycology, providing a rapid and inexpensive method for the direct detection and preliminary identification of fungi. Comparative microscopy is particularly valuable in distinguishing the morphological features of common clinical fungi, many of which exhibit overlapping clinical manifestations but require distinct therapeutic approaches.
Different genera of fungi, including Candida, Aspergillus, Mucorales, Cryptococcus, and Dermatophytes, exhibit characteristic microscopic structures such as yeast cells, pseudohyphae, true hyphae, conidiophores, sporangia, or capsule formation. By comparing these morphological hallmarks under various staining techniques—such as KOH mount, Lactophenol Cotton Blue (LPCB), India Ink, and Histopathological stains—clinicians and microbiologists can achieve a presumptive diagnosis within hours, long before culture and molecular confirmation are available.
In the context of immunocompromised patients, especially those with malignancies, timely recognition of fungal structures is crucial. Comparative microscopy not only aids in early diagnosis but also enhances laboratory training by reinforcing the ability to differentiate opportunistic pathogens from colonizers or contaminants.
Table
| Feature | Mucorales (Mucor/Rhizopus) | Aspergillus spp. | Candida spp. | Fusarium spp. |
| Hyphal Width | Broad (6–15 µm) | Narrow (3–6 µm) | Pseudohyphae (2–4 µm) | Narrow (3–6 µm) |
| Septation | Aseptate or sparsely septate | Regular septation | Pseudohyphae + budding yeast | Septate |
| Branching | Irregular, right-angle (≈90°) | Dichotomous, 45° angle | Not prominent, pseudohyphae show constrictions | Right-angle to acute |
| Shape | Ribbon-like, folded | Uniform, tubular | Round/oval yeast cells + pseudohyphae | Thin, curved |
| Conidia (Culture LPCB) | Sporangiospores in sporangia | Chains of conidia at vesicle | Blastoconidia + chlamydospores | Macroconidia (banana-shaped) |
| Pathogenic Clues (Cancer patient) | Rapidly invasive → mucormycosis | Chronic invasive pulmonary aspergillosis | Thrush, bloodstream infections | Fusariosis, resistant, mimics Aspergillus |
Appearance at Different Magnifications
400X (Routine Microscopy)
- Mucorales: Broad, ribbon-like aseptate hyphae, visible even at low power.
- Aspergillus: Narrow septate hyphae, need closer focus.
- Candida: Small budding yeast, pseudohyphae.
- Fusarium: Similar to Aspergillus but often with slightly curved hyphae.
1000X (Oil Immersion)
- Mucorales: Clear absence of septa, irregular width.
- Aspergillus: Septa and 45° branching are easily visible.
- Candida: Pseudohyphae with constrictions, budding yeast.
- Fusarium: Septate hyphae + occasional phialides seen.
1600X (High-Definition Microscopy)
- Mucorales: Details of aseptation and folds in hyphae are very clear.
- Aspergillus: Septa appear crisp, with a uniform hyphal wall.
- Candida: Intricate budding sites and chlamydospores are observable.
- Fusarium: Banana-shaped macroconidia in culture; hyphal curvature clearer.
Further Readings
- https://www.ncbi.nlm.nih.gov/books/NBK8125/
- https://en.fungaleducation.org/microscopy/
- https://www.researchgate.net/figure/Comparison-of-direct-microscopic-examination-culture-and-high-resolution-melting_fig6_228331882
- https://pmc.ncbi.nlm.nih.gov/articles/PMC5143727/
- https://www.researchgate.net/publication/348740221_Mini_Review_on_Dermatomycosis
- https://ijdvl.com/comparison-of-potassium-hydroxide-mount-and-mycological-culture-with-histopathologic-examination-using-periodic-acid-schiff-staining-of-the-nail-clippings-in-the-diagnosis-of-onychomycosis/
- https://onlinelibrary.wiley.com/doi/pdf/10.1111/ijd.16857
- https://www.sciencedirect.com/science/article/pii/S2213007124000236
- https://pmc.ncbi.nlm.nih.gov/articles/PMC3968782/
- https://www.ijord.com/index.php/ijord/article/view/714
- https://www.adelaide.edu.au/mycology/ua/media/1596/fungus3-book.pdf
- https://www.sciencedirect.com/science/article/pii/S1198743X23002367
- http://www.jmatonline.com/PDF/383-387-PB-11442.pdf
- https://www.jdsjournal.com/article/S0923-1811(06)00260-X/fulltext
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