Overview
Fungi are eukaryotic organisms comprising yeasts, molds and mushrooms, classified in their own kingdom, Fungi, separate from plants, animals and bacteria. Their cell walls contain chitin, distinguishing them from plant (cellulose) and bacterial cell walls.
Fungi feed by absorption after secreting digestive enzymes into their surroundings. This lesson covers fungal morphology (molds vs yeasts), physiology, classification into four taxonomic classes, colony morphology terminology, and an overview of medically important pathogenic fungi and the mycoses they cause.
Learning Objectives
After this lesson you will be able to…- Describe the morphology of fungi, including molds and yeasts
- Explain the physiology of fungi — nutrition, reproduction and growth
- Classify fungi into their four major taxonomic classes
- Describe colony morphology terminology used to identify fungi
- Describe the pathogenicity of fungi and categories of mycoses
Clinical Story
Why This MattersA 32-year-old uncontrolled diabetic presents with facial pain, nasal discharge and a black necrotic lesion on the nasal septum. The physician suspects mucormycosis (a Phycomycete infection) and requests urgent KOH mount and fungal culture from a nasal biopsy — a race against time given the rapidly invasive nature of this infection.
Core Concepts
Laboratory Principle
Fungal identification depends on integrating macroscopic colony morphology (form, elevation, margin, colour, growth rate) with microscopic hyphal and spore characteristics (septate vs aseptate, hyaline vs pigmented, spore arrangement). Because morphology can overlap between genera, biochemical and, increasingly, molecular methods supplement classical morphological identification.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Lactophenol Cotton Blue (LPCB) | Standard stain | Mounting mold colonies for microscopic examination of hyphae/spores | Room temperature |
| Sabouraud's Dextrose Agar | Standard formulation | General fungal culture medium | Room temperature (unpoured), refrigerate poured plates |
| Potassium Hydroxide (KOH) | 10–40% | Direct examination of skin/nail scrapings by dissolving keratin | Room temperature |
Step-by-Step Procedure
Collect skin scrapings, hair, nail clippings or other relevant clinical material as appropriate.
Prepare a KOH mount (10% for skin, 40% skin/nail per protocol) or LPCB mount for direct visualisation.
Inoculate onto Sabouraud's dextrose agar or enriched media depending on suspected organism.
Incubate at 25–30°C (and 35–37°C in parallel for dimorphic fungi) and observe colony growth over days to weeks.
Assess colony morphology (form, elevation, margin, colour) and microscopic features (hyphae, spore type) for presumptive identification.
Flow Diagram
Quality Control
Include known reference fungal strains (positive controls) with each new batch of culture media to confirm growth-supporting properties. Verify incubator temperature accuracy (25–30°C and 35–37°C) with a calibrated thermometer.
Participate in external quality assessment mycology panels that circulate coded slides/cultures to check consistency of morphological identification across participating laboratories.
Reference Values
Normal Ranges⚠️ Values summarised from standard parasitology/microbiology references. Always confirm with your laboratory's SOP.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Broad, ribbon-like, aseptate hyphae on KOH mount | Suggestive of Zygomycetes (e.g., Rhizopus, Mucor) | Urgent clinical correlation — risk of invasive mucormycosis, especially in diabetics |
| Narrow, septate, colourless hyphae | Suggestive of Hyalohyphomyces (e.g., Aspergillus, Penicillium) | Correlate with clinical site and culture confirmation |
| Dark, pigmented hyphae/colonies | Suggestive of Dematiaceous fungi | Consider chromoblastomycosis or mycetoma |
| Colony grows as mold at 25–30°C, yeast at 35–37°C | Suggestive of a dimorphic pathogenic fungus | Handle with caution; refer to reference mycology lab |
Common Errors & How to Avoid Them
Cause: Underestimating the infectivity of mold-phase dimorphic fungi (e.g., Histoplasma, Coccidioides)
Prevention: Always handle suspected dimorphic fungal cultures within a certified biosafety cabinet.
Cause: Cotton fibres, hair or plant material mistaken for hyphae on direct KOH mount
Prevention: Confirm true septation, branching pattern and refractility before reporting fungal elements present.
Cause: Discarding fungal cultures as negative before adequate incubation time
Prevention: Incubate cultures for a minimum of the recommended period (often up to 4 weeks) before finalising a negative report.
Laboratory Tips from the Bench
Use a Lactophenol Cotton Blue mount for gentle handling of fragile mold colonies to preserve spore arrangement for identification.
Always incubate cultures at two temperatures (room temperature and 35–37°C) in parallel when dimorphic fungi are suspected.
'Broad and bare' for Zygomycetes — broad, aseptate ('bare' of septa) hyphae, unlike the narrow, septate hyphae of Hyalohyphomyces.
Important Notes
The mold phase of dimorphic fungi (e.g., Histoplasma capsulatum, Coccidioides immitis) is highly infectious via inhalation of spores — cultures must be handled only within a certified biosafety cabinet by trained personnel.
This chemical distinction from plant cellulose cell walls is key to why antifungal drugs (targeting chitin/ergosterol synthesis) are structurally different from antibacterial agents.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer. Use arrow keys to navigate in single-card mode.
Clinical Case Study
Apply Your KnowledgePresents with three days of facial pain, black nasal discharge, and periorbital swelling. Blood glucose is markedly elevated with ketosis.
Broad aseptate hyphae on direct KOH mount, combined with rapid tissue necrosis and uncontrolled diabetes, are strongly indicative of rhino-orbito-cerebral mucormycosis, a medical emergency.
- →Uncontrolled diabetes is a major risk factor for mucormycosis.
- →Broad, aseptate hyphae distinguish Zygomycetes from other fungal classes.
- →Mucormycosis requires urgent surgical debridement plus antifungal therapy — time is critical.
Frequently Asked Questions
Septation pattern is a key morphological criterion separating fungal classes with very different clinical behaviour and treatment — for example, aseptate hyphae point toward Zygomycetes, which cause aggressive, often fatal infections requiring urgent management.
Their ability to switch between a relatively safe mold form in the environment and an invasive yeast form at body temperature allows them to establish systemic infection after inhalation of spores, making cultures of the mold phase a significant laboratory biohazard.
No — most environmental molds are saprophytic and non-pathogenic; only specific genera and species, often opportunistic in immunocompromised hosts, cause human disease.
Quick Revision
10-Minute ReviewKey Takeaways
- Fungi are eukaryotic, chitin-walled organisms existing as molds, yeasts, or dimorphic forms.
- Fungal nutrition occurs via extracellular enzymatic digestion followed by absorption.
- Classification into four classes is based on hypha type, spore type and mode of reproduction.
- Colony morphology and microscopic hyphal features together guide presumptive identification.
- Mycoses range from superficial skin infections to life-threatening systemic disease.
- Dimorphic fungal cultures pose a significant laboratory biosafety hazard and require careful handling.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Microbiology Module — Lesson 51: Morphology and General Properties of Fungi.
- Larone DH. Medically Important Fungi: A Guide to Identification. 5th ed. ASM Press.
- Kwon-Chung KJ, Bennett JE. Medical Mycology.