Mycology

Learning Objectives

By the end of this page, you should be able to:

• Classify fungi by morphology (yeasts, molds, dimorphic fungi) and explain what determines each growth form.
• Distinguish superficial, cutaneous, subcutaneous, and systemic mycoses by tissue depth and typical organism.
• Describe the mechanism of action and clinical use of each major antifungal drug class (polyenes, azoles, echinocandins, allylamines, others).
• Recognize the three medically dominant genera — Candida, Aspergillus, Cryptococcus — and their signature infections.
• Explain why fungal cell wall/membrane targets make antifungals selective, and why this is harder than targeting bacteria.
• Avoid the common mix-ups examiners test: hyphae vs pseudohyphae, dimorphism, and drug mechanism confusion.

Quick Answer

Mycology is the study of fungi and the diseases they cause. Medically, fungi split into two growth forms: yeasts (single cells, reproduce by budding, e.g., Candida, Cryptococcus) and molds (branching filaments called hyphae, e.g., Aspergillus). Some fungi are dimorphic — mold in the environment (25°C), yeast in the body (37°C) — a temperature switch that is a favorite exam point. Fungal infections (mycoses) are classified by how deep they invade: superficial (skin surface), cutaneous (skin/hair/nails), subcutaneous (dermis after inoculation), and systemic (internal organs, often in immunocompromised patients). Antifungals work because fungi, being eukaryotic, share few drug targets with bacteria — most drugs exploit ergosterol, the fungal equivalent of cholesterol, which human cells don't make.

Overview

Fungi are eukaryotes — they have a nucleus, mitochondria, and a rigid cell wall made of chitin and glucans (not peptidoglycan, which is why penicillins do nothing to them). This single fact explains most of medical mycology: because fungal cells are built more like our own cells than bacteria are, finding drugs that kill fungi without harming the patient is genuinely harder than finding antibacterials. That's why the antifungal armamentarium is smaller and the drugs are, on average, more toxic than antibiotics.

Clinically, fungi matter for two reasons. First, a handful of species (Candida, Aspergillus, Cryptococcus, dermatophytes) account for the overwhelming majority of human disease, so exam and ward relevance concentrates heavily on them. Second, fungal disease severity tracks the host's immune status almost linearly — a healthy person gets athlete's foot; a neutropenic transplant patient can get invasive pulmonary aspergillosis. Understanding who gets which fungus, and how deep it goes, is the organizing logic of this entire topic.

Fungal Classification

By morphology

Form	Structure	Reproduction	Example
Yeast	Single, round/oval cells	Budding	Candida albicans, Cryptococcus neoformans
Mold (mould)	Branching filaments (hyphae) forming a mycelium	Spore formation at hyphal tips	Aspergillus fumigatus, dermatophytes
Dimorphic fungus	Mold at 25°C (environment), yeast at 37°C (host tissue)	Switches form with temperature	Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis (forms spherules, not yeast)

A detail examiners love: Candida is technically a yeast, but under stress (like invading tissue) it can form pseudohyphae — elongated buds that stay attached end-to-end, mimicking true hyphae but lacking the parallel-walled, septate structure of real hyphae. True hyphae with acute-angle branching and septa point to Aspergillus; wide, ribbon-like, non-septate hyphae with wide-angle branching point to the Mucorales (mucormycosis).

By ecological niche and virulence

Fungi are also grouped into true pathogens (can infect healthy hosts — e.g., the dimorphic fungi causing histoplasmosis, blastomycosis, coccidioidomycosis) and opportunists (only cause disease when host defenses are down — Candida, Aspergillus, Cryptococcus, Mucorales). This distinction predicts who walks into your clinic with what: a farmer in the Ohio River valley with flu-like illness might have histoplasmosis regardless of immune status; a neutropenic leukemia patient with new pulmonary nodules is worked up for aspergillosis because their defenses, not the fungus's virulence, are the limiting factor.

Superficial vs Systemic Mycoses

Think of fungal infection as a matter of depth — how far past the skin surface the organism gets before the host stops it (or doesn't).

Superficial mycoses stay in dead keratin — no living tissue invasion, no inflammation to speak of. Malassezia furfur causing tinea versicolor (hypo/hyperpigmented patches, classic "spaghetti and meatballs" appearance on KOH prep) is the textbook example.

Cutaneous mycoses are the dermatophyte infections — tinea capitis, corporis, cruris, pedis, unguium — named by body site, all caused by the three dermatophyte genera that digest keratin. These are common, itchy, and rarely dangerous, but symptomatic and socially bothersome.

Subcutaneous mycoses require traumatic inoculation (a thorn prick, a splinter) that drives the fungus into the dermis. Sporotrichosis is the classic case: nodules track up the lymphatics from the inoculation site, a pattern called lymphocutaneous spread.

Systemic (invasive) mycoses are where mortality lives. These happen when a fungus that's normally either a low-virulence commensal (Candida in the gut) or an environmental mold (Aspergillus spores, ubiquitous in air) crosses into blood or deep organs — almost always because the host's defenses (neutrophils, T-cells, or mucosal barriers) are compromised. This is the category tested most heavily because it's where clinical decision-making (which antifungal, how urgently) actually matters.

The big three: Candida, Aspergillus, Cryptococcus

• Candida albicans — normal flora of mouth, gut, and vagina. Disease occurs when the balance shifts: antibiotics wipe out competing bacteria (vaginal candidiasis), immunosuppression allows oral thrush (white plaques that scrape off, unlike leukoplakia), or a central line lets it enter blood directly (candidemia — the most common invasive fungal infection in hospitalized patients). Germ tube test and pseudohyphae on wet mount are classic identifiers.
• Aspergillus fumigatus — inhaled as ubiquitous environmental spores (conidia). In a healthy person, alveolar macrophages clear them without incident. In neutropenic patients, spores germinate into hyphae that invade blood vessels, causing invasive pulmonary aspergillosis with a classic "halo sign" on CT. In patients with pre-existing lung cavities (old TB), it can colonize as an aspergilloma ("fungus ball"). In asthmatics/cystic fibrosis patients, it can trigger a hypersensitivity reaction (allergic bronchopulmonary aspergillosis, ABPA) without actual tissue invasion — three very different diseases, same organism, determined entirely by host immune context.
• Cryptococcus neoformans — encapsulated yeast found in pigeon droppings and soil. Inhaled, usually asymptomatic in healthy hosts, but in advanced HIV (CD4 <100) it disseminates to the meninges, causing cryptococcal meningitis — often subacute, with headache and minimal fever, and a strikingly high opening pressure on lumbar puncture. India ink stain shows the yeast's thick capsule as a clear halo; cryptococcal antigen (CrAg) testing is faster and more sensitive.

Antifungal Drug Classes

Nearly every antifungal drug class exploits ergosterol, the sterol that stabilizes fungal cell membranes the way cholesterol stabilizes ours. Human cells don't have ergosterol, which is what gives these drugs selective toxicity — though imperfectly, since ergosterol synthesis enzymes resemble our own cholesterol enzymes closely enough to cause side effects.

Class	Mechanism	Example drugs	Key use / notable toxicity
Polyenes	Bind ergosterol directly, punching pores in the fungal membrane	Amphotericin B, nystatin	Broad-spectrum, IV for severe systemic mycoses; nephrotoxic, infusion-related fever/chills ("shake and bake")
Azoles	Inhibit lanosterol 14-α-demethylase (a cytochrome P450 enzyme), blocking ergosterol synthesis	Fluconazole, itraconazole, voriconazole, posaconazole	Oral options for candidiasis, cryptococcosis (fluconazole); voriconazole is first-line for invasive aspergillosis; strong CYP450 drug interactions
Echinocandins	Inhibit β-(1,3)-glucan synthase, blocking fungal cell wall glucan synthesis	Caspofungin, micafungin, anidulafungin	IV only; first-line for candidemia; very safe (no fungal wall target in human cells)
Allylamines	Inhibit squalene epoxidase, an earlier step in ergosterol synthesis	Terbinafine	Oral/topical for dermatophyte infections (nail, skin)
Others	Flucytosine: disrupts fungal RNA/DNA synthesis (converted intracellularly to 5-FU); Griseofulvin: disrupts fungal microtubule function	Flucytosine, griseofulvin	Flucytosine used with amphotericin B for cryptococcal meningitis (synergy, lower resistance); griseofulvin for tinea capitis in children

The mechanism hierarchy is worth memorizing as a sequence: allylamines and azoles both block ergosterol synthesis at different steps (squalene epoxidase vs 14-α-demethylase), while polyenes act on the finished product (ergosterol itself), and echinocandins skip the membrane entirely and attack the cell wall. That's why echinocandins have the cleanest side-effect profile — the target simply doesn't exist in human physiology.

Key Terms

Term	Definition
Hypha	A branching, filamentous fungal structure; septate (has cross-walls) in most molds, non-septate in the Mucorales
Pseudohypha	Elongated yeast cells that remain attached after budding, mimicking hyphae; characteristic of Candida under stress
Dimorphism	The ability of certain fungi to switch between mold and yeast form depending on temperature (environment vs host)
Mycosis	Any disease caused by fungal infection
Ergosterol	The primary sterol in fungal cell membranes, analogous to cholesterol in human cells; the target of most antifungals
Opportunistic pathogen	An organism that causes disease only when host defenses are impaired (e.g., Candida, Aspergillus)
Conidia	Asexual fungal spores produced by molds like Aspergillus, responsible for airborne transmission
Germ tube test	A rapid lab test in which Candida albicans forms a tube-like outgrowth in serum within 2 hours, distinguishing it from other Candida species
KOH prep	A microscopy technique using potassium hydroxide to dissolve host tissue and reveal fungal elements

Common Mistakes

Misconception 1: "All antifungals work the same way, they just kill fungi." Why it's wrong: This ignores that different classes hit different targets (membrane sterol, sterol synthesis enzymes, cell wall glucan, nucleic acid synthesis), which is exactly why some drugs are combined (amphotericin B + flucytosine) and why resistance to one class doesn't imply resistance to another. Correct explanation: Match the drug to its mechanism — polyenes bind ergosterol, azoles and allylamines block ergosterol synthesis at different steps, echinocandins attack the cell wall, flucytosine disrupts nucleic acids.

Misconception 2: "Candida found in a stool or vaginal swab always needs treatment." Why it's wrong: Candida is normal flora of the gut and vagina in most people. Its mere presence on a culture is not diagnostic of disease; treatment decisions depend on symptoms and clinical context, not colonization alone. Correct explanation: Distinguish colonization (no treatment needed) from infection (symptomatic overgrowth, e.g., thrush, vulvovaginal candidiasis, or invasive candidemia), which requires evidence of tissue invasion or systemic spread, not just a positive culture.

Misconception 3: "Dimorphic fungi are yeasts, so they behave like Candida." Why it's wrong: Dimorphism is a temperature-dependent switch (mold outside the body, yeast inside), which is fundamentally different from Candida, which is a yeast that occasionally forms pseudohyphae under stress — it never grows as a true mold. Correct explanation: True dimorphic fungi (Histoplasma, Blastomyces, Coccidioides) exist as molds at 25°C in soil and convert to yeast (or spherules, for Coccidioides) only once inside the warmer host — this switch is often the basis of diagnostic lab identification.

Comparison and Connections

Feature	Candida albicans	Aspergillus fumigatus	Cryptococcus neoformans
Morphology	Yeast, forms pseudohyphae	Septate hyphae, acute-angle branching	Encapsulated yeast
Habitat/source	Normal human flora	Environmental spores (soil, decaying matter)	Soil, pigeon/bird droppings
At-risk host	Antibiotic use, diabetes, catheters, immunosuppression	Neutropenia, structural lung disease, asthma/CF	Advanced HIV (CD4 <100), other T-cell defects
Classic disease	Thrush, vulvovaginal candidiasis, candidemia	Invasive aspergillosis, aspergilloma, ABPA	Cryptococcal meningitis
Key diagnostic test	Germ tube test, KOH/wet mount	Culture + galactomannan antigen, CT halo sign	India ink stain, CrAg (cryptococcal antigen) test
First-line therapy	Fluconazole (mild) / echinocandin (invasive)	Voriconazole	Amphotericin B + flucytosine, then fluconazole

Practice Questions

Recall

1. What structural feature distinguishes true hyphae from pseudohyphae? Answer guidance: True hyphae are septate (or non-septate in Mucorales) filaments with parallel walls; pseudohyphae are elongated yeast cells joined end-to-end at constrictions, produced by budding yeasts like Candida under stress.

2. Name the enzyme inhibited by azole antifungals. Answer guidance: Lanosterol 14-α-demethylase, a cytochrome P450 enzyme required for ergosterol synthesis.

Understanding

3. Explain why echinocandins have fewer side effects than amphotericin B. Answer guidance: Echinocandins target β-(1,3)-glucan synthase, an enzyme that builds the fungal cell wall — a structure absent in human cells entirely. Amphotericin B binds ergosterol in the fungal membrane, but its imperfect selectivity lets it also interact with cholesterol in human cell membranes (notably in the kidney), causing nephrotoxicity.

4. Why does the same organism, Aspergillus fumigatus, cause three completely different diseases (ABPA, aspergilloma, invasive aspergillosis)? Answer guidance: The disease manifestation depends on host context, not the organism: hypersensitivity in atopic/asthmatic airways (ABPA), colonization of a pre-existing lung cavity without invasion (aspergilloma), and active tissue invasion in neutropenic hosts who cannot mount a phagocytic response (invasive aspergillosis).

Application

5. A patient with poorly controlled diabetes on broad-spectrum antibiotics develops white plaques in the mouth that scrape off, leaving a raw base. What is the likely diagnosis and first-line treatment? Answer guidance: Oropharyngeal candidiasis (thrush), caused by Candida albicans overgrowth after antibiotics disrupted competing bacterial flora, worsened by diabetic immune dysfunction. Treat with topical nystatin or oral fluconazole.

6. A gardener develops a nodule at a thorn-prick site on the hand that ulcerates, followed by similar nodules appearing in a line up the forearm over two weeks. What organism and what mechanism explains the spreading pattern? Answer guidance: Sporothrix schenckii (sporotrichosis, "rose gardener's disease"). The linear spread reflects lymphocutaneous spread — the fungus tracks along lymphatic channels draining the initial inoculation site.

Analysis

7. Compare and contrast why fluconazole is effective for both vulvovaginal candidiasis and cryptococcal meningitis, but is not first-line for invasive aspergillosis. Answer guidance: Fluconazole inhibits ergosterol synthesis broadly across susceptible yeasts (Candida, Cryptococcus), and it crosses the blood-brain barrier well, making it useful for CNS cryptococcosis (usually after induction therapy) and for candidiasis. However, Aspergillus is intrinsically less susceptible to fluconazole (poor spectrum against molds), so voriconazole, which has better mold coverage, is preferred for invasive aspergillosis.

8. A neutropenic leukemia patient and an HIV patient with CD4 count of 40 both develop fever and headache. Explain why the differential and workup differ despite both being "immunocompromised." Answer guidance: Neutropenia impairs phagocytic clearance of molds and yeasts (raising suspicion for invasive aspergillosis or candidemia, workup includes blood cultures, galactomannan, CT chest), while advanced HIV specifically depletes CD4 T-cells needed to contain intracellular/encapsulated pathogens like Cryptococcus (raising suspicion for cryptococcal meningitis, workup includes lumbar puncture with opening pressure, CrAg, India ink). The type of immune defect that is broken predicts which class of pathogen becomes dangerous.

FAQ

Why don't antibiotics like penicillin work against fungi? Penicillin and other beta-lactams target peptidoglycan synthesis, a structural component of bacterial cell walls. Fungal cell walls are made of chitin and glucans instead, so there's no target for these drugs — fungi are simply immune to antibacterial mechanisms by cell wall composition alone.

Is Candida always dangerous? No. It's part of normal human flora in the mouth, gut, and vagina in most healthy people. It only becomes a problem when local defenses are disrupted (antibiotics killing competing bacteria) or when systemic immunity is impaired, allowing overgrowth or invasion.

Why is amphotericin B nicknamed "amphoterrible"? Because its selectivity for fungal ergosterol over human cholesterol is imperfect — it still binds cholesterol in human cell membranes, especially in the kidney, causing significant nephrotoxicity, plus notorious infusion reactions (fever, chills, rigors). Lipid formulations reduce but don't eliminate this toxicity.

How is a fungal infection actually diagnosed in the lab? Common approaches include direct microscopy (KOH prep, India ink for Cryptococcus), culture on Sabouraud agar, antigen tests (galactomannan for Aspergillus, CrAg for Cryptococcus), and increasingly, molecular/PCR-based methods. The right test depends on the suspected organism and site.

Why are dimorphic fungi geographically clustered (e.g., Histoplasma in the Ohio/Mississippi River valleys)? Because these fungi live in soil under specific environmental conditions (often enriched by bird or bat droppings), their mold form's distribution is tied to climate, soil composition, and animal reservoirs — which is why travel and occupational history (spelunking, farming, construction) are key parts of the diagnostic workup.

Quick Revision

• Fungi are eukaryotes with chitin/glucan cell walls — this is why beta-lactam antibiotics never work on them.
• Two basic morphologies: yeasts (single cells, bud) and molds (branching hyphae); dimorphic fungi switch between the two based on temperature.
• Candida is a yeast that forms pseudohyphae under stress; true septate hyphae with acute-angle branching = Aspergillus; wide, non-septate, wide-angle branching = Mucorales.
• Mycoses are classified by depth: superficial (Malassezia, tinea versicolor) → cutaneous (dermatophytes, tinea) → subcutaneous (Sporothrix, lymphocutaneous spread) → systemic/invasive (Candida, Aspergillus, Cryptococcus, endemic dimorphic fungi).
• Systemic mycosis risk tracks immune status: neutropenia favors Aspergillus/Candida; advanced HIV (CD4 <100) favors Cryptococcus.
• Ergosterol is the fungal membrane sterol (analogous to human cholesterol) and the target of most antifungal classes.
• Polyenes (amphotericin B) bind ergosterol directly; azoles inhibit 14-α-demethylase; allylamines (terbinafine) inhibit squalene epoxidase — both block ergosterol synthesis, at different steps.
• Echinocandins (caspofungin) inhibit β-(1,3)-glucan synthase, attacking the fungal cell wall — a target absent in humans, hence a cleaner side-effect profile.
• Flucytosine + amphotericin B is classic combination therapy for cryptococcal meningitis.
• Diagnostic shortcuts: germ tube test for C. albicans; India ink/CrAg for Cryptococcus; galactomannan/CT halo sign for Aspergillus.
• Voriconazole, not fluconazole, is first-line for invasive aspergillosis because fluconazole has poor mold coverage.
• Amphotericin B's major toxicity is nephrotoxicity, due to imperfect selectivity between fungal ergosterol and human cholesterol.

Related Topics

Prerequisites

• Basic microbial classification (prokaryotes vs eukaryotes)
• Cell membrane and cell wall structure
• Innate and adaptive immune response basics (neutrophils, T-cells)

Related Topics

• Bacteriology (contrast in cell wall structure and antibiotic targets)
• Virology (contrast in structure and treatment approach)
• HIV/AIDS and opportunistic infections
• Pharmacology of antimicrobial agents

Next Topics

• Parasitology (protozoa and helminths)
• Clinical infectious disease syndromes (sepsis, meningitis workup)
• Antimicrobial resistance mechanisms