6.1 Mycology: Dermatophytes, Onychomycosis & Systemic Mycoses

6.1 Mycology: Dermatophytes, Onychomycosis & Systemic Mycoses

Independent study guide by OpenExamPrep.

Core Examination Pearl: Board examiners rigorously test the biochemical distinctions between fungal and bacterial cell structures, the definitive cultural and microscopic characteristics of dermatophytes (notably Trichophyton rubrum versus Trichophyton mentagrophytes), the clinical patterns of tinea pedis and onychomycosis, and the diagnostic gold standard for onychomycosis (PAS staining of nail plate clippings).

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1. Fungal Cytology & Cell Wall Biochemistry

Fungi are eukaryotic, heterotrophic organisms that possess complex membrane-bound organelles and distinct biochemical structures separating them from both mammalian human host cells and prokaryotic bacteria.

Cell Membrane: Ergosterol vs. Cholesterol

Unlike mammalian cell membranes, which utilize cholesterol as their primary stabilizing sterol, fungal cell membranes incorporate ergosterol to regulate membrane fluidity, integrity, and transport protein function.

• Allylamines (e.g., Terbinafine): Inhibit the enzyme squalene epoxidase, preventing the conversion of squalene to lanosterol. This causes toxic intracellular accumulation of squalene and depletion of ergosterol, exerting a fungicidal effect against dermatophytes.
• Azoles (e.g., Fluconazole, Itraconazole, Ketoconazole): Inhibit the cytochrome P450-dependent enzyme 14-alpha-demethylase, blocking the conversion of lanosterol to ergosterol. This destabilizes the membrane and results in accumulation of toxic 14-methylated sterols, exerting primarily fungistatic effects.
• Polyenes (e.g., Amphotericin B, Nystatin): Bind directly to ergosterol in the fungal membrane, creating trans-membrane channels (pores) that leak intracellular potassium, magnesium, and essential metabolites, resulting in osmotic lysis.

Cell Wall: Chitin & Glucan Matrix

External to the plasma membrane sits a rigid multi-layered cell wall that provides structural shape, osmotic protection, and antigenic identity. It contains no peptidoglycan:

• Chitin: A linear homopolymer of $\beta$-(1,4)-linked N-acetylglucosamine (NAG) arranged in microfibrils near the inner membrane layer, imparting exceptional tensile strength.
• $\beta$-Glucans: Complex branched polymers of D-glucose. $\beta$-(1,3)-D-glucan forms the primary structural scaffold, cross-linked to $\beta$-(1,6)-glucan. Echinocandins (e.g., Caspofungin, Micafungin) non-competitively inhibit $\beta$-(1,3)-D-glucan synthase, disrupting cell wall integrity and causing osmotic lysis (particularly in Candida and Aspergillus).
• Mannoproteins: Outer surface glycoproteins linked to asparagine (N-linked) or serine/threonine (O-linked) residues that govern cellular adhesion, immune evasion, and cell wall permeability.

+-----------------------------------------------------------------------------------------+
|                            FUNGAL CELL STRUCTURE & DRUG TARGETS                         |
+-----------------------+-----------------------------+-----------------------------------+
| Cellular Layer        | Primary Biochemical Biomarkers| Primary Therapeutic Class         |
+-----------------------+-----------------------------+-----------------------------------+
| Fungal Cell Wall      | • Chitin (poly-NAG)         | • Echinocandins (Caspofungin)     |
| (Rigid Outer Matrix)  | • Beta-(1,3)-D-glucan       |   Inhibit beta-(1,3)-glucan synth |
|                       | • Mannoproteins             |   (Effective vs Candida, Asperg)  |
+-----------------------+-----------------------------+-----------------------------------+
| Fungal Cell Membrane  | • Ergosterol                | • Allylamines (Terbinafine):      |
| (Inner Lipid Bilayer) |   (Fluidity & stability)    |   Inhibit squalene epoxidase      |
|                       |                             | • Azoles (Itraconazole):          |
|                       |                             |   Inhibit 14-alpha-demethylase    |
|                       |                             | • Polyenes (Amphotericin B):      |
|                       |                             |   Bind ergosterol to form pores   |
+-----------------------+-----------------------------+-----------------------------------+

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2. Dermatophytes: Speciation, Morphology & Biochemical Profiling

Dermatophytes are specialized, keratinophilic filamentous fungi belonging to three distinct anamorphic genera: Trichophyton, Epidermophyton, and Microsporum. They produce specialized extracellular proteases (keratinases) that digest keratin in non-viable cornified tissues (stratum corneum of skin, nail plate, and hair shafts).

Genera Differentiation & Morphological Characteristics

1. Trichophyton:

   • Tissue Tropism: Infects skin, hair, and nails.
   • Microscopic Hallmarks: Microconidia are typically numerous and prominent; macroconidia are rare, smooth-walled, and pencil- or cigar-shaped.
   • Trichophyton rubrum:
     • The single most common cause of tinea pedis, onychomycosis, tinea cruris, and tinea corporis worldwide (>70% of podiatric fungal isolates).
     • Microconidia: Abundant, tear-shaped to pyriform (pear-shaped), arranged singly along the sides of hyphae ("birds on a telephone wire").
     • Macroconidia: Rare, pencil-shaped, thin-walled, with 3 to 8 transverse septa.
     • Colony Appearance: Fluffy, cottony white surface with a pathognomonic deep wine-red to burgundy reverse pigment on potato dextrose agar (PDA) or Sabouraud dextrose agar (SDA).
     • Biochemical Tests: Urease negative within 7 days; hair perforation test negative (cannot penetrate hair shafts in vitro).
   • Trichophyton mentagrophytes:
     • Second most prevalent cause of tinea pedis; classically associated with acute, inflammatory, and vesiculobullous presentations.
     • Microconidia: Abundant, round to globose, arranged in dense, grape-like clusters.
     • Macroconidia: Cigar-shaped to club-shaped, thin-walled, with 4 to 6 septa.
     • Hyphal Structures: Characteristically exhibits spiral hyphae (coiled, spring-like hyphal coils).
     • Colony Appearance: Powdery to granular, cream-to-tan surface; yellow-brown to tan reverse pigment.
     • Biochemical Tests: Urease positive within 48 to 72 hours; hair perforation test positive (produces wedge-shaped transverse perforations in hair shafts in vitro).

2. Epidermophyton:

   • Tissue Tropism: Infects skin and nails only (strictly never infects hair).
   • Epidermophyton floccosum:
     • Macroconidia: Abundant, smooth, thick-walled, club-shaped to clavate ("beaver-tail"), arranged singly or in clusters of two or three, containing 2 to 4 cells.
     • Microconidia: Completely absent (the definitive diagnostic microscopic feature).
     • Colony Appearance: Velvety to powdery, greenish-khaki to mustard-yellow surface with a tan-orange reverse.

3. Microsporum:

   • Tissue Tropism: Infects hair and skin (rarely infects nails).
   • Microscopic Hallmarks: Macroconidia are abundant, large, spindle-shaped (fusiform) with thick, rough, echinulate (spiny/verrucous) walls containing 5 to 15 septa; microconidia are sparse.
   • Microsporum canis / audouinii: Zoophilic (M. canis, dogs/cats) or anthropophilic (M. audouinii). Cause tinea capitis and tinea corporis.
   • Wood's Lamp Examination: Infected hair shafts demonstrate bright green-yellow ectothrix fluorescence under long-wave ultraviolet light (365 nm), caused by the metabolic production of pteridine.

Table 1: Dermatophyte Comparative Characteristics & Diagnostic Profiles

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3. Clinical Syndromes of Tinea Pedis (Athlete's Foot)

Tinea pedis is the most prevalent superficial fungal infection in humans, favored by the warm, dark, occlusive environment of modern footwear. Infection is classified into four distinct clinical phenotypes:

1. Interdigital Tinea Pedis

• Epidemiology & Location: The most common clinical presentation. Most frequently localizes to the fourth interdigital web space (between the 4th and 5th toes), followed by the third web space. This distribution reflects the anatomical tightness of the 4th interdigital sulcus and greater accumulation of moisture.
• Clinical Presentation: Erythema, maceration, scaling, peeling, and painful fissuring of the interdigital skin, accompanied by pruritus and malodor.
• Complications (Dermatophytosis Complex): Chronic fungal breakdown of the stratum corneum facilitates secondary bacterial invasion by Staphylococcus aureus, Pseudomonas aeruginosa, or Corynebacterium minutissimum (the causative agent of erythrasma, which fluoresces coral-red under Wood's lamp). This bacterial superinfection can precipitate acute ascending cellulitis and lymphangitis, particularly in diabetic or immunocompromised patients.

2. Moccasin-Type (Chronic Hyperkeratotic) Tinea Pedis

• Pathogen: Almost universally caused by Trichophyton rubrum.
• Clinical Presentation: Diffuse, chronic, non-inflammatory fine silvery scaling and mild erythema involving the plantar surfaces, medial and lateral borders of the feet, and heels in a "moccasin" distribution. Pruritus is typically mild or absent; patients frequently dismiss the condition as "dry skin" or calluses.
• "Two Feet, One Hand Syndrome": A classic clinical phenomenon in which bilateral moccasin-type tinea pedis is accompanied by unilateral tinea manuum involving the patient's dominant hand (the hand used to scratch or pick at the feet). T. rubrum is cultured from all three anatomical sites.

3. Vesiculobullous (Inflammatory) Tinea Pedis

• Pathogen: Most commonly caused by Trichophyton mentagrophytes.
• Clinical Presentation: Acute emergence of intensely pruritic, tense vesicles, bullae, or pustules filled with clear or straw-colored fluid, localizing predominantly to the plantar medial arch (instep) and submetatarsal areas.
• Dermatophytid ("Id") Reaction: An immunologically mediated, sterile secondary vesicular eruption occurring distant to the primary infection site (most commonly on the lateral aspects of fingers, palms, or trunk). The id reaction represents a delayed-type hypersensitivity (Type IV) response to circulating fungal antigens; vesicular fluid from id lesions is strictly sterile and contains no fungal elements.

4. Ulcerative Tinea Pedis

• Rapidly progressive, denuded, macerated, and painful ulcerations occurring within the interdigital web spaces and spreading onto the plantar foot. Typically represents an aggressive, synergistic polymicrobial infection combining dermatophyte hyphae with virulent Gram-negative bacilli (Pseudomonas, Proteus).

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4. Onychomycosis (Tinea Unguium) Classification

Onychomycosis accounts for approximately 50% of all nail disorders evaluated in clinical podiatry. Dermatophytes cause roughly 90% of toenail infections, with non-dermatophyte molds and yeasts accounting for the remainder.

+-----------------------------------------------------------------------------------------+
|                            CLINICAL PATTERNS OF ONYCHOMYCOSIS                           |
+-----------------------------------+-----------------------------------------------------+
| Clinical Subtype                  | Primary Pathogen & Key Anatomical Hallmarks         |
+-----------------------------------+-----------------------------------------------------+
| Distal Lateral Subungual (DLSO)   | • Trichophyton rubrum (>85% of cases)               |
|                                   | • Invades distal hyponychium & lateral nail groove  |
|                                   | • Subungual hyperkeratosis, yellowing, onycholysis  |
+-----------------------------------+-----------------------------------------------------+
| Proximal Subungual (PSO)          | • Trichophyton rubrum                               |
|                                   | • Invades proximal nail fold & matrix under lunula  |
|                                   | • Marker of immunosuppression (e.g., HIV)           |
+-----------------------------------+-----------------------------------------------------+
| White Superficial (WSO)           | • Trichophyton mentagrophytes                       |
|                                   | • Direct invasion of dorsal nail plate surface      |
|                                   | • Chalky white patches, readily scraped with blade  |
+-----------------------------------+-----------------------------------------------------+
| Total Dystrophic (TDO)            | • End-stage progression of DLSO or PSO              |
|                                   | • Entire nail plate & matrix ruined; thick, crumbly |
+-----------------------------------+-----------------------------------------------------+

Subtypes & Pathological Manifestations

1. Distal Lateral Subungual Onychomycosis (DLSO):

   • The most prevalent clinical variant (>85% of toenail onychomycosis).
   • Etiology: Predominantly Trichophyton rubrum.
   • Pathophysiology: Fungi penetrate the hyponychium and lateral nail groves, advancing proximally toward the matrix in the subungual space.
   • Clinical Features: Subungual hyperkeratosis (accumulation of crumbly keratinaceous debris beneath the nail plate), yellow-to-brownish discoloration, onycholysis (separation of the nail plate from the underlying bed), and progressive nail plate thickening (onychauxis) and brittleness.

2. Proximal Subungual Onychomycosis (PSO):

   • Etiology: Trichophyton rubrum.
   • Pathophysiology: Organisms breach the cuticle, invade the proximal nail fold, and penetrate the underlying nail matrix. Fungal elements become incorporated into the newly synthesized ventral nail plate and migrate distally as the nail grows.
   • Clinical Features: Opaque white, chalky leukonychia appearing initially beneath the lunula of the proximal nail plate. The overlying dorsal surface of the nail plate remains intact, smooth, and firm.
   • Board Correlation: In the absence of direct trauma, PSO is an extraordinary clinical rarity in immunocompetent individuals. Its emergence serves as a classic, high-yield clinical indicator of cellular immunodeficiency, specifically HIV infection or immunosuppression after organ transplantation, and it should prompt HIV testing when no other explanation is evident.

3. White Superficial Onychomycosis (WSO):

   • Etiology: Classically Trichophyton mentagrophytes (or non-dermatophytes such as Fusarium and Aspergillus).
   • Pathophysiology: Direct fungal invasion of the superficial dorsal surface of the nail plate; the underlying nail bed and ventral nail plate are uninvolved.
   • Clinical Features: Discrete, chalky white, dull, friable islands or patches on the dorsal nail plate. A unique diagnostic feature is that the white patches can be readily scraped away with a scalpel blade or curette, exposing healthy underlying nail keratin.

4. Total Dystrophic Onychomycosis (TDO):

   • The end-stage culmination of prolonged, untreated DLSO or PSO. The entire nail unit (matrix, bed, and plate) is destroyed, leaving a thickened, opaque, yellow-brown, severely fragmented, and crumbling mass of keratinaceous debris.

5. Endonyx Onychomycosis:

   • Fungi invade the interior substance of the nail plate without provoking subungual bed hyperkeratosis or onycholysis (e.g., Trichophyton soudanense).

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5. Laboratory Diagnostics in Podiatric Mycology

Clinical examination alone misdiagnoses onychomycosis in approximately 30–50% of dystrophic nails, as non-fungal conditions (psoriasis, lichen planus, microtrauma, contact dermatitis, yellow nail syndrome) mimic fungal dystrophy. Confirmatory laboratory testing is essential prior to initiating systemic antifungal therapy (e.g., oral terbinafine).

1. Potassium Hydroxide (KOH) Preparation

• Mechanism: Direct microscopic examination. A 10% to 20% aqueous solution of potassium hydroxide is applied to subungual debris or skin scales. KOH digests and dissolves host keratin, cellular membranes, and protein debris while leaving the alkali-resistant chitin and $\beta$-glucan fungal cell walls intact.
• Microscopic Findings: Dermatophytes appear as refractile, branching, uniform, septate hyphae with parallel cell walls and spherical arthroconidia.
• Enhancements:
  • Calcofluor White: A fluorochrome dye that binds specifically to $\beta$-(1,3) and $\beta$-(1,4) polysaccharides (chitin and cellulose). Under fluorescent microscopy, fungal elements fluoresce a bright blue-green, providing greater diagnostic sensitivity than standard brightfield KOH.
  • Chlorazol Black E: An amphoteric dye that stains chitin blue-black, improving contrast under ordinary light microscopy.

2. Fungal Culture & Dermatophyte Test Medium (DTM)

• Sabouraud Dextrose Agar (SDA): Standard fungal culture medium containing peptone and dextrose at pH 5.6. Commonly supplemented with chloramphenicol (to inhibit bacterial growth) and cycloheximide (to inhibit saprophytic environmental molds).
• Dermatophyte Test Medium (DTM):
  • A specialized modified SDA medium designed for rapid screening of dermatophytes.
  • Indicator: Contains the pH indicator phenol red (yellow at acidic pH <6.8; red at alkaline pH >8.2).
  • Biochemical Mechanism: Dermatophytes preferentially metabolize peptones/proteins as their initial energy source, liberating alkaline metabolites (ammonium ions). This raises the agar pH and causes the medium to turn from yellow to deep red simultaneously with early colony growth (within 7 to 14 days).
  • In contrast, environmental non-dermatophyte molds preferentially metabolize carbohydrates (dextrose) first, producing acidic metabolites that keep the agar yellow. (Non-dermatophyte molds turn DTM red only after several weeks when carbohydrates are completely exhausted and proteins are consumed).

3. Periodic Acid-Schiff (PAS) Histopathology of Nail Clippings

• The Clinical Gold Standard: PAS staining of full-thickness distal nail plate clippings combined with subungual debris exhibits the highest diagnostic sensitivity (>85–90%) of all diagnostic modalities (superior to fungal culture [~50–60%] and KOH prep [~70%]).
• Biochemical Mechanism: Periodic acid oxidizes 1,2-glycol groups in cell wall glucans and chitin to dialdehydes. The dialdehydes react with colorless Schiff reagent (leucofuchsin) to produce a vivid magenta to bright pink staining of fungal hyphae and pseudohyphae within the nail plate keratin.
• Grocott's Methenamine Silver (GMS): Stains fungal cell walls brownish-black via silver reduction, also providing exceptional histological contrast.

Table 2: Diagnostic Modality Comparison in Podiatric Mycology

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6. Non-Dermatophyte Molds & Opportunistic Yeasts

Non-Dermatophyte Molds (NDMs)

Non-dermatophyte molds account for approximately 5–10% of onychomycosis cases. They are environmental saprophytes that primarily act as secondary colonizers of damaged nails or primary pathogens in warm climates. Most NDMs respond poorly to standard oral terbinafine:

• Scopulariopsis brevicaulis: Produces characteristic lemon-shaped (annelloconidia) with truncate bases in chains; causes chalky-white to brown-yellow subungual hyperkeratosis.
• Fusarium species: Soil molds causing onychomycosis, keratitis, and opportunistic disseminated infections in neutropenic patients; produces canoe- or sickle-shaped multicelled macroconidia.
• Aspergillus species (A. niger, A. fumigatus): Produces black, brown, or green subungual discoloration with conidiophores bearing radiating phialides and chains of conidia.
• Scytalidium (Neoscytalidium dimidiatum): Common in tropical regions; causes dark, melanin-pigmented tinea pedis and DLSO that fails terbinafine therapy.

Candida albicans & Opportunistic Yeasts

• Microbiology: Candida albicans is a diploid eukaryotic yeast. It exhibits thermal dimorphism in reverse of classic systemic mycoses: it grows as a budding yeast (blastoconidia) with pseudohyphae at 20°C, and produces true hyphae and germ tubes when incubated at 37°C in mammalian serum for 2 to 3 hours (Germ Tube Test Positive).
• Podiatric Manifestations:
  • Candida Paronychia: Chronic erythematous, tender, swollen proximal nail folds with retraction or loss of the eponychium (cuticle). Frequently develops in individuals with chronic wet-work exposure (bartenders, dishwashers). Squeezing the nail fold often expresses purulent exudate.
  • Candida Onychia: Direct invasion of the nail bed and plate, frequently secondary to chronic paronychia or in patients with chronic mucocutaneous candidiasis (CMC) (associated with endocrine deficiencies and T-cell defects).
  • Interdigital Candidiasis (Erosio Interdigitalis Blastomycetica): Macerated, glistening white plaque with a peeling erythematous border in the web spaces, particularly between the 3rd and 4th toes.

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7. Subcutaneous & Deep Mycoses of the Lower Extremity

Traumatic transcutaneous implantation of environmental fungi into the deep dermis, subcutaneous fat, and osseous structures of the foot produces chronic, destructive granulomatous diseases.

Sporothrix schenckii (Sporotrichosis)

• Mycology: A thermally dimorphic fungus found in soil, sphagnum moss, decaying wood, and rose thorns.
  • At 25°C (room temperature): Grows as a mold with thin, septate hyphae producing delicate, pear-shaped conidia clustered in a daisy-wheel (rosette) pattern.
  • At 37°C (in host tissue): Transforms into cigar-shaped budding yeasts (pleomorphic, 2 to 6 $\mu$m).
• Pathogenesis: Inoculation occurs via penetrating plant trauma ("rose gardener's thorn").
• Clinical Presentation: An initial painless papule or nodule erupts at the inoculation site (foot, ankle, or hand) and ulcerates (sporotrichotic chancre). Within weeks, secondary subcutaneous nodules develop along the ascending lymphatic drainage pathway (lymphocutaneous / sporotrichoid spread). The nodules become fluctuant, ulcerate, and drain seropurulent fluid.
• Treatment: Drug of choice is oral Itraconazole (historical therapy was saturated solution of potassium iodide, SSKI).

Mycetoma (Madura Foot)

A chronic, slowly progressive granulomatous infection of the subcutaneous tissues, fascia, and bones of the foot and lower leg, endemic to arid tropical regions (the "mycetoma belt").

• The Diagnostic Triad of Mycetoma:

  1. Tumefaction: Massive, localized, indurated, non-tender subcutaneous swelling with architectural deformity of the foot.
  2. Multiple Draining Sinuses: Chronic fistulae that erupt through the skin surface.
  3. Discharge of Grains / Granules: Purulent exudate discharging visible macroscopic fungal or bacterial aggregates (grains) whose color provides crucial etiologic clues.

• Eumycetoma vs. Actinomycetoma:

  • Eumycetoma (True Fungal Mycetoma): Caused by fungi such as Madurella mycetomatis (produces black grains) or Acremonium / Fusarium (produce white/pale grains). Biopsy reveals wide, septate hyphae (2–6 $\mu$m) embedded in cement-like matrix. Requires long-term antifungal therapy (itraconazole) coupled with aggressive surgical debridement or amputation.
  • Actinomycetoma (Bacterial Mycetoma): Caused by filamentous higher bacteria belonging to the order Actinomycetales (e.g., Actinomadura madurae [red/pink or white grains], Nocardia brasiliensis [small white/yellow grains], Streptomyces somaliensis [yellow/brown grains]). Gram stain reveals delicate, branching Gram-positive filaments (0.5–1.0 $\mu$m). Responds to prolonged high-dose antibiotic therapy (trimethoprim-sulfamethoxazole [TMP-SMX], amikacin, dapsone) without extensive surgery.

Systemic, Subcutaneous and Opportunistic Mycoses

Beyond dermatophytes, the outline's mycology heading includes fungi that invade deeper tissues.

Dimorphic systemic fungi grow as molds in the environment at 25°C and as yeasts or spherules in tissue at 37°C. They are acquired by inhaling spores, cause a primary lung infection and can disseminate to skin and bone.

Subcutaneous mycoses follow traumatic implantation, often in the foot or leg:

• Sporothrix schenckii causes lymphocutaneous sporotrichosis: nodules spread up the lymphatics from a thorn or splinter wound (rose gardener's disease). It is treated with itraconazole.
• Chromoblastomycosis causes verrucous plaques on the lower leg with sclerotic (Medlar) bodies, also called copper pennies.
• Eumycetoma (Madura foot) causes chronic swelling, sinus tracts and fungal grains that destroy bone.

Opportunistic fungi:

• Candida albicans forms pseudohyphae and germ tubes. It causes intertrigo, paronychia, onychomycosis of the fingernails, and candidemia linked to catheters.
• Candida auris is an emerging, often multidrug-resistant yeast that spreads in health care settings.
• Aspergillus has acute-angle (45°) branching septate hyphae. It causes invasive disease in people with neutropenia.
• Mucor and Rhizopus have broad, ribbon-like, nonseptate hyphae with right-angle branching. They invade vessels in diabetic ketoacidosis and cause rhinocerebral or necrotizing wound infections that need surgical debridement and amphotericin B.
• Cryptococcus neoformans has a polysaccharide capsule seen on India ink stain. It causes meningitis in AIDS.

Systemic infections are treated with azoles (itraconazole, fluconazole, voriconazole) or amphotericin B for severe disease (11.2).