Comprehensive Mycology: Introduction, Morphology, Diagnosis, and Clinical Classification of Fungi

Overview and General Objectives of Mycology

• Lecture Identity: MFMII: Lectures #90-91, Dr. Mondal.
• Reading Reference: Murray Medical Microbiology, Chapter 57.
• Key Objectives:     - Define structures of the fungal cell and their differences from other eukaryotic cells.     - Describe the basic forms of fungi and terminologies associated with different forms.     - Describe laboratory techniques employed to diagnose fungal infections.     - List antifungal agents and their mechanisms of action (MOAs).     - Identify characteristics and causative agents of superficial, cutaneous, and subcutaneous mycoses.     - Define the "Seven Questions" for each organism.
• Clinical Context: There is a notable increase in fungal infections among immunocompromised patients, including:     - Elderly individuals.     - Transplant recipients.     - Cancer patients.     - HIV-positive patients.
• Complications: Fungal infections commonly lead to opportunistic infections, relapses, and super-infections.

Fungal Cell Structure and Biology

• Eukaryotic Nature: Fungi possess organelles, a cytoskeleton, a nucleus, and mitochondria.
• Plasmalemma (Cell Membrane):     - Structure: A lipid bilayered structure composed of phospholipids and unique sterols.     - Sterols: Primarily ergosterol and zymosterol (distinct from cholesterol in animal cells).     - Functions:         - Protects cytoplasmic contents.         - Regulates intake and secretion of solutes.         - Facilitates the synthesis of the capsule and cell wall.     - Drug Targets:         - Polyenes (e.g., Amphotericin B): Target ergosterol directly.         - Azoles (e.g., Fluconazole): Inhibit ergosterol synthesis.
• Cell Wall:     - Structure: A multilayered structure that is antigenic in nature.     - Composition:         - Polysaccharides: Approximately $90\%$; composed of hexose and hexosamine polymers.         - Proteins and Glycoproteins: Approximately $10\%$.     - Functions: Provides shape, rigidity, strength, and protection from osmotic shock.     - Polymers and Monomers:         - Chitin: Monomer is N-acetyl glucosamine.         - $\alpha$-Glucan: Monomer is D-Glucose.         - $\beta$-Glucan: Monomer is D-Glucose.         - Mannan: Monomer is D-Mannose.     - Variability: Polysaccharide types vary from one fungal species to another.     - Drug Target: Echinocandins (e.g., Caspofungin). These drugs inhibit the synthesis of $\alpha$-1,3-d-glucan.
• Capsule:     - Structure: Polysaccharide.     - Function: Acts as an anti-phagocytic virulence factor.     - Presence: Exists only in some fungi.     - Example: Cryptococcus neoformans (an encapsulated yeast). The capsule can be visualized using the India ink stain.

Fungal Morphology and Growth Forms

• General Classifications:     - Yeast: Unicellular organisms. Reproduce by nuclear fission or budding. A bud is referred to as blastoconidia. Structures include germ tubes and pseudohyphae.     - Mold: Multicellular organisms.
• Defining Structures:     - Hypha(e): Branching filaments.     - Mycelium: A network of filaments.     - Spores: Reproductive, unicellular units. Spores are rarely observed in skin scrapings.
• Types of Hyphae:     - Septate: Divided by crosswalls (e.g., Aspergillus).     - Aseptate (Coenocytic): Multinucleate without crosswalls (e.g., Zygomycetes).
• Dimorphism:     - Fungi capable of growing in either mold or yeast forms depending on environmental conditions (temperature, $CO_2$, nutrients).     - Thermal Dimorphism: Characteristic of many pathogenic fungi.         - $25^{\circ}C$: Grows as filamentous fungi (mold) in the environment.         - $37^{\circ}C$: Grows as a single-cell organism (yeast) in the host.

Reproduction and Spore Types

• Spores: Function in reproduction; normally haploid and unicellular.
• Sexual Reproduction (Teleomorph):     - Involves meiosis of two compatible mating types.     - Results in a fruiting body.
• Asexual Reproduction (Anamorph):     - Involves mitosis; often represented by the mold-like stage.
• Specific Asexual Forms:     - Conidia: Spores at the tip of hyphae.     - Blastoconidia: Buds produced by yeasts.     - Arthroconidia: Formed by the segmentation of hyphae.     - Spherules and Endospores: Specialized structures found in organisms like Coccidioides.

Laboratory Diagnosis of Mycoses

• Microscopy:     - Direct Microscopic Examination (Wet Mounts):         - Potassium Hydroxide (KOH): Dissolves keratin to reveal colorless cells or hyphae.         - India Ink: Used to visualize colorless cells with halos, indicating a capsule.         - Calcofluor White: A fluorescent stain that specifically targets glucans in the cell wall.     - Examination Conditions: Microscopy conducted at $25^{\circ}C$ or $37^{\circ}C$ to assess morphology.
• Culture: Fungi are typically slow-growing.     - Media: Sabouraud dextrose agar (SDA) or Mycobiotic agar.
• Serology: Screening for specific antibodies.
• Molecular Techniques: Antigen detection or Polymerase Chain Reaction (PCR).

Diagnostic Morphologies of Specific Fungi

• Candida spp.: Characterized by budding yeast cells (blastoconidia), pseudohyphae, and septate hyphae.
• Cryptococcus neoformans: Pleomorphic yeast-like cells, formation of narrow-based buds, and encapsulated budding yeast.
• Aspergillus (e.g., A. niger, A. fumigatus): Hyaline, septate, dichotomously branched hyphae with uniform diameter, branching at acute angles.
• Coccidioides immitis: Characteristic spherules containing endospores.
• Histoplasma capsulatum: Typical intracellular budding yeast cells.
• Blastomyces dermatitidis: Broad-based budding yeast.

Clinical Classification of Mycoses

• Pathogen Types:     - Primary Pathogens: Infect immunocompetent hosts.     - Opportunistic Pathogens: Infect hosts with impaired defenses (e.g., due to steroids, chemotherapy, radiation, HIV, or antibiotics).
• Immunity: Cell-mediated immunity is critical for maintenance against fungal infections.
• Anatomical Classification:     - Superficial and Cutaneous: Involves hair, skin, nails, and cornea.     - Subcutaneous: Involves deeper skin layers, often associated with trauma.     - Systemic (Deep Seated): Can be opportunistic or endemic; affects lungs, meninges, liver, lymphatics, bone, and bloodstream.

Superficial Mycoses

• Pityriasis (Tinea) Versicolor:     - Causative Agent: Malassezia furfur (part of normal skin flora).     - Clinical Presentation: Small, irregular, well-demarcated macules (hypopigmented or hyperpigmented). Patches remain white when skin around them tans in sunlight.     - Transmission: Transfer of infected keratinous material between persons.     - Diagnosis:         - Microscopy (KOH): Features "Spaghetti and Meatballs" appearance (short, curved hyphae with spherical yeast cells).         - Wood Lamp (UV): Lesions fluoresce a pale yellowish color.         - Culture: Requires supplemental oil/lipid source as Malassezia cannot synthesize fatty acids.     - Treatment: Selenium sulfide (Selsun Shampoo) or topical ketoconazole.
• Tinea Nigra:     - Causative Agent: Exophiala werneckii (also known as Hortae werneckii).     - Clinical Presentation: Solitary, irregular, painless brown-to-black pigmented macules on palms or soles.     - Environment: Saprophytic fungus in soil, compost, and wood in humid/subtropical regions.     - Diagnosis: KOH shows two-celled oval yeast and short hyphae. Black colonies grow on Sabouraud agar.     - Treatment: Topical ketoconazole.
• Piedra:     - White Piedra: Trichosporon spp. (normal flora/soil). White-brown swelling around hair shafts in the groin and axilla.     - Black Piedra: Piedraia hortae. Small dark nodules surrounding hair shafts on the scalp. Characterized by barrel-shaped arthroconidia.     - Treatment: Shaving or clipping infected hair.

Cutaneous Mycoses (Dermatophytosis)

• General Characteristics: Also called keratinophilic fungi. They secrete keratinase to digest keratin. They do not invade living tissue.
• Etiologic Agents: Three genera of dermatophytes:     1. Microsporum (Skin and hair; rarely nails).     2. Epidermophyton (Skin and nails; no hair).     3. Trichophyton (Skin, hair, and nails).
• Habitat Groups:     - Anthropophilic: Human-to-human transmission.     - Geophilic: Soil-to-human transmission.     - Zoophilic: Animal-to-human transmission.
• Morphological Traits:     - Microsporum: Spindle-shaped, rough, thick-walled macroconidia. Microconidia are rare. Fluoresces "bright green" under Wood lamp (UV).     - Epidermophyton floccosum: Smooth, club-shaped macroconidia in bifurcated hyphae. No microconidia. Forms yellow, cottony cultures.     - Trichophyton: Numerous microconidia (teardrops, spheres, or peg-shaped). Rare thin-walled macroconidia. Branching septate hyphae.
• Clinical Types of Tinea:     - Tinea corporis: Glabrous (hairless) skin (Ringworm).     - Tinea barbae: Beard area.     - Tinea cruris: Groin (Jock itch).     - Tinea pedis: Feet/between toes (Athlete's foot; most common dermatophytosis).     - Tinea unguium: Nails (Onychomycosis; "Green Nail disease").     - Tinea capitis: Scalp.     - Favus (Tinea favosa): Caused by Tri. schoenleinii; permanent hair loss, highly contagious.
• Treatment:     - Mild/Skin: Topical terbinafine or azoles.     - Hair, Nail, or Severe: Oral terbinafine, griseofulvin, or itraconazole.
• Diagnosis Note: Dermatophytes are resistant to cycloheximide (a protein synthesis inhibitor), which is used in selective media.

Subcutaneous Mycoses

• Sporotrichosis ("Rose Handler's Disease"):     - Causative Agent: Sporothrix schenckii.     - Environment: Soil, tree bark, shrubs, rose thorns.     - Dimorphism:         - $37^{\circ}C$: Round or cigar-shaped yeast cells.         - $25^{\circ}C$: Septate hyphae with rosette-like clusters of conidia at conidiophore tips.     - Clinical Presentation:         - Lymphocutaneous: Linear chain of nodules extending proximally along lymphatic channels from the primary inoculation site (e.g., a thorn prick).         - Pulmonary: Subacute or chronic pneumonia in COPD patients.         - Osteoarticular: Chronic involvement.         - Disseminated: Rare; involves multiple organs.     - Diagnosis:         - Histopathology: Asteroid bodies (Splendore-Hoeppli material) – yeast cells surrounded by eosinophilic, star-shaped antigen-antibody complexes.         - Microscopy: Cigar-shaped yeasts.     - Treatment:         - Lymphocutaneous: Oral itraconazole for 3 to 6 months.         - Disseminated: IV Amphotericin B followed by oral itraconazole for 1 year.
• Chromoblastomycosis:     - Causative Agents: Phialophora verrucosa, Cladosporium carrionii, Fonsecaea species.     - Features: Copper-colored soil saprophytes found on rotting wood.     - Morphology: Sclerotic bodies (copper-colored cells).     - Clinical: Following a puncture wound, a violet wart-like lesion develops, eventually resembling cauliflower.     - Treatment: Itraconazole or local excision.

Candidiasis

• Causative Agent: Candida albicans.
• Reservoir: Normal inhabitant of the skin, mouth, and GI tract. Not found in blood normally.
• Morphology: Pseudohyphae and yeast.
• Clinical Manifestations (Normal Host):     - Oral thrush.     - Vulvovaginal candidiasis.     - Cutaneous diaper rash.
• Clinical Manifestations (Immunocompromised):     - Esophageal: Retrosternal chest pain, dysphagia, fever.     - Disseminated: Multi-organ failure in hospitalized patients.     - Chronic mucocutaneous candidiasis.
• Treatment:     - Thrush: Oral fluconazole, nystatin swish and spit, clotrimazole candies.     - Cutaneous: Topical imidazole or oral fluconazole.     - Esophageal: Fluconazole or caspofungin.     - Systemic: IV Amphotericin B, fluconazole, or caspofungin.     - Diagnosis: KOH or Silver stain; blood assay for beta-D-glucan.

Questions & Discussion

• Patient Case 1: A 25-year-old man with light and dark patches on chest/back after sun exposure. No itching. KOH shows "spaghetti and meatballs."     - Answer: Malassezia furfur.
• Patient Case 2: A 35-year-old runner with a pruritic, scaly rash between toes. KOH reveals branching septate hyphae.     - Answer: Tinea pedis (likely Trichophyton).
• Patient Case 3: A 9-year-old boy with a "bump" on the back of his head. Culture positive for Microsporum.     - Physical Exam Clue: Ultraviolet light lamp.     - Wood Lamp Appearance: Bright green.     - Microscopy: Numerous macroconidia (spindle-shaped).
• Patient Case 4: A 38-year-old landscaper pricked by a thorn develops a linear chain of nodules on the forearm.     - Answer: Sporothrix schenckii.
• Question Clue - Acute Angles: Which fungi shows septate, dichotomously branched hyphae in acute angles?     - Answer: Aspergillus fumigatus.
• Question Clue - Broad-Based Budding: Associated with which fungus?     - Answer: Blastomyces dermatitidis.
• Complex Diagnostic Case: Mario Gonzales from Mexico presenting with FUO, malaise, confusion, itchy facial hair, ptosis, and diplopia. Culture of facial hair showed pseudohyphae and "spaghetti and meatballs."     - Outcome: The physician was stumped because both are normal flora. Why was Mario arrested?     - Diagnosis: Fungal Ingestion (hallucinogenic) rather than infection.