Medical Mycology Overview

Mycology

• Mycology is the study of fungi
• Key aspects include taxonomy, environmental impact, and genetic/biochemical properties

Medical Mycology

• Focuses on fungi and their relationship to human disease
• Encompasses single-celled yeasts and filamentous molds
• Includes agents causing superficial skin infections (cutaneous mycosis) and disseminated deep-seated visceral disease (systemic mycosis).
• True pathogens are historically pathogenic fungi—opportunistic.
• Pathogens—Saprophytic fungi
• Immunocompromised hosts (HIV/AIDS, immunosuppressive infections, high-dose cancer chemotherapy, solid organ transplant) are at risk.

Common Terms in Medical Mycology (Table of Terms)

• (A comprehensive list of terms and related definitions)

Overview of Fungi

• Diverse group of organisms, including molds (filamentous) and yeasts (moist)
• Categorization includes classic pathogens and environmental saprobes (living on nonliving material)
• Can cause a range of signs and symptoms, from mild to life-threatening, significant in immunocompromised patients

General Characteristics

• Eukaryotic organisms
• Most fungi are saprophytic (live on dead organic matter)
• Fungi can be yeasts (moist) or molds (filamentous).
• Dimorphic organisms can exist in both yeast and mold forms, often dependent on temperature.
• Polymorphic fungi have more than one form or stage.
• Fungi have thick cell walls made of chitin.
• Mostly obligate aerobes (require oxygen)
• Prefer neutral pH.

Shared Characteristics

• Contain chitin in the cell wall
• Ergosterol in the cell membrane
• Reproduce via spores (sexually or asexually)
• Lack chlorophyll
• Lack susceptibility to antibacterial agents

Epidemiology

• Increased fungal infections worldwide, particularly in immunocompromised individuals.

Yeasts

• Single vegetative cells
• Form a smooth, creamy, bacterial-like colony without aerial hyphae
• Identification relies on biochemical testing and molecular diagnostic methods
• Reproduction via budding or fission

Molds

• Filamentous mycelium (long strands of tubelike structures called hyphae)
• Reproduction by spores (asexually or sexually)
• Vegetative mycelium composed of arthrospores, chlamydospores, blastospores, and rhizoids
• Aerial mycelium composed of conidia and sporangia

Mold Morphology

• Hyphae can be diverse in shape, including antler, racquet, spiral, or rhizoid shapes.
• Septate hyphae have frequent perpendicular cross-walls
• Sparsely septate hyphae have frequent perpendicular cross-walls

Hyaline Versus Phaeoid

• Hyaline fungi are nonpigmented or lightly pigmented
• Phaeoid (dematiaceous) fungi have darkly pigmented cell walls due to melanin production

Dimorphism and Polymorphism

• Dimorphism is the ability to exist in two forms based on growth conditions (mold phase at room temperature/ yeast phase at 37C with increased CO2)
• Polymorphism is an ability to form both yeast and mold forms in the same culture.

Asexual Reproduction

• Asexual reproduction involves conidia (conidium) formation following mitosis
• Types include phialoconidia (produced in fruiting structures or phialides), annellides (produced in structures called annellides), and arthroconidia (formed by fragmentation of specialized hyphae.)

Sexual Reproduction

• Sexual Reproduction involves the teleomorph stage.
• Some fungi can reproduce asexually as occasionally observed.
• Synanamorphs are a name given to strains that display more than one anamorph (form) in the same teleomorph.

Zygospore Formation

• Sexual reproduction involving the fusion of hyphae and nuclei

Taxonomy Nomenclature

• Historically, fungi had multiple names associated with reproductive forms
• Current practice uses a "one fungus, one name policy". (International Codes of Nomenclature for Algae, Fungi, and Plants)

Taxonomy of Clinically Significant Fungi

• Four groups of fungi cause clinical infections including Phylum Ascomycota, Basidiomycota, Mucoromycotina, and Entomophthoromycotina.

Mucorales

• Rapid-growing fungi often causing opportunistic infections.
• Characterized by a gray to white aerial mycelium with hyaline, sparsely septate hyphae.
• Reproduce asexually.
• Results in zygospores (not commonly seen in clinical labs).

Ascomycota

• Sexual spores are ascospores.
• Ascospores are contained within sac-like structures (asci).
• Common species include Microsporum, Trichophyton, and Scedosporium.

Basidiomycota

• Filobasidiella neoformans is a significant pathogen (causing cryptococcosis).
• Other genera associated with human infections are Malassezia and Trichosporon.
• Identification involves clamp connections at septations.

Fungi Imperfecti

• Category applied when mode of sexual reproduction is not known.
• Identified based on characteristic asexual reproductive structures.
• Examples: Ulocladium and Alternaria tenuis.

Parafungal Organisms

• Resemble yeast and protozoans
• Remain unculturable
• Atypical agents of human infections.

Macroconidia

• Larger conidia produced by some molds