Clinical meaning
Fungal infections (mycoses) represent a diverse group of diseases caused by eukaryotic organisms that possess cell walls containing chitin and cell membranes containing ergosterol -- key molecular targets that distinguish fungi from human cells and form the basis for antifungal pharmacotherapy. Fungi are classified into three major morphological groups relevant to clinical practice: yeasts (unicellular organisms that reproduce by budding, such as Candida and Cryptococcus), molds (multicellular organisms that grow as branching filamentous hyphae, such as Aspergillus and Mucor), and dimorphic fungi (organisms that exist as molds in the environment at 25 degrees Celsius and convert to yeast forms at body temperature of 37 degrees Celsius, such as Histoplasma, Blastomyces, and Coccidioides). This thermal dimorphism is remembered by the mnemonic: mold in the cold, yeast in the heat (or beast). Fungal infections are categorized by depth of tissue involvement. Superficial mycoses affect only the outermost layers of skin and hair (tinea versicolor caused by Malassezia furfur). Cutaneous mycoses (dermatophytoses) involve the keratinized layers of skin, hair, and nails, caused by three genera of dermatophytes: Trichophyton, Microsporum, and Epidermophyton. These organisms possess keratinase enzymes that allow them to digest keratin for nutrition. Common dermatophyte infections include tinea corporis (body ringworm), tinea pedis (athlete's foot), tinea cruris (jock itch), tinea capitis (scalp ringworm), and tinea unguium/onychomycosis (nail fungus). Subcutaneous mycoses involve deeper skin layers and subcutaneous tissue, typically introduced through traumatic inoculation (thorn pricks, splinters), including sporotrichosis (rose gardener's disease caused by Sporothrix schenckii) and chromoblastomycosis. Systemic (deep) mycoses affect internal organs and represent the most life-threatening fungal infections. Opportunistic systemic mycoses occur primarily in immunocompromised hosts and include candidiasis, aspergillosis, cryptococcosis, mucormycosis, and Pneumocystis jirovecii pneumonia (PCP). Endemic systemic mycoses are caused by dimorphic fungi with specific geographic distributions and can cause disease in immunocompetent individuals: histoplasmosis (Ohio and Mississippi River Valleys, Central America), blastomycosis (Great Lakes region, Ohio and Mississippi River Valleys), coccidioidomycosis (Southwestern US, Mexico -- San Joaquin Valley fever), and paracoccidioidomycosis (Latin America). Laboratory diagnosis of fungal infections relies on several techniques. The potassium hydroxide (KOH) preparation is the most widely used rapid diagnostic test: skin scrapings, nail clippings, or hair samples are placed on a glass slide with 10-20% KOH solution, which dissolves keratin and other tissue elements while leaving fungal elements intact for microscopic visualization. The Wood lamp (ultraviolet light at 365 nm wavelength) examination causes certain dermatophyte species to fluoresce: Microsporum canis and Microsporum audouinii produce a bright green fluorescence, while Trichophyton species generally do not fluoresce (making the Wood lamp useful for Microsporum but not Trichophyton). Fungal culture on Sabouraud dextrose agar at 25-30 degrees Celsius remains the gold standard for species identification but requires 2-6 weeks for growth. For systemic mycoses, histopathological examination with special stains (GMS -- Grocott methenamine silver, and PAS -- periodic acid-Schiff) demonstrates fungal elements in tissue. The practical nurse must recognize that immunocompromised patients are at significantly elevated risk for fungal infections and that seemingly minor superficial infections in these patients can rapidly progress to life-threatening invasive disease. Risk factors for invasive fungal infections include neutropenia, organ transplantation, HIV/AIDS, prolonged corticosteroid use, broad-spectrum antibiotic therapy (disrupts normal bacterial flora allowing fungal overgrowth), central venous catheter placement, total parenteral nutrition, and diabetes mellitus (especially poorly controlled).