Dematiceous Molds

Introduction to Dematiaceous Fungi

  • The lecture focuses on melanized molds, also referred to as dematiaceous fungi.

  • Characteristics of dematiaceous fungi:

    • Produce dark pigmentation due to melanin in their cell walls.

    • Melanin is a pigment also present in human skin, contributing to skin color and offering protection from UV radiation.

    • Increased melanin results in darker skin, while reduced melanin results in lighter skin.

    • Melanin provides protection against host immune defenses and antifungal agents.

Morphological Characteristics

  • Dematiaceous molds are morphologically characterized by:

    • Dark brown to black color due to cell wall melanin.

    • Observed both macroscopically on cultures and microscopically through hyphae and conidia under a microscope.

    • This pigmentation acts as a virulence factor, assisting in resistance to oxidative killing by the host.

Growth Rates

  • The growth rates of dematiaceous molds are relevant in clinical contexts:

    • Molds are categorized into:

    • Slow Growers: Require 7-10 days or longer to grow.

      • Typically associated with chronic or subcutaneous infections (e.g., chromoblastomycosis, mycetoma).

    • Rapid Growers: Grow in less than 7 days.

      • Often opportunistic or contaminants, associated with sinonasal infections especially in immunocompromised patients.

    • Infections primarily occur in immunocompromised patients, not commonly seen in healthy individuals.

Important Diseases Caused by Dematiaceous Fungi

  • Three major disease terms associated with these fungi are:

    • Pheohyphomycosis: Presence of pigmented hyphae in infected tissue.

    • Chromoblastomycosis: Chronic skin infections with characteristic sclerotic bodies.

    • Mycetoma: Granulomatous infections with grains in pus.

Superficial Infections

  • Tinea Nigra:

    • Causative organism: Hortaea werneckii.

    • Often mistaken for melanoma or moles; manifests as a dark brown-black mole that is not raised.

    • Typically found on palms or soles of feet, can be scraped off revealing pigmented hyphae on KOH prep.

    • More commonly seen in tropical/subtropical climates.

  • Black Piedra:

    • Causative organism: Piedraia hortae.

    • Occurs in long untreated hair and wraps around hair shafts, creating gritty nodules; may feel like sand.

    • Common in regions like South America, Asia, and tropical areas.

    • Treatment options include topical antifungals or shaving the hair off.

Mycetoma

  • Characteristics:

    • Chronic granulomatous infections, primarily occurring on feet or legs.

    • Caused by traumatic implantation of environmental organisms through soil, thorns, etc.

    • Symptoms include immense swelling, purple discoloration, small wounds leading to chronic infections.

    • Diagnostic feature: Multiple sinuses draining pus containing grains, which are clumps of the causative agent.

  • Types of Mycetoma:

    • Actinomycetomas: Form yellow or white grains, usually caused by bacterial agents (e.g., Nocardia, Streptomyces).

    • Eumycetomas: Cause black or white grains and are primarily caused by dematiaceous fungi.

    • Organisms associated include:

      • White grains: can be from both bacteria and fungi (e.g., Fusarium, Acremonium).

      • Black grains: Almost always fungi, specifically dematiaceous molds; includes genera such as Curvularia, Madurella.

Chromoblastomycosis

  • Caused by melanized molds entering through trauma (thorns, splinters).

  • Common in rural warm climates like South America, Africa, Southeast Asia.

  • Often unnoticed initially; lesions grow and can be mistaken for warts or calluses.

  • Lesions develop into cauliflower-like crusted growths with black dots representing emerging fungal structures.

  • Histological hallmark: Sclerotic bodies (Medlar bodies), round, thick-walled structures that exhibit divisions in multiple planes, visible under microscopy.

  • Common causative organisms include Fonsecaea pedrosoi, Phialophora, and Cladophialophora.

Pheohyphomycosis

  • A catch-all term for infections caused by different types of melanized fungi, characterized histologically by pigmented septate hyphae.

  • May present with yeast-like forms; absence of sclerotic bodies or grains.

  • Common in immunocompromised patients but also seen in cases of trauma in immunocompetent individuals.

  • Notable serious cases include cerebral pheohyphomycosis.

Clinically Relevant Organisms

  • Pseudalsharia boydii:

    • Also known as Scedosporium boydii.

    • Associated with white grain mycetoma and emerging opportunist in immunocompromised patients.

    • Can lead to sino-pulmonary diseases, brain abscesses, or cutaneous infections.

    • Rapid grower (5-10 days) and changes from white to gray/brown with age.

    • Microscopically shows oval conidia in clusters or singly.

  • Acremonium:

    • Often viewed as a lab contaminant but can cause severe infections in patients with central lines/neutropenia.

    • Known to cause white grain mycetoma.

    • Forms powdery/velvety colonies, colors may vary.

    • Long slender conidia with irregular shapes.

  • Cladophialophora & Phialophora:

    • Both are slow growers, classic agents of chromoblastomycosis.

    • Diagnosis based on the presence of sclerotic bodies.

    • Cladophialophora: Forms long chains of smooth oval conidia without branching.

    • Phialophora: Distinctive flask-shaped conidiophores with a collarette.

  • Funsecchia:

    • The most common cause of chromoblastomycosis worldwide.

    • Exhibits a unique conidiation pattern, grows slowly, and produces dark colonies that can appear olive or black.

  • Alternaria:

    • Common environmental mold and recognized allergen.

    • Can cause opportunistic infections: sinusitis, cutaneous infections.

    • Fast-growing and exhibits suede-like texture and dark coloration.

    • Microscopically shows large, muriform conidia with transverse and longitudinal septa.

  • Bipolaris:

    • Clinically significant; can cause fungal sinusitis and subcutaneous infections.

    • Rapid-growing and morphologically dark.

    • Conidia are distoseptate with scythe-like or curved shapes, capable of germ tube formation at both ends.

  • Curvularia:

    • Recognizable by the dramatic curvature of its conidia.

    • Associated with fungal sinusitis or skin/nail infections.

    • Colonies are suede-like, while conidia display significant curvature resembling boomerangs.

Conclusion

  • Dematiaceous fungi represent a diverse group of pathogens primarily affecting immunocompromised individuals but can also infect healthy hosts through trauma.

  • Understanding the morphological traits, growth rates, and disease implications of these fungi is crucial for accurate diagnoses and effective treatments in a clinical setting.