Fungi Morphological Features

Morphological Features of Fungi

• Three basic morphologies of fungi: yeast, mould, and dimorphic fungi
• Yeast:
  • Unicellular organism that reproduces by budding or binary fission
  • Forms pseudohyphae through continued budding
  • Larger than bacteria, spherical to egg-shaped
  • Facultative anaerobe, prefers warmer temperatures (37°C)
  • Grows rapidly (24-48 hours) and forms creamy, opaque, or pasty colonies
• Mould:
  • Has branching tubular structures called hyphae
  • Forms mycelia on culture media
  • Colonies are fluffy, cottony, woolly, or powdery
  • Grows slowly, requiring about 6 weeks
  • Pattern of branching and width of mycelium aid in morphological identification
  • Can be coenocytic (non-septate) or septate
• Dimorphic fungi:
  • Exist as yeasts or spherules in tissues and exudates at 35-37°C
  • Conversion to yeast form appears essential for pathogenicity
  • Identified by morphological or biochemical characteristics and fruiting bodies

Hyphae

• May be coenocytic (aseptate and multinucleated) or septate
• Grow at their tips by apical extension
• Divided into two main types: vegetative and aerial hyphae
• Vegetative hyphae: submerged, responsible for nutrition
• Aerial hyphae: project above the surface, produce specialized structures called conidia for asexual reproduction

Fungal Reproduction

• Fungi reproduce by budding, hyphal extension, or spore formation
• Spores are resistant to heat, cold, acids, bases, and other chemicals

Fungal Spores

• Can cause tissue inflammation, necrosis, and gangrene (e.g., ergot alkaloids of Claviceps purpurea)
• Can cause liver damage and may be carcinogenic (e.g., aflatoxin of Aspergillus flavus)
• Can trigger hypersensitivity reactions, such as asthmatic attacks, rhinitis, pneumonitis, and alveolitis

Infections Secondary to Colonization

• Classified based on the part of the body affected
• Includes superficial mycosis, cutaneous mycosis, subcutaneous mycosis, systemic mycosis, and opportunistic mycosis
• Opportunistic mycosis caused by organisms of low pathogenicity that produce disease only under conditions of lowered immunity

Superficial Mycoses

• Affect the outermost layers of the skin and hair without eliciting host/pathogen hypersensitivity reactions
• Two types: Tinea (Pityriasis and Nigra) and Piedra (White and Black)
• Tinea affects the skin, while Piedra affects the hair

White Piedra

• Caused by Trichosporon beigelii
• Pathology: soft, loosely attached white or light brown nodule on axillary, pubic, beard, and moustache hairs
• Culture: cream-colored yeast-like colony with radial furrows and irregular folds
• Microscopy: septate hyaline hyphae, arthrospores, and blastospores
• Treatment: remove affected hairs and apply topical antifungal, 3% sulphur ointment

Black Piedra

• Caused by Piedraia hortae
• Pathology: firmly attached, hard, black nodule around scalp hairs
• Culture: small brown to black colonies with elevated center and flat periphery, sometimes with red pigmentation
• Microscopy: dark, thick-walled, septate hyphae with swollen irregular cells (Chlamydoconidia)
• Treatment: remove affected hairs and apply topical antifungal, 3% sulphur ointment

Tinea Nigra Palmaris

• Caused by Exophiala werneckii
• Pathology: chronic infection of stratum corneum, appearing as light brown to black non-scaly patches on palmar surface of hands and fingers
• Culture: black yeast colony that may develop into short olive-gray mycelium
• Microscopy: brown, pigmented budding yeast cells with branched septate hyphae
• Treatment: daily application of Whitfield's ointment, 3% sulphur, 1% salicylic acid, or azole drug

Tinea Versicolor (Pityriasis Versicolor)

• Caused by Malassezia furfur (Pityrosporum orbiculare)
• Pathology: chronic, non-irritating infection of stratum corneum, characterized by superficial white, brown, or fawn-colored lesions
• Culture: not usually necessary, diagnosis is mostly clinical
• Microscopy: short, unbranched hyphae or spherical cells, diagnostic with 10-20% KOH digested skin scraping
• Treatment: use of 2% selenium sulphide applied daily for a few days, or selsun shampoo

Cutaneous Mycoses

• Fungal diseases confined to outer layers of skin, nail, or hair
• Only affect keratinized layers of the body, rarely invading deeper tissues or viscera
• Fungi involved are called Dermatophytes, belonging to three genera: Microsporum, Trichophyton, and Epidermophyton
• Dermatophytes are probably restricted to non-viable skin because most are unable to grow at 37°C or in the presence of serum
• Classified as anthropophilic, geophilic, and zoophilic, depending on their usual habitat: humans, soil, or animals