Fungi

IMMUNOLOGY HOST INVASION BY MICROORGANISMS INFECTIOUS AGENTS AND PARASITES

Introduction

Although there are over 50,000 identified species of fungi, only a few hundred are known to be pathogenic, causing infections known as mycoses (mycology being the study of fungi). Fungi are nonphotosynthetic organisms which are restricted to a parasitic or saphrophytic exsistence. They are abundant in soil, vegetation, water and on decaying vegetation and wood. Fungi have a eukaryotic cell structure and are able to grow as branching or filamentous forms (mycelia) or as single cells (yeasts).

Types of Fungi

There are two principle types of fungi, the moulds and the yeasts. Fungal hyphae are the branching tubular structures of moulds which become entwined as the fungi grows to form a mycelium. Hyphae are divided by cross walls into septa. There are arial (above the surface) and surface hyphae. The aerial hyphae produce spores which are reproductive fungal cells. There are two main types of spore produced, conidiospores and sporangiospores. These produce asexual spores called conidia which are relatively resistant to physical and chemical agents.

The yeasts range from 3µm to 5µm and are oval or spherical cells. Pseudohyphae are produced by some yeasts which are chains of irregular yeast cells. Dimorphic fungi change from the mycelial form into yeasts in certain conditions, e.g. 37°C or when inside animal tissue.

Major pathogenic fungi

Superficial: These infections affect only the skin, hair, nails etc.

• Generally not dangerous
• Examples- Hortaea werneckii, causing tinea nigra, and Piedraia hortae, causing black piedra

Cutaneous: Also known as dermatophytoses

• Only affect superficial keratinized tissue- skin, hair and nails
• Again generally not dangerous
• Examples- Candida albicans, causing candidiasis of the skin and nails, Microsporum species

Subcutaneous: Usually cause chronic localised infections of the skin and underlying dermis

• Normally reside in soil and vegetation; they enter skin through cuts and grazes
• Can occasionally affect the deep tissues, e.g. bones and muscles
• Examples- Sporothrix schenkii, causing sporotrichosis, Madurella mycetomatis causing mycetoma

Systemic: Also known as deep, endemic or primary

• Usually serious; sometimes fatal
• Examples- Histoplasma capsulatum causing histoplasmosis, Blastomyces dermatitidis causing blastomycosis

Opportunistic: Ubiquitous fungi that affect only susceptible hosts; immunocompetent individuals are normally resistant

• Examples- Aspergillus fumigatus, causing aspergilliosis, Cryptococcus neoformans, causing cryptococcosis

Immunity to fungi

Cutaneous and superficial fungal infections are normally self-limiting, with recovery associated with a resistance to re-infection. As hosts often develop delayed-type hypersensitivity, this resistance appears to be cell-mediated.

• T cell immunity- it is thought that helper T cells release cytokines to activate macrophages to fight the infection
• Neutrophils- there is now growing evidence for the role of neutrophils in some infections of the respiratory system, with defensin and the nitric oxide pathways implicated