Fungi
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 with a eukaryotic cell structure and are able to grow as branching or filamentous forms (mycelia) or as single cells (yeasts).
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