Major groups of disease-causing fungi

Importance of fungi in causing disease

There are more than 70 000 species of fungi but only about 300 are identified as pathogens in humans and animals. Some of these are cosmopolitan, others are found mainly in tropical regions. Some, those that infect superficially, cause only minor health problems but those that invade deeper tissues can be life threatening. These systemic forms have become much more serious problems as medical advances have taken place, e.g. immunosuppressive and antibiotic therapies, transplantation, invasive procedures and AIDS, such that opportunistic infections are now significant components of hospital-acquired infection.

Fungal pathogens can be classified on the basis of their growth forms or the type of infection they cause

Fungi were reclassified down to the level of order in 2007 following advances in fungal molecular taxonomy. Whilst this has no immediate effect on the practice of clinical microbiology, it will lead to greater understanding of the biology of the Kingdom Fungi and the diseases its members may cause.

Fungal pathogens may exist as branched filamentous forms or as yeasts (Fig. 4.1); some show both growth forms in their cycle and are known as dimorphic fungi. In filamentous forms (e.g. Trichophyton), the mass of hyphae forms a mycelium. Asexual reproduction results in the formation of sporangia, which are sacs that contain and then liberate the spores by which the fungus is dispersed; spores are a common cause of infection after inhalation. In yeast-like forms (e.g. Cryptococcus) the characteristic form is the single cell, which reproduces by division. Budding may also occur, with the bud remaining attached, forming pseudohyphae. Dimorphic forms (e.g. Histoplasma) form hyphae at environmental temperatures, but occur as yeast cells in the body, the switch being temperature-induced. Candida is an important exception in the dimorphic group, showing the reverse and forming hyphae within the body.