The Biology of Fungi Impacting Human Health

• Habitat Relationships • The primary and opportunistic pathogens that infect human beings are associated with plants, animals, organic debris and soil particles in various habitats. The exact nature of their interactions and associations is unknown to a large extent, as information collected has relied heavily on identification of the organisms isolated in pure culture. With the application of molecular techniques, it is confidently expected that major advances will be possible in the coming decades. Besides the human pathogenic fungi, there is another group of fungi that invade plant products producing mycotoxins. These organisms are often soil fungi, which can live indefinitely as saprophytes. They may act as unspecialized pathogens or occasionally they may be more specialized pathogens (e.g., Claviceps) that are unable to survive in the soil as free-living organisms. The soil is a complex environment that continually receives living and dead tissues from animal and plant sources. These tissues may be occupied by microorganisms on their entry to the soil or they essentially may be virgin. As microbes colonize and degrade substrates their nature alters and consequently the ability of a substrate to support various groups of microbes changes over time. For many, early colonization will give a competitive advantage. Eventually however a substrate disappears so that the organism must either move on (growth or spore dispersal) or it enters a dormant phase. • Ecological Classification • The ecological classification of soil and other organisms is challenging. For example, resident soil organisms are consistently found in climax communities in a particular location and can become dominant colonizers of plant and animal tissues reaching the soil. Some are characterised by an ability to use simple sugars and to germinate and grow quickly. These organisms are well represented among the Mucorales. They bridge the gap between the appearance of successive suitable substrates by the formation of dormant structures—asexual and sexual spores and modifications of the hyphae (Garrett 1951, Atlas & Bartha 1998, White et al. 2006). At the other end of the ecological spectrum are the fungi capable of decomposing lignin. The Basidiomycetes are well represented among this group and with them the genus Schizophyllum. The tendency among this group is to develop mycelial strands and rhizomorphs allowing the organisms to colonize new food bases somewhat removed from their original base (Garrett 1951). Both these groups of fungi belong to the resident or indigenous biota. Some microbes that colonize a habitat as indigenous members of the biota are potentially pathogenic. When conditions change, they may become pathogens (Mackie 1996). The ecological concept of resident (indigenous or autochthonous) and non-resident or transient (cf. allochthonous—do not colonize and grow) is valuable whether we are dealing with the animal, plant, soil or water systems. • Ascomycetous and Basidiomycetous Yeasts • The yeasts will be discussed under a single heading for the simple reason that both groups occur frequently as unicellular organisms, commonly reproduce asexually via budding and share some common elements of biology. We will discuss their general features first and then highlight the special features of the two distinct phylogenetic groups. General Features. Yeasts are found in association with nutrient rich substrates where they are among the primary decomposers. They are located in all biomes and are abundant in the soil and the phyllosphere. They enter into a variety of inter-relationships with other organisms. Yeasts are particularly successful early competitors as they have a high surface to volume ratio that allows them to acquire nutrients rapidly (Starmer & Lachance 2011). This ability is surpassed by the bacteria that are smaller in size. Yeasts are able to derive energy from substrates through respiratory and fermentative metabolism. Substrates such as pentose sugars, sugar alcohols, amino sugars, organic acids and even aromatic compounds may be utilized. • Basidiomycetous Yeasts • Under this heading we will deal with the genera Malessizia and Cryptococcus (Filobasidiella). The latter is found on the skin of warm blooded animals and is phylogentically positioned in the Tremellomycetes while the former is placed in the subphylum Ustilaginomycotina (possibly class Exobasidiomycetes). Cryptococcus. Not all Cryptococcus species are pathogenic, but there are several notable exceptions. C. neoformans (serotypes A, D and AD) is most frequent encountered in immunocompromised individuals, such as HIV-infected patients. C. gattii (serotypes B and C), on the other hand, appears to be more common among individuals with competent immune systems. C. gattii tends to be found mainly in tropical and subtropical regions whereas C. neoformans is worldwide in its distribution. However, recent outbreaks of C. gattii in western Canada and northwestern United States and investigations elsewhere have put paid to such neat geographical boundary statements (Cooper 2011). The identification of strains of C. gattii in North America has led to a new emphasis on the natural relationships of the organism beyond an association with leaves, bark and wood of Eucalyptus and other selected trees. It is apparently able to grow in soil, particularly where moisture levels and organic carbon content are low, and has been isolated from water where it can survive for at least a year. It does not survive in areas experiencing temperatures below 0 °C. Dispersal of the organism may occur via footwear, the wheels of vehicles and forestry activity and can be aerosolized readily through wood chipping, mulching and disturbances of the soil (Kidd et al. 2007, Bartlett et al. 2012). Its range of ecological niches now extends to soil, vegetation, insect frass, bird excreta, water and it is frequently associated from decayed wood inside tree hollows (genera involved are diverse), cacti, bark and plant debris (Baddley & Dismukes 2011, Chowdhary et al. 2011, Harris et al. 2012). The organism is able to grow profusely on wood and wood extracts and isolates are highly melanized on these substrates. Loss of melanization results in loss of virulence, which suggests that selection of ecologically fit isolates are selections for virulence. The fungus is able to infect wounded Arabidopsis thaliana plants using extracellular fibrils for adhesion. This attachment device is necessary for infection and is also significant to virulence in mice (Springer et al. 2010).