The Biology of Fungi

Abstract

Fungi are found ubiquitously in the environment such as soil, water, and food. There are an estimated 1.5 million fungal species worldwide, although this number is felt to be grossly underestimated and is regularly updated. Of these vast numbers, around 500 fungi to date have been implicated in human disease. As opposed to bacteria, which are prokaryotes, fungi are eukaryotes, meaning they have a well-defined nucleus and have membrane- bound organelles in the cytoplasm, including an endoplasmic reticulum and a golgi apparatus. In 1969, the scientist R. H. Whittaker first proposed that organisms be classified into five kingdoms: Monera (Bacteria), Protista (Algae and Protozoans), Plantae (Plants), Mycetae (Fungi), and Animalia (Animals). Since then, there have been dramatic changes to the classifications of fungi, largely due to the appliance of phylogenetic molecular techniques. This has resulted in variances to the number of phylums, and the species assigned to them. Table 3.1 shows the seven phyla of the Fungi Kingdom. The majority of fungi pathogenic to humans inhabit the Ascomycota and Basidiomycota phyla. Fungi used to be dually named if they had a pleomorphic life cycle with sexual/ asexual stages (teleomorph/ anamorph, respectively), which meant that fungi often had two names and were classed differently. This practice was discontinued in January 2013 after the International Commission on the Taxonomy of Fungi decided that a ‘one fungus, one name’ approach should be followed. Fungi can be unicellular (yeast) or multicellular (fungi). Yeasts may look globose in nature when grown, whereas multicellular fungi grow as tubular, filamentous material called hyphae that can create a branching, hyphal network called a mycelium. Hyphae may have septa that cross their walls or be nonseptate, which is a method of differentiating fungi. An early hyphal outgrowth from a spore is called a germ tube. The germ tube test can be used to differentiate the yeasts Candida albicans and Candida dubliniensis from other Candida species. The fungal cell wall is composed of chitin and glucans, which are different components to the human cell wall. This means that they can be an effective target for antifungal therapy.