Fungi

FUNGI

What are fungi?

Yeast, such as brewers’ yeast, and moulds, such as Penicillium chrysogenum which produces the antibiotic penicillin, are classified as fungi. Yeast cells tend to grow as single cells which reproduce asexually in a process known as budding, although a minority of species (e.g.Schizosaccharomyces pombe) reproduce by fission. Many yeast species are capable of sexual reproduction and the formation of spores. In contrast, moulds grow as masses of overlapping and interlinking hyphal filaments and reproduce by producing masses of spores in a variety of structures. This division between yeast and moulds based on growth morphology is not clear-cut since some yeast can produce hyphae under specific conditions (e.g. Candida albicans) while many normally filamentous fungi possess a yeast-like phase at some point in their life cycle. Fungi are eukaryotic organisms, i.e. their cells possess a nuclear membrane, consequently there are many similarities between the biochemistry of fungal cells and vertebrate (human) cells. Fungi are widely distributed in nature, occurring as part of the normal flora on the body of warm-blooded animals, as decomposers of organic matter and as animal and plant pathogens. Medically, fungi are an extremely important group of microbes, being responsible for a number of potentially fatal diseases in humans (Table 4.1), but a significant number of fungi are of great benefit to humanity in terms of the production of alcoholic beverages, bread, enzymes, antibiotics and recombinant proteins (Table 4.2). Fungi have also been utilized for a range of molecular biological applications.

Fungal taxonomy is extremely difficult and there is much discussion as to the number and interrelatedness of fungal classes. Using one classification system, the kingdom Fungi can be subdivided into six classes. The class Oomycetes contains the mildews and water moulds, the class Ascomycetes contains the mildews, some moulds and most yeast species (including Saccharomyces cerevisiae), the class Basidiomycetes contains the mushrooms and bracket fungi, the class Teliomycetes contains the rust fungi (plant pathogens), the class Ustomycetes contains the smuts (plant pathogens) and the class Deuteromycetes contains species such as Aspergillus, Fusarium and Penicillium (see Figure 4.1). In contrast, phylogenetic analysis has revealed that there are four distinct phyla within the fungal kingdom; these are the Chytridiomycota, Zygomycota, Ascomycota and Basidiomycota.