All About Fungi: Part 1

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Fungi include a range of different forms including moulds, yeasts and higher fungi. All fungi are eukaryotic, that’s to say they have their genetic material organized into a membrane-bound nucleus. They are distinct from bacteria in that they have sterols but not peptidoglycan in their cell membrane. Fungi are chemoheterotrophs (they require organic matter for nutrition) and most are aerobic. Many fungi are saprophytes, which means that the live off dead organic matter, and are found in soil and water. Characteristically, they produce sexual and asexual spores. There are over 100,000 species of fungi known, but only around 100 of these are infectious agents of humans. Moulds are composed of numerous microscopic, branching hyphae known collectively as a mycelium.

Septa, (the plural of septum) are cross-walls which divide fungal hyphae into cells. They may add strength to the hyphae or serve to isolate adjacent parts to allow differentiation, such as during production of the reproductive structures.
The structure of the septa is of taxonomic importance. They may consist of a solid plate, have a single pore at the centre, or have multiple pores creating a sieve-like appearance. In most moulds the hyphae have septa (crosswalls), but in some have no septa and these are known as coenocytic or aseptate. An example of a mould is Penicillium, the source of penicillin, the first antibiotic to be discovered.

Fungal growth occurs from the apical tip of the hypha. Apical vesicles contain materials and enzymes for the formation of new hyphal wall are transported to the growing tip and new cell wall is laid down. Older hyphae are less biochemically active and may contain storage vacuoles. Part of the mycelium is involved in gathering nutrients, the vegetative mycelium, and part in growth and reproduction, the aerial or reproductive mycelium. Spores are formed from the reproductive mycelium. Asexual spores are produced by the aerial mycelium of a single organism, but sexual spores are formed by the fusion of cells containing nuclei from opposite mating strains. There are many different types of spore with long names such as chlamydospore and sporangiospore.

Yeasts are unicellular fungi, normally oval or spherical in shape. Typically, yeasts replicate by budding rather than by binary fission like bacteria. During budding, the cytoplasm and dividing nucleus from the parent is initially continuous with the bud, or daughter yeast, before a new cell wall forms to separate the two. In some cells, these buds fail to detach and may form a short chain of cells called a pseudohypha. Although yeasts are single cells and produce smooth bacterial-like colonies on laboratory agar media, unlike bacteria they have a distinct nucleus and are thus clearly eukaryotes. An example of a yeast is Saccharamyces cerevisiae that produces alcohol during anaerobic fermentation.