AN INTRODUCTION TO MEDICAL MYCOLOGY
David H. Ellis, Mycology Unit, Women's and Children's Hospital, Adelaide.
Definition of fungi.
The living world is divided into the five kingdoms of Planta, Animalia, Fungi, Protista and Monera. It is important to recognize that the fungi are not related to bacteria (Monera). To define the exact limits of the groups in a few words is virtually impossible, however generally speaking fungi are eukaryotica, heterotrophicb, unicellular to filamentous, rigid cell walledc, sporebearing organisms that usually reproduce by both sexual and asexual means. Further they are insensitive to antibacterial antibiotics. a Eukaryotic cells contain membrane bound cell organelles including nuclei, mitochondria, golgi apparatus, endoplasmic reticulum, lysosomes etc. Eukaryotes also exhibit mitosis. These features separate fungi from bacterial which are prokaryotic cells lacking the above structures. Heterotrophic - fungi lack chlorophyll and are therefore not autotrophic (photosynthetic) like plants and algae; rather they are heterotrophic absorptive organisms that are either saprophytes (living on dead organic matter) or parasites (utilizing living tissue). Like plants, fungi have rigid cell walls and are therefore non-motile, a feature which separates them from animals.
Structure of Fungi.
Fungi occur in two basic growth forms or stages: (a) A unicellular or yeast form which is defined morphologically, as a single-celled fungus that reproduces by simple budding to form blastoconidia. Colonies are usually moist or mucoid. Yeast-like fungi may be basidiomycetes, such as Cryptococcus neoformans or ascomycetes such as Candida albicans. A filamentous or mould form which is a vegetative growth of filaments. Structures such as mushrooms consist simply of a number of filaments packed tightly together, and reproduction is by spores or conidia. Moulds produce a great variety of conidia which are borne on specialized hyphae or conidiophores. Many moulds can be identified by the morphology of these spores and by their arrangement on the hyphae.
Fungal filaments are known as hyphae and a mass of hyphae collectively make up the mycelium. The terms "hyphae" and "mycelium" are used interchangeably. There are two kinds of hyphae; non-septate (coenocytic) and septate. The septa divide the hyphae into compartments but not into cells. In some groups nuclei and/or cytoplasm can flow through a hole or pore in the centre of these septa.
Fungi with non-septate hyphae typically belong to the Zygomycetes. Non-septate hyphae are considered to be more primitive because if a hyphal strand is damaged the entire strand dies. Septate hyphae are typically found in the Basidiomycetes and Ascomycetes, the latter also includes the hyphomycetes or conidial fungi. When a septate hyphal strand is damaged, the pores between adjacent compartments can be plugged, thus preventing death of the whole hyphal strand. Two basic types of reproductive propagules are found in the fungi: (a) Sexual propagules are produced by the fusion of two nuclei that then generally undergo meiosis. Sexual methods of reproduction involve plasmogamy (cytoplasmic fusion of two cells), karyogamy (fusion of two nuclei), genetic recombination and meiosis. The resulting haploid spore is said to be a sexual spore, e.g. zygospores, ascospores and basidiospores. If a sexual spore is produced only by fusion of a nucleus of one mating type with a nucleus of another mating type (+ and - strains), the fungus is said to be heterothallic. In contrast, homothallic moulds produce sexual spores following the fusion of two nuclei from the same strain. (b) Asexual propagules are termed either spores or conidia depending on their mode of production, and which arise following mitosis of a parent nucleus. Conidia arise either by budding off conidiogenous hyphae or by differentiation of preformed hyphae. Asexual spores are commonly formed by consecutive...
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