Introduction to Mycology: Basic Structure & Classification of Fungi INNOCENT AFEKE SCHOOL OF ALLIED HEALTH SCIENCES UHAS Upon completion of this topic, the student should be able to: • Establish familiarity with the scientific terminology peculiar to mycology • Understand the ecology and physiology of fungi • Emphasis the eukaryotic nature of fungi and relate structure to function • Know the phylogeny or classify fungi and fungal diseases into groups • Recognise the two modes of reproduction in fungi; lifecycles • Appreciate fungi as agents of diseases in humans • Identify the portal of entry into the human host • Know how to prevent fungal infections (a) The cup-shaped ascocarps (fruiting bodies) of Aleuria aurantia give this species its common name: orange peel fungus. (b) The edible ascocarp of Morchella esculenta, the succulent morel, is often found under trees in orchards. 10 m (c) Tuber melanosporum is a truffle, an ascocarp that grows (d) Neurospora crassa feeds as underground and emits strong odors. These ascocarps a mold on bread and other have been dug up and the middle one sliced open. food (SEM). Definitions • Mycology: scientific discipline dealing with fungi • Mycologists: scientists who study fungi • Mycoses: diseases caused in animals by fungi • Mycorrhizae: Fungi associated with plants (symbiosis) Ecology and Physiology of Fungi • Fungi are widely distributed in air, dust, fomites and normal flora • They are adapted to grow anywhere; Water, Soil and Vegetation • Fungi have heterotrophic life; they could survive in nature as: Saprophytic: live on dead or decaying matter (produce exoenzymes) Symbiotic: live together and have mutual advantage Commensal: one benefits and other neither benefits nor harmed. Parasitic: live on or within a host, they get benefit and harm the other • Most fungi store their food as glycogen(like animals)- plants store food as starch • They are also biochemically versatile: -Primary metabolites: citric acid, ethanol, glycerol -Secondary : antibiotics [as penicillin], aflatoxins , Ergot alkaloids (these are produced by Claviceps purpurea) and Psychotropic agents – e.g. psilocybin, psilocin and lysergic acid diethylamide (LSD) • They produce chitin, a polymer of N-acetyl glucosamine, and other compounds, for use in forming the cell wall. These induce immune hypersensitivity • The synthesis of ergosterol for incorporation into the plasma membrane. This makes the plasma membrane sensitive to those antimicrobial agents which either block the synthesis of ergosterol or prevent its incorporation into the membrane or bind to it, e.g. amphotericin B. • The synthesis of proteins on ribosomes that are different from those found in bacteria. This makes the fungi immune to those antimicrobial agents that are directed against the bacterial ribosome, e.g., chloramphenicol • Most fungi are Aerobic, some are facultative anaerobic, or strict anaerobic • Most fungi have very small nuclei, with little repetitive DNA sequences • Fungi are slow growing with cell-doubling times in hours • Fungal growth requirements: In contrast to bacteria, fungi tend grow in places that are: . more acidic . have higher osmotic pressures . are lower in moisture . are low in nitrogen . contain complex carbohydrates Structure and Functions • Fungi are more evolutionarily advanced forms of microorganisms, as compared to the prokaryotes (prions, viruses, bacteria, archaea) • Eukaryotic organisms, distinguished by a rigid cell wall composed of chitin and glucan, and a cell membrane in which ergosterol is substituted for cholesterol as the major sterol component • Fungi can be divided into two basic morphological forms, yeasts and hyphae. • Yeasts are unicellular fungi which reproduce asexually by blastoconidia formation (budding) or fission. • Hyphae are multi-cellular fungi which reproduce asexually and/or sexually Vegetative hyphae, grow under surface of culture medium or substrate Aerial Hyphae: project above surface of medium or substrate • Most fungi occur in the hyphae form as branching, threadlike tubular filaments • These filamentous structures either lack cross walls (coenocytic) or have cross walls (septate) depending on the species. In some cases septate hyphae develop clamp connections at the septa which connect the hyphal elements • Dimorphism is the condition where by a fungus can exhibit either the yeast form or the hyphal form, depending on growth conditions • Very few fungi exhibit dimorphism Structure and Function • Some unique fungi have specialized hyphae that allow them to penetrate the tissues of their host Hyphae Nematode 25 m (a) Hyphae adapted for trapping and killing prey Fungal hypha Plant cell wall Plant cell Plant cell Haustorium plasma membrane (b) Haustoria • A mass of hyphal elements is termed the mycelium (synonymous with mold) • Aerial hyphae often produce asexual reproduction propagules termed conidia(synonymous with spores) • Relatively large and complex conidia are termed macroconidia while the smaller and more simple conidia are termed microconidia • When the conidia are enclosed in a sac (the sporangium), they are called endospores • The presence/absence of conidia and their size, shape and location are major features used in the laboratory to identify the species of fungus in clinical specimens. Structure and Functions Sporangiospores of Mucor Aspergillus niger Penicillium conidia conidiophore and conidia Classification of Fungi Zygote No known Club Sac Chytrids fungi sexual fungi fungi state Ascomycota Basidiomycota Deuteromycota Zygomycota Chytridiomycota • Chytridiomycota -Are found in freshwater and terrestrial habitats -Can be saprobic or parasitic -Unique to other fungi in having flagellated spores called zoospores Hyphae • Zygomycota: -Sexual spores are thick walled resting spores called zygospores; asexual spores are borne internally in a sporangium -saprophytic” molds -common bread mold (Rhizopus) -coenocytic hyphae -Asexual spores: Sporangiospores Approximately 600 species including bread molds (such as Rhizopus) exist Ascomycota: -Sexual spores borne internally in a sac called an ascus; asexual spores are borne externally as conidia -sac” fungi, Septate hyphae and yeasts ,Asexual spore: Conidiospores Approximately 30,000 species including yeasts (Saccharomyces cerevisiae), some of the molds (Aspergillus), morels, and truffles. Ascomycetes have typical reproductive structures called asci • Basidiomycota: -Sexual spores borne externally on a club-shaped structure called a basidium; usually no asexual spores e.g. club”fungi &Mushrooms Approximately 25,000 species including many macroscopic fungi (mushrooms, puffballs, shelf fungi) as well as a number of plant pathogens (rusts, smuts) Some of the medical important fungi you must know -Histoplasma -Microsporum (dematophytes) -Blastomyces - Cryptococcus -Coccidiodes -Candida Paracoccidiodes -Pneumocystis - Aspergillus Classification of Mycoses • Four groups of mycoses based on degree of tissue involvement and mode of entry Systemic: Deep within the body; effects a number of different tissues and organs Subcutaneous: Infections beneath the skin; Hyphae or spores in wounds Cutaneous (Superficial):Infections strictly confined to keratinized epidermis (skin, hair, nails) are called dermatophytoses e.g. ringworm and tinea Opportunistic: affects Transplant, AIDS and Cancer patients Reproduction in Fungi • Fungi propagate themselves : -By asexually-fragmentation of hyphae OR -By producing vast numbers of spores, either sexually (basidiospores & ascospores) or asexually • Asexual reproduction, via conidia formation, does not involve genetic recombination between two sexual types whereas sexual reproduction does involve genetic recombination between two sexual types Asexual spores: Conidiospores, Blastospores, Chlamydospores and Sporangiospores Hyphae fragmentation: Arthrospores Generalised Lifecycle of Fungi Key Haploid (n) Heterokaryotic stage Heterokaryotic (unfused nuclei from PLASMOGAMY different parents) (fusion of cytoplasm) Diploid (2n) KARYOGAMY (fusion of nuclei) Spore-producing structures SEXUAL Zygote REPRODUCTION Spores ASEXUAL Mycelium REPRODUCTION MEIOSIS GERMINATION GERMINATION Spore-producing structures Spores Sexual Reproduction oThe sexual life cycle involves • Cell fusion, plasmogamy • Nuclear fusion, karyogamy oAn intervening heterokaryotic stage • Occurs between plasmogamy and karyogamy in which cells have haploid nuclei from two parents • Sometimes can be dikaryotic and have two different, separate nuclei oThe diploid phase following karyogamy • Is short-lived and undergoes meiosis, producing haploid spores • Sexual Reproduction in Fungi 2 Diploid nuclei 3 Spores released 1 Fruiting body (mushroom) Haploid nucleus DIPLOID Spore HAPLOID DIKARYOTIC 4 Germination of spores and growth of mycelia 6 Growth of dikaryotic mycelium 5 Fusion of two hyphae of compatible mating types Fungal Diseases -Mycoses • In general, humans have a high level of innate immunity to fungi and most of the infections they cause are mild and self-limiting • This resistance is due to: -Fatty acid content of the skin: unsuitable environment for microbes - pH of the skin, mucosal surfaces and body fluids -Epithelial turnover :clean up surfaces - Normal flora: e.g. S. epidermidis -Transferrin (binds with Iron; creating low free iron environment) - Cilia of respiratory tract : traps microbes • Sometimes fungi do pass the resistance barriers of the human body and establish infections • fungi associated diseases are rising, due to nosocomial infections