2.26_Week 9 - Fungi.pdf

An introduction to Fungi Week 9 – Microbiology SCI1013-N-GJ1-2020 Dr Helen carney 1 What we will be covering today • What are fungi and why are they important • Characteristics of fungi • Morphology • Symbiotic fungi • Pathogenic fungi • Asomycota • Saccharomyces cerevisiae • Microsporidia 2 Wha...

An introduction to Fungi Week 9 – Microbiology SCI1013-N-GJ1-2020 Dr Helen carney 1 What we will be covering today • What are fungi and why are they important • Characteristics of fungi • Morphology • Symbiotic fungi • Pathogenic fungi • Asomycota • Saccharomyces cerevisiae • Microsporidia 2 What are fungi? • A large, diverse and widespread group of organisms • Moulds • Mushrooms • Yeasts • Number of fungal species described: 100,000 • Number of potential fungal species: 1,500,000 3 Why are fungi important? • Fungi perform integral roles in a multitude of environments • Without wood-decomposing fungi, fallen trees would render forests impenetrable • Without dung-loving fungi, the landscape would be contoured by mountains of herbivore faeces • Without aquatic fungi, ponds and waterways would be clogged by plant debris • They are the masters of recycling! 4 Fungal phylogeny • The last common ancestor of fungi was between 450 million and 1.5 billion years ago • The majority of described species belong to Ascomycota and Basidiomycota • The Basidiomycota include your typical mushrooms in addition to plant pathogens 5 Why are fungi important? • Fungi have been used as sources of food and for food processing for millennia • Edible fruiting bodies, such as mushrooms and various fungi have been used to supplement and add flavour to foods • Yeasts are used in the fermentation of fruits to produce wines, cereals to make beer, in bread manufacture and flavouring in the form of yeast extract • Filamentous fungi are used in traditional processes for the ripening of cheeses and in the production of enzymes used in the food industry. 6 A fungus sparked the greatest development in modern medicine • The first antibiotic was derived from a fungus – Penicillium rubens (sometimes referred to as Penicillium notum but they are the same species) • The fungus produces penicillin that has a bactericidal activity against multiple bacterial pathogens • Cephalosporins also originated in fungi and the class is one of the most important in human medicine 7 Fungi are eukaryotes • Some forms of fungi have a visual likeness to bacteria, but fungi are eukaryotes • They contain organelles • Rigid cell wall comprised of chitin and glucan • Some forms of fungi may look like plants, but they are genetically closer to animals 8 Fungi characteristics • Fungi are chemoorganotrophs • This means they derive energy from chemicals (just like you!) • They are primarily aerobic and have simple nutritional requirements • They obtain these nutrients by secreting enzymes to digest polymeric materials • These are things like polysaccharides and proteins • These are broken down into smaller molecules that can be absorbed 9 Fungi nutrition • The ways fungi obtain their food can be split into three broad categories • Parasite • Absorbs the food from the cells of a live host (e.g yeast infection, ringworm) • Symbiotic relationship • E.g Lichen, provides protection and moisture. In return it receives nutrients from an organism capable of photosynthesis • Saprophyte • Uses dead or dying organisms for a food source 10 Morphology • The majority of fungi are multicellular and form a network of filaments called hyphae (the exception are single-celled fungi, yeasts) • Hyphae are tubular cell walls that surround a cytoplasmic membrane • They are often septate meaning cross-walls dividing each hypha into separate cells • Each filament grows by tip extension of the terminal cell 11 Morphology • Some species of fungi form coenocytic hyphae instead of septate • This forms when a hypha contains more than one nucleus and repeated nuclear divisions can result in hundreds of nuclei forming without cell walls 12 Morphology • The hyphae grow together across and above a surface to form a visible tuft called a mycelium • From here, aerial hyphae reach up into the air and asexual spores (conidia) are formed • It is the conidia that give mycelium a ‘fuzzy’ or ‘dusty’ appearance and they are often pigmented black, brown, green, red or yellow • These spores function to disperse the fungus to new habitats 13 Reproduction • Fungi reproduce asexually in three ways • Growth and spread of hyphal filaments • Asexual production of conidia • Simple cell division • The majority of fungi also produce sexual spores during their lifecycle • These spores can develop from unicellular gametes or through specialised hyphae, which are called gametangia • Sexual spores can also originate from the fusion of two haploid cells to form a diploid cell • This then undergoes meiosis and mitosis to yield individual haploid cells • This is how baker’s and brewer’s yeast reproduces 14 Symbiosis • The majority of plants rely on specific fungi to facilitate the uptake of nutrients from the soil • These fungi form a symbiotic relationship with the plant roots called mycorrhizae • Mycorrhizal fungi establish a close physical contact wit the roots to help the plant obtain minerals such as phosphate in addition to water from the soil • Presence of mycorrhizal fungi increase the surface area of the root up to 700 times • In return the fungi obtain nutrients such as sugars from the roots 15 Pathogenic fungi • The majority of fungi are harmless to humans with only 50 species known to cause diseases • In healthy people the rate of serious infections is very low • However, superficial fungal infections are common More on this later • In people in compromised immune systems fungal infections can penetrate tissues and become life threatening • One species, Candida albicans, is an important nosocomial pathogen Trichophyton – causes ringworm Epidermophyton floccosum – causes athlete’s foot 16 Pathogenic fungi • Fungal pathogens include moulds and yeasts but many possess the ability to be both (this is called dimorphic) • This change in form is often due to a change in temperature • In the environment (25 °C) the fungi are filamentous with hyphae but in a host (37 °C) switch to a yeast form • Dimorphic switching requires the fungus to sense and respond to the host environment and is essential for pathogenicity • For fungi that cause disease, infections are usually initiated by the inhalation of dormant spores 17 Why would you want to be dimorphic? • Dimorphism allows the fungi to circumvent the effectiveness of host defence responses • A number of these fungi are happy to reside within phagocytotic cells of the host where they are shielded from the rest of the immune system • If the fungi were to continue with hyphal growth it would rupture the phagocyte and expose it to the host immune system again • They are like trojan horses, waiting to break out and disseminate when they have multiplied enough • Dimorphic switching allows for the colonisation of unique environmental niches within the host and the failure to switch almost always attenuates pathogenicity in these fungi 18 Fungal disease classes • There are three primary mechanisms for fungal disease: • Inappropriate immune response – some fungal species like Aspergillus that are found in nature produce allergens that can trigger asthma attacks or other hypersensitivities • Toxin production – the production of mycotoxins, such as the aflatoxin by Aspergillus flavus that common grows on improperly stored dry foods • These are highly toxic and carcinogenic and can cause liver damage including cancer, they are especially dangerous if consumed by children 19 Fungal disease classes • The third mechanism is via host infection and this is called mycosis • These can be superficial or life threatening and are split into three classes • Superficial • Subcutaneous • Systemic • The systemic infections are the most serious of all the fungal infections and are categorised by growth in internal organs • Treatment of systemic infections is very difficult due to the toxicity of fungal treatments 20 Asomycota • Ascomycota are the largest and most diverse group of fungi • It contains single celled species, Saccharomyces cerevisiae – baker’s yeast • Filamentous species – Aspergillus – common mould • Saprophytic and pathogenic yeasts – Candida albicans 21 Saccharomyces cerevisiae • Saccharomyces cells can be spherical, oval or cylindrical and division of the cells takes place through budding • This process involves the new cell forming as a small outgrowth before separating from the parent • This process often leaves scars on the parent cell • It is unicellular and around 6 µm in diameter 22 Saccharomyces cerevisiae • It is the best studied of the eukaryotes and is a valuable tool for most aspects of basic research on eukaryotes • It is very easy to genetically manipulate (as briefly covered in week 10) and this makes it a often used host for biotechnical applications • They like to grow in sugar-rich habitats such as flowers, fruits and the bark of trees • It was originally isolated from the skin of fruit • Economically important species as brewer’s and baker’s yeast 23 Microsporidia • They are closely related to fungi and 2-5 µm in size and lack mitochondria • Microsporidia are ancient groups of parasitic or saprophytic fungi • They are obligate parasites of animals and infect every vertebrate and invertebrate • Over 1,500 species have been characterised since first discovered in an infected silkworks in 1857 • They are obligate intracellular pathogens • Smallest genome size of the eukaryotes GENOME SIZES Human 30000 genes Arabidopsis Yeast E.coli E.cunicili E.intestinalis 25000 genes 6300 genes 4000 genes 1999 genes 1833 genes 50MBp 150MBp 300MBp 26 Microsporidia infect a wide range of hosts Microsporidia species Honey Bee Silkworm Colony Collapse Pébrine disease Nosema apis Nosema ceranae Nosema bombycis Spoiled fish Spraguea lophii 27 Microsporidia • There are three common Microsporidia that infect humans • Trachipleistophora hominis • Encephalitozoon intestinalis • Enterocytozoon bieneusi • Symptoms include chronic, non-bloody diarrhoea and respiratory infections • This is rare in healthy adults with functioning immune systems • Increasing issue for those that are immunocompromised such as those with HIV/AIDS, or have had organ transplants 28 Microsporidia spore • Spores are ubiquitous in nature • Non-motile • Small - 1-5um length • Highly environmentally resistant • Inner wall(endospore) of chitin (long chain polysaccharide) • Ingested or inhaled 29 Microsporidia • When they are outside the body they can only exist as spores but when close to a host cell they extend a helical polar tubule • This tubule penetrates into the host cytoplasmic membrane • The spore then injects its sporoplasm into the host cell and begins replication, forming new spores 30 Recapping what we discussed • Fungi are eukaryotes that are genetically closer to animals than plants • They are incredibly useful and act as the recyclers of the world • Primarily aerobic but some are able to ferment and they secrete enzymes to break down polymeric materials • They can be multicellular with hyphae or single cellular like yeasts • Pathogenic fungi can often switch between the two and this is called dimorphism • The hyphae can be septate (with cell walls) or coenocytic (without cell walls) • Conidia (spores) form on the edge of the hyphae and this helps the fungi to spread Tasks for this week • Read sections 18.8-18.9, 18.12 and 33.1 of Brock Biology of Microorganisms • See what you can find out about the nosocomial pathogen, Candida albicans • • • • How can we grow it What size is it What cell shape does it form How can we treat infections? 32