Fungal Pathogens Lecture Notes PDF

Fungi as pathogens ➢Fungi of medical importance Mycoses Allergies Mycotoxins ➢Fungi as phytopathogens Primary cause of disease in plants Increasing prevalence of fungal infections Increasing prevalence of fungal infections (2) “Emerging fungal threats to animals, plant and ecosystem health” (20...

Fungi as pathogens ➢Fungi of medical importance Mycoses Allergies Mycotoxins ➢Fungi as phytopathogens Primary cause of disease in plants Increasing prevalence of fungal infections Increasing prevalence of fungal infections (2) “Emerging fungal threats to animals, plant and ecosystem health” (2012) Nature 484, 186 – 194 IMPACT: ➢ ➢ ➢ (a Species extinction Food security Loss of fixed carbon (CO2 release) reference to global warming) True pathogens ➢Healthy host ➢Adapted to high temperature and low oxygen tension ➢Restricted to specific geographical regions – limited number (approx. 10) of species known to cause systemic infections in humans ➢Display thermal dimorphism ➢Usually asymptomatic in healthy host ➢Not obligate parasites Thermal Dimorphism – True Pathogen Examples of true pathogens Events in Histoplasma infection Examples of true pathogens (2) Coccidioidomycosis (Coccidioides immitis) Most common in alkaline desert soils of southwestern US 60% no symptoms; 40% fever; <0.5% growth in lungs (fungoma) Biology: <35 ºC; filamentous, arthrospores 37 ºC; Spherules (sporangia) Endospores in sporangia Events in Coccidioides infection Opportunistic pathogens ➢Host defences need to be impaired (immunocompromised host) ➢Distributed world-wide ➢Many species and genera / increasing all the time. ➢They show no specific adaptations to the host environment but are hardy to it. ➢Superficial/benign to chronic systemic infections ➢Prognosis – poor Examples of opportunistic pathogens Candida albicans infection (Candidiosis) ➢Widespread – 20% of us are inhabited by this yeast ➢Immunocompetent: non invasive local infection. ➢Immunocompromised: systemic causing organ failure ➢Most common fungal pathogen causing loss of life ➢Fourth biggest killer in tertiary care hospitals ➢Invades wounds/burns ➢Dimorphic yeast - hyphal form is aggressive Examples of opportunistic pathogens (2) Aspergillosis (Aspergillus spp.) – filamentous fungi ➢ 600 Aspergillus spp. / 8 species cause human disease ➢ A. fumigatus (>80 % of cases) – thermophile / small spores ➢ Spore germination in lungs – fungal ball, “aspergilloma” ➢ Aggressive invasion using hyphae – tissue necrosis ➢ Colonises lung / disseminates to brain/heart Dermatophytes ➢Superficial infections on healthy host (non invasive) ➢World-wide distribution ➢Most common are those causing Ringworm and Athletes Foot - Trichophyta spp. ➢ > 1 million nail infections in UK ➢Not life threatening – discomfort/unsightly ➢“Keratin Lovers” (hair, skin, nails) Why are human mycoses a mounting clinical problem? More immunocompromised hospital patients A relatively poor understanding of fundamental fungal biology – limited antifungal treatments Control of mycoses of humans Azoles Ketoconazole Fluconazole Polyenes Nystatin Amphotericin B Ergosterol-dependent Control of mycoses of humans (2) Phytopathogenic fungi Major Groups: 1) Pathogens of immature (or compromised) tissue 2) Pathogens of mature and non compromised tissue Necrotrophic pathogens Biotrophic pathogens Pathogens of immature tissue ➢Broad host range ➢Soil-borne pathogens attack roots - e.g. Phythium and Phytothphora spp. [originally classed as fungi due to their filamentous appearance but based on biochemical and molecular analysis they are more closely related to algae and have been reclassified as oomycete protists] ➢Aggressive invasion of root tips with extensive destruction ➢Water-logged soils Pathogens of mature tissue Usually high degree of host specificity Necrotrophic pathogens ➢Excrete toxins and cell wall digesting enzymes ➢Host cell death ➢Tissue invasion ➢Evoke host resistance mechanisms Example of a necrotrophic pathogen Armillaria mellea (honey fungus) - necrotrophic pathogen Major root rot pathogen of broad leaved trees Excrete ligninases, cellulase and pectinases Pathogens of mature tissue (2) Biotrophic pathogens (e.g. rust and powdery mildew) ➢Maintain host viability ➢Limited tissue invasion ➢Do not evoke resistance mechanisms ➢Life cycle depends on living host ➢Arrive as spores on leaves Pathogens of mature tissue (3) Biotrophic pathogens - the haustorial biotrophs Puccinia graminis Via stomatal opening Infect mesophyll cells Erysiphe graminis Infect directly through cuticle Epidermal layer Haustoria – structure/function A nutrient sink Erysiphe graminis in wheat Scale of the problem Scale of problem: Puccinia graminis infected wheat – spore to spore in 6 –10 days An infected crop can produce 1.5 trillion spores per day for several weeks Overwhelming! Rust spores bursting through at infection site