Fungi Structure, Nutrition & Infections (SOM302) PDF

Fungi Structure, nutrition & infections SOM302 N.B. Mannathoko  Phylogeny of living organisms Figure 12-7: Biology of Plants, 7th ed.2005. W.H. Freeman & Co. Garcia-Hermoso, D. National Ref. Center for Invasive...

Fungi Structure, nutrition & infections SOM302 N.B. Mannathoko  Phylogeny of living organisms Figure 12-7: Biology of Plants, 7th ed.2005. W.H. Freeman & Co. Garcia-Hermoso, D. National Ref. Center for Invasive Mycosis & Antifungal, Institut Pasteur, France Fungi are unique eukaryotes Estimation of fungal biodiversity: 1.5 – 2million species ~150 000 species have been described  Introduction Mostly saprophytes i.e. decay of dead animal & plant materials, providing nutrients for living plants Beneficial mycorrhizal relationship with roots of green plants Enable plant to better attain H2O, phosphates & other minerals Symbiotic relationship with cyanobacteria or green algae- forming ‘lichens’ (crusty growths on surfaces of trees & rocks) Production of antibiotics & enzymes (used in fermentation): Penicillium species produce the famous penicillin G Food-producing processes e.g. bread, wine, beer & cheese Food e.g. certain mushrooms  But they can be agricultural challenge: Many fungi spp. are plant parasites Botrytis on raspberries, dusting Brown rot of a peach fruit of gray spores apparent Cedar-apple rust on top & Phytophthora root & stem rot bottom of an apple leaf of soybean Beneficial in food production Bread rising due to yeast activity Cornish Blue Cheese: Penicillium spp. added for flavouring, resulting in ‘blue veins’ Edible mushrooms Bread (yeast) spread Contamination of damp areas & food spoilage Mold growth on food Ceiling mold growth Black mold in between shower tiles So what is their relevance in affecting human health?  Oral thrush Ring worm Pulmonary aspergillosis Fungi are increasingly becoming leading pathogens especially amongst immunocompromised patients i.e. cancer, AIDS Cause various infections ranging from superficial to invasive Fungi metabolites are a source of antimicrobials & other medicines Beekman, A. M & Barrow, R. A (2014) Aust. J. Chem. 2014, 67, 827–843 Structure / Morphology  Structure/ Morphology Thallus (body) of fungi is diverse but generally two forms: o Yeasts o Filamentous 1. Yeasts Unicellular fungi, which reproduce asexually by budding Electron micrograph Pseudomycelium of Candida albicans in of a yeast nutrient-limiting conditions.  Structure/ Morphology 2. Filamentous Multicellular strands termed hypha/ hyphae & mycelium Septate: divided into discreet cells by walls (septa) Coenocytic: continuous tubular network not divided by septa Aimanianda, V. Group leader, Molecular Mycology Unit, Mycology Department, Institut Pasteur, PaRris, France Morphology Multicellular filamentous fungi (molds) Hyphae Grow at tip, thus younger more active towards periphery Bound by rigid hyphal cell wall Hyphae anastomose on surface or within substrate, forming compact, macroscopic tuft -‘mycelium’  Morphology Aerial hyphae produce spores – ‘conidia’ Asexual or sexual Often pigmented e.g. black, blue-green, yellow Mycellium & conidia of an Aspergillus Give mycelium dusty appearance species on agar. Function to disperse fungus to new habitats Some fungi form macroscopic reproductive structures/ ‘fruiting bodies’ i.e. mushrooms, Spores produced within Fruiting bodies can release millions of spores spread by wind, water or animals Dimorphic / biphasic Unicellular to filamentous transitions influenced by changes in environmental conditions Cell wall structure  Fungal cell membrane Phospholipid bi-layer interspersed with ‘Ergosterol’ (a sterol) Ergosterol, key component in regulating structural integrity Key enzymes in ergosterol synthesis include squalene epoxidase & 14–α–demethylase which convert lanosterol to ergosterol Ergosterol not found in mammalian cells, so ideal target of certain antifungal drugs e.g. azoles, polyenes o azole inhibit 14α-demethylase & results in the accumulation of 14α- methylsterols, which disrupt the fungal cell membrane o polyene macrolides bind to ergosterol form, complexes & disrupt the structural integrity of the fungal membrane  Fungi cell wall Cell wall Cell membrane Three major components– ‘polysaccharides’ (90%): Chitin (polymer of N-acetyl-glucosamine-NAG) Glucans (polymers of glucose) Mannans (chains of mannoses bound to cell wall proteins)  1,3- glucan synthase Chitin Immediately exterior to cell membrane (minor component) o ~1-2% of cell wall in yeasts & 10-20% in filamentous fungi Long linear polymers of -1,4-linked NAG o Confers enormous tensile strength to cell wall o Unique (also found as component of shells of molluscs e.g. snails) Synthesized in cytoplasm by chitin synthase (integral membrane enzyme) - target for some anti-fungal drugs  Hardison & Brown (2012): Nature Immunology. 13: 817–822 Glucans Main structural polysaccharide (50-60%) to which other cell wall components are attached Repeating glucose assembled into branched chains linked by -glucosidic linkages (mainly -(1,3) o Other fungal walls may comprise: -(1,3)-glucan & -(1,6)-glucan (mixed) or -(1,4)-glycan or -(1,3)-glucan Synthesized in cytoplasm by -1,3-glucan synthase Glucan synthase target for anti-fungal drugs i.e. echinocandins Mannans Oligopolysaccharide of mannose. Diff. types e.g. galactomannan, glucomannan, galactoglucomannan Fungi cell wall: mannans bound to proteins via N- or O- linkages. Different manno-proteins within & across fungal species  Gow, N. A. R., van de Veerdonk, F. L., Brown, A. J. P. & Netea, M. G. (2012). Nature Reviews Microbiology. 10: 112-122 Proteins Most fungi cell wall proteins are glycoproteins i.e. mostly with N- & O-linked mannans o Differ amongst fungi e.g. S. cerevisiae & C. albicans (yeasts) contain glycoproteins glycosylated with mannose vs. A. fumigatus glycoproteins which may have both mannose & galactose residues  Gow, N. A. R., van de Veerdonk, F. L., Brown, A. J. P. & Netea, M. G. (2012). Nature Reviews Microbiology. 10: 112-122 Proteins Glycoproteins covalently attach to the glucans or chitin Also anchored to cell membrane via glycosylphosphatidylinositol (GPI) links Many fungi also have other proteins i.e. heat shock proteins; pigments Proteins - 2 broad categories i) Wall-associated enzymes (WAEs) ii) Structural proteins WAEs have catalytic activities: cell wall synthesis & remodeling e.g. chitinases; glucanases; peptidases (breakdown cell wall components) & glycosyltransferases (synthesis & crosslinking cell wall polysaccharides) Structural proteins: cell migration, adhesion Cell walls of different fungi a. Candida spp.: mainly O & N-linked mannans & phosphorylated mannans b. Pneumocystis spp: lack chitin synthesis enzymes c. Aspergillus spp.: conidial spore wall has outer hydrophobin layer (rodlets) & inner melanin layer Erwig, L. P. & Gow, N.A. R. (2016) Nature Reviews Microbiology. 14: 163–176 Diversity within fungi cell walls & also different from bacteria cell walls Brown, L., Wolf, J. M., Prados-Rosales, R. & Casadevall, A. (2015). Nature Reviews Microbiology. 13: 620–630 Interesting but relevance …..?  Latgé, J. Cellular Microbiology (2010) 12(7), 863–872 Antigen presenting cells e.g. dendritic cells & macrophages of the immune system have “Pattern recognition receptors (PRRs)” Recognise ‘Pathogen Associated Microbial Patterns’ in fungi cell wall Receptors involved in innate immune recognition of fungal polysaccharides & the evoked responses to the fungal antigens Nutrition  Fungal nutrition Diverse nutritional status across fungi Use of O2 - obligate aerobes & facultative anaerobes Heterotrophs: obtain carbon from organic matter Secrete extracellular enzymes, digesting complex organic material into sugars, peptides, amino acids. These are assimilated as sources of carbon & energy E.g. decomposers- digest dead animal & plant materials E.g. parasites- take up nutrients from living calls of plants & animals pH 4-7 & 20-30C optimal for growth. But many can grow in extreme conditions too Fungi phyla Subkingdom Contain clinically important fungi Garcia-Hermoso, D. National Ref. Center for Invasive Mycosis & Antifungal, Institut Pasteur, France Phyla that contain clinically important fungi Genera Phylum Phyla that contain clinically important fungi Order Phylum Incl. genera: Rhizopus Lichtheimia Rhizomucor Mucor Mucormycosis ‘The black fungus’ Recently caused opportunistic infections in Covid-19 patients in some countries Phyla that contain clinically important fungi Genera Phylum Most are wood saprophytes or plant pathogens But a few species cause human infections  Sometimes it is a case of beautiful hair to itchy, flaky scalp Culprit? Often it is a fungus Malassezia Fungi (Basidiomycota) Commensal fungus of scalps of most healthy adults, feeds on oils secreted by hair follicles But Sometimes grows out of control, irritates scalp, causes skin cells to die, fall off, clumping with oil from hair & scalp Appear white & flaky in hair or clothes Associated with dandruff, eczema & other skin diseases Fungal infections & Virulence factors  Virulence factors in fungi Virulence factors allow fungal survival, persistence & eventually tissue damage in host Adherence structures: allow attachment to host tissues Cause damage of host tissues & impairment of host defenses i.e. production of phospholipases, proteases, elastases (elastin significant component of the lungs, elastases implicated in pulmonary infections i.e. in aspergillosis): Ability to reversibly switch to different morphology forms during infection e.g. dimorphic fungi Thermotolerance  Virulence factors in fungi Some fungi reversibly transform between filamentous & unicellular structures in the host. This allows adaptation to different environments Examples: Some Candida spp. grow in different forms i.e. yeasts, blastospores, pseudohyphae & hyphae depending on infection sites Candida albicans stimulates production of a cytokine GM-CSF, which can suppress complement production by monocytes & macrophages. This facilitates a decrease in the chemotaxis of phagocytes.  Virulence factors in fungi Examples: Cryptococcus neoformans In the yeast form it expresses a capsule inhibiting phagocytosis (also Histoplasma capsulatum) Inhibit production of cytokines TNF- & IL-12 while stimulating IL-10 production. TNF & IL-12 activate macrophages while IL-10 suppresses their activation  Virulence factors in fungi Some filamentous fungi e.g. Aspergillus spp., Fusarium spp. & Zygomycota are inhaled as unicellular conidia but transform into branching hyphae in the lungs & produce elastases which destroy lung parenchyma Fungal infections  Fungal infections Generally fungi are harmless but some can cause human infections BUT Human fungal pathogens have become increasingly important due to increase in nos. of immunocompromised patients i.e. cancer, AIDS. Range: mild/easily cured e.g. athletes foot, ring-worm & thrush to life-threatening pulmonary & systemic infections Fungal disease may be generally classified into: 1. Allergy or hypersensitivity 2. Mycotoxins (fungal toxins) i.e. aflatoxins 3. Mycosis or growth of fungus on the skin or in the body:  Aspergillus flavus on peanut Mycotoxins Aspergillus flavus on corn Fungal exotoxins e.g. ‘aflatoxins’ by Aspergillus flavus (commonly grows in improperly stored grains) are ‘carcinogenic’ Aflatoxins metabolized by cytochrome p450 & converted to several metabolic products including: Aflatoxicol, Aflatoxin Q1, Aflatoxin P1 & Aflatoxin M1 & ‘Aflatoxin 8,9 epoxide’ Aflatoxin 8,9 epoxide intercalates with DNA, forming an adduct with guanine. Causes G:T transversion at codon 249 in p53 gene in liver. Mutation associated with hepatic carcinoma  Mycoses Mycoses range from superficial lesions to serious life- threatening diseases Categories: Superficial Subcutaneous Systemic Opportunistic Spread by contact with infected person or contaminated surfaces e.g. bathtub & shared items e.g. a) Superficial mycosis of foot ‘athletes foot’ infection by towels, bed-linen Trichophyton rubrum b) Sporotrichosis, subcutaneous infection by Sporothrix schenckii Superficial mycoses  Superficial mycoses Infections of epidermis of skin, hair/scalp or nails as require keratin for growth. Generally termed ‘Dermatophytes Causing inflammation, pruritus & often flaking of the skin 3 genera are leading causes: Epidermophyton, Microsporum & Trichophyton. Diseases by these genera referred to as ‘tineas' (ringworm) of various parts of the body: T. corporis (body ringworm), T. cruris (groin/ jock itch), T. unguium (nails), T. capitis (scalp), T. barbae (beard/chin) Disease Pathogen Site Ringworm Epidermophyton, Microsporum, Scalp of children, skin Trichophyton Favus Trichophyton Scalp Athletes foot Epidermophyton, Trichophyton Between toes, skin Jock itch Epidermophyton, Trichophyton Genital area Keratitis Fusarium Eye (cornea) Vaginal candidiasis Candida albicans Vagina Ringworm on forehead Ringworm on scalp (Tinea corporis) (Tinea capitis) Athletes foot/ fungal infection between toes (Tinea pedis) Other superficial mycoses Pityriasis versicolor (previously termed ‘Tinea versicolor’) presents with hypopigmented, hyperpigmented or erythematous macules commonly on proximal upper extremities. Mild itching. Although referred to as tinea versicolor, it’s not a dermatophyte infection Common causes: Malassezia species especially: M. globosa, M. restricta & M sympodialis. Produce azelaic acid, which impairs the function of the melanocytes. Typically Malassezia spp. are commensals in sebaceous gland-rich regions e.g. scalp, face & chest. But trigger(s) for their overgrowth & subsequent cause of pityriasis versicolor not well understood Is more common in hot, humid climates or in those who sweat heavily, Malassezia spp. is a yeast but can transform to a mycelial form which is assoc. with disease  Other superficial mycoses Malassezia folliculitis / Pityriasis folliculitis: overgrowth of Malassezia furfur in the hair follicles & causing pustules on skin. M. furfur is lipophilic, so predisposed to oily skin areas Seborrhoeic dermatitis (dandruff): overgrowth of Malassezia spp. on skin triggering inflammation & causing patches that flake or peel Clin. Micro. Reviews (2012): 25(1):106-41. doi: 10.1128/CMR.00021-11 Seborrheic dermatitis (dandruff) affecting scalp Malassezia folliculitis diagnosed in Seborrheic a construction worker. Condition dermatitis in developed after working in a hot, nasolabial humid environment for a few days. folds (A) Back of the patient (B) Close-up view of the lesions Subcutaneous mycoses  Subcutaneous mycoses Infections within deeper layers of the skin Disease Pathogen Site Sporotrichosis Sporothrix schenckii Arms, hands Blastomycosis Blastomyces dermatitidis Lungs, arms, legs Sporotrichosis: by Sporothrix schenckii - ‘rose gardeners disease’ Dimorphic, ubiquitous saprophyte of wood, bark of trees & soil Occupational hazard of agricultural workers Transmission: fungal contamination of wounds or abrasion sites At body TC, conidia germinate into yeasts & cause infection characterised by suppurative, granulomatous cutaneous nodules along local lymphatic channels Symptoms: nodular lesions/ bumps under skin, may be ulcerous at point of entry or along lymph nodes & vessels  Sporotrichosis Cutaneous nodules in a lymphangitic (sporotrichoid): cutaneous lesions progressing along dermal and subcutaneous lymphatics Typically initially present as papules or subcutaneous nodules that then progressively develop along lymphatics and may then ulcerate or necrose www.msdmanuals.com/professional/infectious- diseases/fungi/sporotrichosis Blastomyces dermatitidis (Blastomycosis) Dimorphic fungus common in soil of central & south eastern U.S.A. Conidia, can become aerosolized when the fungal colony is disturbed & cause infection Primarily pulmonary infection: after conidia are inhaled goes to lower respiratory tract. Normally killed by neutrophils, phagocytized by bronchopulmonary mononuclear cells & macrophages (‘asymptomatic’ or mild infection) o Initial symptoms are typically mild ‘flulike’, resolving within days. Mild or asymptomatic infection occurs in 50% of infected persons BUT, B. dermatitidis can transform to yeast form & thick wall hinders phagocytosis, resulting in symptomatic pulmonary infection o Causing acute respiratory distress syndrome in the elderly &/or immunocompromised patients Extrapulmonary disease can occur after systemic dissemination to other organs, with the skin being the most commonly infected. o Cutaneous blastomycosis starts as papules, progressing into vegetative plaques often with central clearing or ulceration. Lymphangitis & lymphadenopathy may also be present Blastomyces dermatitidis (Blastomycosis) Blastomycosis lesions Extrapulmonary Blastomycosis: presenting as raised, irregular border ulcerative skin lesion with small microabscesses at the periphery Blastomycosis is a pulmonary disease (pneumonia), occasionally, it can spread hematogenously, causing extrapulmonary disease. Diagnosis is clinical & laboratory-based i.e. chest X-ray, & confirmation with laboratory identification of the fungus hwww.msdmanuals.com/professional/infectious- diseases/fungi/blastomycosis  Systemic mycoses Serious infections affecting internal organs & blood vessels, generally by filamentous fungi Leading fungal pathogens include: Aspergillus, Histoplasma, Coccidioides, Blastomyces, Paracoccidioides  Risk factors of invasive mycoses Transplantation procedures e.g. organ; bone marrow; haematopoietic stem cells Neutropaenia (acute or chronic) Colonisation by fungi spp. due to: o Use of immune-suppressant agents for inflammatory conditions o &/or antimicrobial prophylactic regimens for immune- suppressed patient groups Poorly controlled diabetes results in reduced neutrophil function NB. leading fungal pathogens: Aspergillus fumigatus; Candida albicans (& species Candida albicans from Mucorales family) Aspergillus fumigatus Limper, A.H (2010). Proc Am Thorac Soc : 7:163–168, Disease Pathogen Site Aspergillosis Aspergillus speciesa Lungs Blastomycosis Blastomyces dermatitidis Lungs, skin Candidiasis Candida albicansb Oral cavity, intestinal tract Coccidiodomycosis Coccidioides immitisb Lungs Cryptococcosis Cryptoccus neoformansb Lungs & meninges Histoplasmosis Histoplasma capsulatumb Lungs Pneumocystis pneumonia Pneumocystis jirovecib Lungs a. Aspergillus also causes allergies, toxemia & other limited infections b. Opportunistic pathogens frequently associated with HIV/ AIDS Aspergillus fumigatus (Aspergillosis/ farmers lung disease) Spores inhaled from rotting vegetation or compost may cause clinical allergy or asthma or invasive lower respiratory tract infection Mycelium may grow large enough to be seen on X-ray i.e. fungus ball/ aspergilloma, which may cause asphyxiation  Aspergillus species Most cases of aspergillosis caused by A. fumigatus, followed by A. flavus & A. niger e.g. allergic aspergillosis, pulmonary & invasive aspergillosis aspergilloma Invasive aspergillosis rarely occurs in the immunocompetent host BUT In susceptible host: after inhalation of airborne conidia (small enough for alveolar deposition) there is germination of conidia into hyphae in alveoli causing invasive disease Chronic pulmonary aspergillosis Virulence factors of Aspergillus spp Conidia bind to human fibrinogen & laminin: vital step allowing adherence to host tissues Hyphae produce proteases which degrade fibrinogen & laminin Elastases: serine protease & metalloprotease: degradative activity against ‘elastin’-constitutes 30% Chronic pulmonary aspergillosis lung tissue A. fumigatus also produces gliotoxin: inhibits macrophage phagocytosis &Tcell activation Resulting in disease Endemic systemic mycoses Coccidioides immitis (Coccidioidoymycosis) Indigenous in U.S.A. especially in warm, arid regions e.g. Ohio, Mississippi River valleys - ‘Valley Fever’ On inhalation of the conidia, it typically causes lung infection , the time from exposure to the development of symptoms may take several wks Typically affects the lung BUT in fewer cases may spread systemically & affect other body parts causing serious Coccidiodomycosis complications “disseminated coccidioidomycosis.” skin rash on face Histoplasma capsulatum (Histoplasmosis) Endemic in certain regions of U.S.A - Ohio, Mississipi, Ottawa river valleys Affecting >80% young adults from states bordering Ohio & Mississippi rivers Fungus associated with bat or bird droppings Inhalation of airborne spores: germinate in lungs causing pneumonia like symptoms Other important systemic mycoses  Cryptococcus neoformans Cryptococcosis Common cause of meningitis in immunocompromised adults & can also cause lung infections or pneumonia *Clinically relevant to Botswana: leading cause of meningitis in AIDS patients (AIDS defining illness) Frequently found in soil containing bird droppings (high conc. in bird faeces) Clinically important variants Var. neoformans Var. gattii Var. grubii (often considered part of neoformans) C. neoformans  C. neoformans virulence factors Encapsulated budding yeast, becoming thickly encapsulated in mammalian tissues o Capsule: viscous polysaccharide composed primarily of glucuronoxylomannan Capsule envelopes cell wall, thus phagocytes cannot bind via mannose & -glucan receptors & yeast subverts phagocytosis o Its been observed that antibodies to C. neoformans glucuronoxylomannan may be present in serum but opsonization of yeast cells fails & no resultant phagocytosis occurs  C. neoformans virulence factors Melanin synthesis: can occur in C. neoformans cell wall: Antimicrobial oxidants produced by phagocytes e.g. superoxide anions & singlet oxygen. BUT Melanins are scavengers of reactive O2 intermediaries, thus organisms producing them become relatively resistant to destruction by immune cells  Pneumocystis jiroveci Clinically relevant to Botswana, due to association with HIV infection & AIDS Causes severe, often fatal pneumonia (Pneunocystis pneumonia-PCP) in immunocompromised individuals i.e. HIV & AIDS Invasion & inflammation of lungs, causes epithelium to rupture & alveoli septa to thicken Foamy exudate collects in alveoli but can also cause extrapulmonary infections especially in immunocomprimised Clinical samples collected: sputum, biopsy of lung tissue or sample f bronchoalveolar lavage. Diagnosis: microscopic detection for cysts in respiratory specimens &/or PCR Difficult to study because P. jiroveci cannot be cultured yet Opportunistic mycoses  Mucormycosis/ ‘black fungus’ (previously called Zygomycosis) Etiology: order Mucorales: commonly by Rhizopus oryzae & Mucor spp. (also Lichtheimia spp & Rhizomucor spp.) Transmission: inhalation of mucormycete spores, causing infections in sinuses & lungs, which can spread throughout the body RARE but life threatening. Mostly affects immunocompromised patients.**Risk factor – Diabetes (uncontrolled) &neutropenia 1. Rhinocerebral mycosis (sinus and brain) Black lesions on nasal bridge or upper portion of oral cavity. Fever 2. Pulmonary mucormycosis Fever; cough; chest pains; shortness of breath 3. Cutaneous mucormycosis Paranasal Blisters or ulcers on skin which can turn black mucormycosis 4. Disseminated mucormycosis & 5. Gastrointestinal Systemic, can reach brain & cause coma mucormycosis  Opportunistic fungal infections A B (A) Thrush in oral cavity (white patches). (B) Candida infections of nails very difficult to eradicate Candida albicans (Candidiasis) Dimorphic - oval budding yeast on mucosal surfaces; normal flora of intestinal & urogenital tracts. BUT invasive strains – form hyphae Superficial candidiasis ‘thrush’ – milky/ white patches of inflammation in mucous membranes i.e. mouth, vagina. Especially in infants, diabetic patients, debilitated patients & those on prolonged antibiotic therapy Also causes ‘vaginitis’ – vaginal secretions containing high amounts of sugar i.e. during pregnancy, poorly controlled diabetes, wearing tight synthetic undergarments (promote heat & moisture) Can be sexually transmitted  Candida auris (Candidiasis) An emerging fungal pathogen associated with nosocomial infections & considered a serious global health threat. Often resistant to multiple antifungal treatments C. auris can cause bloodstream infections, wound infections, respiratory infections and UTIs Risk factors similar to those of other Candida species i.e. Presence of a central venous catheter Indwelling urinary catheter Immunosuppressive state (HIV, hematologic malignancy, solid tumors, transplant recipients, neutropenia, chemotherapy, corticosteroid therapy) Diabetes mellitus Chronic kidney disease Exposure to broad-spectrum antibiotics or previous exposure to antifungal agents within 30 days Thank you To the Lord Jesus Christ be all the Glory