Fungi Flashcards
What do fungi feed on?
Dead plant or animal material in soil and compost heaps
Name some similarities between the genomes of fungi and humans
Eukaryotes
Membrane-bound nucleus with several chromosomes
DNA contains introns
Plasma membrane and cytoplasmic organelles
Name some differences between the genomes of fungi and humans
6000 genes
Normally haploid, only briefly diploid
What is the structure of a fungal cell wall?
Cell wall of polysaccharides
Inner layer of chitin microfibrils (beta-1,4-NAG)
Outer layer of glucans (beta-1,3-linked glucose) and glycoproteins (asp-N-mannose or galactose, ser/thr-O-mannose or galactose)
What are the three categories of fungal morphology?
Yeasts
Filamentous moulds
Dimorphic fungi
Discuss yeasts: shape, growth
Oval or round
Unicellular
Mitotically divide by symmetrical binary fission or asymmetrical budding
Others grow as short cylinders called pseudohyphae
How does Schizosaccharomyces pombe divide?
Symmetrical binary fission
Discuss filamentous moulds: shape, growth
Hyphae - thin branching multicellular cylinder
Apical growth - mitosis at tip or t side branches behind the tip
What are the different structures hyphae can make up, and give examples of fungi of each of these categories?
- Septate: interconnected compartments (Aspergillus)
- Aseptate: many nuclei in common cytoplasm (Mucor)
- Mycelium: many hyphae forming an interwoven mass when compacted together. Can form macroscopic structures (mushrooms)
Dicuss dimorphic fungi
Fungi that are a different shape based on temperature
e.g. Histoplasma
Filamentous mould at 22 degrees in the environment
Yeast at 37 degrees in the body of a mammal
What are spores?
Small, tough, light fungal cells adapted for dispersal to new habitats and/or survival in hostile environments
What are the two states of differentiation of spores?
- Asexual/anamorph/mitotic/imperfect - yeasts produce internal endospores, moulds produce external endospores called conidia or internal spores inside a special hypha called a sporangium
- Sexual/telomorph/meiotic/perfect - motile gametes or specialised sexual hyphae. Means undergoes recombination and chromosome reassortment
What are conidia?
Mould external endospores - asexual spores
What is a sporangium?
A special hypha that some moulds form internal asexual spores in
How do Deuteromycota replicate?
Asexually - incapable of sexual reproduction
Where are sexual spores formed?
- Internally within tubular structure called an ascus
- Externally on specialised club-like structure called basidum
- Externally following fusion of specialised sexual hyphae
Discuss Ascomycota - where are the spores formed, give examples of yeasts and moulds
Sexual spores in an ascus
Asexual spores in a conida
Yeast = schizosaccharomyes, saccharomyces, pneumocystis, candida
Moulds = asperfillus, fusarium, microsporum, trichophyton, epidermophyton
Dimorphic = histoplasma
Discuss Basidomycota - where are the spores formed, give examples
Sexual spores on a basidum
Yeasts = Cryptococcus, malassezia
Discuss Zygomycota - where are the spores formed, give examples
Sexual spores = external fusion of hyphae
Aseuxal = sporangium
Moulds = mucor, rhizopus
Discuss Deuteromycota (imperfect fungi)
Only asexual - conidia
Which 3 lifestyles do fungi use to get nutrition?
- Saprotrophs - feed on dead plant or animal material. Infect by accidental inhalation or implantation
- Parasites of living plants - diseases of crops mainly
- Parasites of humans - can cause disease
How does an intact epithelium inhibit fungal entry?
- Physical barrier
- Secrete anti-microbial peptide
- Commensals inhibit multiplication of fungi on the surface
Which PRRs recognise fungi?
- MBL mannose-rich structures
- Dectin-1 beta1,3glucan
- Dectin-2 mannan
- DC-SIGN mannose-rich structures
- Mincle Mannose-rich structures
- TLR2 - Phospholipomannan
- TLR4- O-linked mannose
- Mannose receptor N-linked mannose
What does MBL recognise?
Mannose-rich structures
What does dectin1 recognise?
Beta1,3glucan
What does dectin2 recognise?
Mannan
What does DC-SIGN recognise?
mannose-rich structures
What does mincle recognise?
mannose-rich structures
What does TLR2 recognise?
Phospholipomannan
What does TLR4 recognise?
O-linked mannose (linked to a ser/thr)
What does the mannose receptor recognise?
N-linked mannose (linked to an asp)
Which PRRs activate the Syk pathway?
Dectin1
Which PRRs activate the Myd88 pathway?
TLR4
What are the key effector cytokines in response to fungi?
IL-17, IL-22
What is the pathway to activate proIL-1beta?
- Dectin1 –> Syk –> Inflammasome NLRP3/ASC/Casp1 –> proIL-1beta to IL1beta
What are the pathways to activate NFkappaB?
- Beta1,3-glucan –> Dectin2 –> Syk –> CARD9 –> MALT –> NFkappaB –> pro-IL1B + secrete IL6, IL23, IL12, TNFalpha
- O-linked mannose –>TLR4 –> Myd88 –> NF-kappaB –> pro-IL1b, secrete IL6, IL23, IL12, TNFalpha
Which cytokines, secreted in response to NFkappaB, stimulate a Th17 response?
IL6, IL23
Which cytokines, secreted in response to NFkappaB, stimulate a Th1 response?
IL12 TNFalpha
Which conditions can cause impaired fungal sensing? What does this make you susceptible to?
Susceptible to superficial fungal infections
- Dectin1 deficiency
- CARD9 caspase recruitment domain-containing protein 9 mutation
What does impaired IL-17 receptor sensing do?
Susceptibility to chronic superficial Candida infection, or where auto-antibodies against IL-17 (or 22) inactivate the cytokines
What does impaired neutrophil number predispose to? When does this occur?
Deep fungal infections
Opportunistic filamentous moulds esp Aspergillus
Cytotoxic chemotherapy, chronic granulomatous disease where there is a mutation in one of the genes of the neutrophil NADPH oxidase system that normally generates superoxide needed to kill fungi after phagocytosis
What does impaired T cells and macrophage number or function predispose to? Why do people get this?
Yeast infection (superficial and deep) (candida, pneumocystitis, cryptococcus) and dimorphic fungi that can invade the body as yeasts (histoplasma) Primary immunodeficiency syndromes, immunosuppressant drug reatment, haematological malignancies, AIDS
How do you diagnose a fungal infection?
- Culture of fungus from clinical sample e.g blood, urine, skin scraping, lung biopsy
- Detection of fungal polysaccharide or fungal DNA in a sample e.g Cryptococcal polysaccharide in blood or CSF, pneumocystitis DNA in lung fluid
- Tissue biopsy for microscopy and culture - morphology to ID moulds
Amphotericin, nystatin
Polyene
Inhibit ergosterol function
Clotrimazole, fluconazole, voriconazole
Azoles
Inhibit ergosterol synthesis
Terbinafine
Allylamine
Inhibit ergosterol synthesis
Grisofulvin
Inhibit fungal microtubule synthesis
Caspofungin
Echinocandin
Inhibits beta-1,3-glucan synthesis of the fungal cell wall
What is ergotamine?
A fungal toxin in rye crops
Potent vasoconstrictor that causes painful ischaemia/gangrene of fingers and toes
Where are filamentous moulds commensal?
NOWHERE
What is ringworm?
Superficial fungal infection of dead keratinised tissues (skin, nails, hair)
What are dermatophyte moulds?
Microsporum, trichophyton, epidermophyton (all cause athlete’s foot)
Very common in humans, wild and domestic animals
Acquired by spores: direct person-to-person contact, indirect sharing contaminated clothing or environment
Not life threatening, don’t invade into deeper tissues
Chronic
Induce local type IV sensitivity (host CD4+ T) so increased skin proliferation –> flaky, scaly skin + itching, leading to scratching and secondary bacterial infection
Hard to eradicate, may recur
What does candida albicans cause superficially?
Disfiguring chronic fingernail condition
What does Malassezia furfur cause?
Pityriasis versicolor (blothy skin rash that doesn’t tan)
What is rose picker’s disease?
Subcutaneous fungal infection
Invades as a yeast at 37 degrees
Forms chronic skin nodules that may spread up the arm
What are the different levels of fungal infection?
- Toxins
- Commensals overgrowing
- Superficial
- Subcutaneous
- Systemic
What are the two main types of systemic fungal infections?
Systemic pathogens and systemic opportunists
How do histoplasma change morphology?
Temperature-sensitive dimorphism regulating kinases DRK1 and RYP1 induce a pattern of gene expression:
- Expression of alpha-1,3-glucan not recognised by host PRRs
- Fungal hsp60 which binds to CD11/CD18 on the surface of human mphages
- Secreted protease-resistant calcium binding protein that facilitates fungal growth within macrophage vacuoles
What are the temperature-sensitive dimorphism regulating kinases called?
DRK1 and RYP1
How are fungal infections controlled?
T cell response –> mphages –> granulomas
How do you identify if people have a memory T cell response against Histoplasma?
Skin test- inject dead histoplasma antigen into the skin
How does histoplasma enter the body?
Lung (accidentally breathed in)
Soil and bat droppings in Mississippi and Ohio valleys
Inhaled by bats
Name the forms of histoplasmosis, the clinical features, and the final outcome
Asymptomatic: positive skin test only, resolution
Acute pulmonary (influenza-like): lung infiltrates, enlarged hilar lymph nodes, resolution
Chronic pulmonary: chronic lung inflammation, fibrosis, cavitation, progressive incapacity
Disseminated (spread through blood): focal spread outside lung or widespread disease (skin, larynx, adrenal), often fatal
Which host defects can predispose to systemic opportunist fungal infection?
Defect in skin barrier (IV, burns)
Neuropenia (leukaemia or its treatment)
Reduced function of neutrophils (chronic granulomatous disease)
Reduced number/function of T cells - lymphoma, AIDS, immunosuppressive drug treatment
NOT LACK OF AB
What is the receptor on dendritic cells used to recognise histoplasma?
VLA-5 fibronectin and fibrinogen receptor
What type of response is the histoplasma response?
Th1
How does candida divide?
Divide by asymmetrical budding
What is the structure of candida?
Commensal yeast on moist mucosal surfaces
Some strains have increased capacity for adhesion and invasion through the epithelial surface by forming pseudohyphae
Forms a biofilm: large communities of organisms embedded in a sticky extracellular matrix at the interface between a liquid medium and a sticky extracellular matrix
Where do you find candida?
Throat, gut, vagina
What is candidalysin?
A secreted 31 aa peptide toxin that damages epithelial cells
When does oral candidiasis occur?
Newborn babies, following inhaled steroid treatment for asthma, following antibiotics
When does vulvo-vaginitis candidiasis occur?
Pregnancy, following antibiotic treatment, nappy rash
When does UTI candidiasis occur?
Following urethral catheter and/or previous antibiotic treatment
When does oesophageal candidiasis occur?
Advanced HIV, organ transplant
When does candidaemia occur?
Bloodstream infection
IV cannula in hospital, IV drug users, endotracheal tube
When does disseminated infection of candida occur?
Spreads through blood to eyes, liver and spleen, heart valves
Mainly in immunosuppressed people
How do you treat candida?
Fluconazole or caspofungin
Name a species of candida resistant to fluconazole
Candida krusei
How is pneumocystitis jirovecci transmitted?
Human-to-human by inhalation, obligate intracellular parasite
Are recurrences of pneumocystitis jirovecci due to re-activation or re-acquisition when individuals have impaired T cell immunity?
Re-acquisition.
What does pneumocystitis jirovecii cause? e.g. pathophysiology, signs, symptoms
Diffuse inflammation of lung alveoli
Clusters of oval fungal organisms in the alveolar airspaces, injury to alveolar epithelial cells, alveoli filled with protein-rich foamy exudate
Impairs gas exchange so arterial hypoxaemia worst during exercise
Dry cough, progressive breathlessless, fever, weight loss
X-ray widespread hazy opacification
Treament of pneumocystitis jirovecii?
Co-trimoxazole - inhibits fungal folic acid synthesis +
Prednisolone = inhibit innate inflammatory response to fungal polysaccharides released by dying organisms
How does pneumocystitis jirovecii replicate?
Trophic form = binary fission
Sexual form = sexual spore/cyst, after maturation the collapsed spore releases trophic forms
How do you prevent pneumocystis pneumonia in high-risk immunocompromised patients?
Regular low dose treatment with co-trimoxazole or inhaled pentamide
Pentamide
Used to prevent pneumocystis pneumonia in high-risk immunocompromised patients
Inhibits topoisomerase enzymes in mitochondria
How do humans acquire Cryptococcus neoformans?
Inhalation of bird droppings, then spreads through lung to CNS
What is a key virulence factor of Cryptococcus neoformans?
Thick polysaccharide caspule
Are the types of infection of Cryptococcus neoformans?
Pneumonia rarely, meningo-encephalitis when T cell immunodeficiency
What does Cryptococcus infection cause?
Slowly worsening meningo-encephalitis Progressively worsening headache Fever Confusion Hydrocephalus Coma 20% fatality Survivors may have persistent neurological disability including blindness
How do you treat Cryptococcus?
Amphotericin + flucytosine, then high dose fluconazole long term
Amphotericin
Polyene antifungal and antiparasitic, forms pores in the membrane by an ergosterol-dependent mechanism. Drug of choice for most systemic mycoses: Cryptococcus, candida, aspergillus. Also antiprotozoal
How is aspergillus fumigatus acquired?
Air-borne spores
Inhaled
Mouldy hay, compost heaps, hospital re-building work
What types of infection does aspergillus cause and under which conditions?
Airway colonisation - asthma
Aspergilloma (localised mass of hyphae) - pre-existing lung cavity e.g. after prev TB
Invasive infection of lung or paranasal sinuses - severe neutropenia e.g. after chemotherapy for leukaemia
V high fatality
Treatment for aspergillus?
Amphotericin or voriconazole, surgery if poss
How do you get mucormycosis?
Inhalation of spores
Direct contamination of wounds (tornado or tsunami)
Invades aggressively through paranasal sinuses or lungs
How do you treat mucormycosis?
Urgent surgery to remove dead tissue + IV amphotericin
