Transcript Document 7155409

Medical Mycology
Dr C. O. Morton
Prof. T. R. Rogers
Dept of Clinical Microbiology
Today’s Lecture
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Introduction to Fungi
Mycotoxicology
Fungal Cell Wall
Fungal Diseases
Diagnosis
Treatment
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Reading List
Fungi are Eukaryotic, spore-bearing, heterotrophic
organisms that produce extracellular enzymes and
absorb their nutrients.
Fungi – Kingdom Mycota
Divisions - Phyla – Chytridiomycotina, Glomeromycota,
Zygomycota, Basidiomycota, Ascomycota
Microsporidia, Neocallimastigomycota
Fungi are Everywhere
In Medicine
Food
Spoilage
Superficial Colonisation
Fungi
•Of the 50-250,000 fungal species less than 200 cause human
disease and only a dozen or so on a regular basis
•Yeasts: unicellular fungi reproduce by budding
• Moulds (filamentous): produce hyphae and mycelium
• Dimorphic: grow as moulds (environment) or yeasts (in
human host)
Yeast
Hyphae
Basic Structure
Woronin body
Hyphal Tip
Aspergillus
(Anamorph)
Mitosporic State
Neosartorya
(Teleomorph)
Sexual State
The Fungal Cell Wall
Rigid structure surrounding the cell
Cell wall
•The fungal cell wall is essential for growth and viability
•Pathogen associated molecular patterns (PAMPS) – the
immune systems of most organisms recognise fungal cell
wall components such as ß-d-glucan and mannans
•Melanin – is an important component of fungal cells walls
especially in spores. It protects against UV radiation and
Reactive Oxygen Species
•Composition can affect action of antimicrobial agents –
Candida mutants lacking mannosylphosphate in their cell
wall displayed enhanced resistance to cationic antimicrobial
peptides via reduced peptide binding (Harris et al. 2009)
Immune Avoidance
•Cryptococcus neoformans – produces a
polysaccharide capsule – possible functions in avoiding
phagocytosis, affects antibodies – capsule deficient
mutants are less virulent (Perfect, 2005)
•Hydrophobin layer of Aspergillus conidia renders them
inert to the immune system (Aimanianda et al., 2009)
Fungi Need to Eat
Heterotrophic
Secrete extracellular enzymes
Absorptive nutrition
(Osmotrophs)
•Saprobes: decay dead
organic matter
•Pathogens: biotroph,
necrotroph
•Symbionts: parasites commensals - mutualists
Fungi Interact with other organisms
•Fungi may gain nutrients
through parasitism of other
organisms
Plant Parasitism
“The Frog
problem”
Chytridiomycosis
Affecting 30% of the
amphibian species of
the world??
Nematode Parasitism
Mycotoxicosis
Mycotoxins
• Mycotoxins are low-molecular-weight secondary
metabolites of fungi
• Often produced by food spoilage organisms or in
basidiocarps (Mushrooms)
• mycotoxins are an important chronic
dietary risk factor
• Aflatoxins - Aspergillus spp.; Citrinin –
Penicillium spp.; Ergot Alkaloids – Claviceps
spp. – Ergotism; Fuminosins – Fusarium spp.
http://www.doctorfungus.org
Amanita muscaria
• Genus/Species: Amanita muscaria
• Slide Reference #: GK 016
• Image Type: Macroscopic Morphology
(Basidiocarps)
• Disease(s): Mycotoxicosis
http://www.doctorfungus.org
Amanita phalloides
• Genus/Species: Amanita phalloides
• Slide Reference #: GK 017
• Image Type: Macroscopic Morphology
(Basidiocarp)
• Disease(s): Mycotoxicosis
Fungal Spores
Humans and Fungi
• Humans present a series of diverse microenvironments
and barriers to nutrient acquisition, including:
• pH – human body has wide pH range – fungi prefer
acidic conditions
• Temperature – 37 °C is inhospitable to many fungi
• Nutritional immunity – sequestration of essential
micronutrients such as iron
• Physical barriers – skin is composed of polymers that
many fungi cannot degrade
• Gaseous tension – the ration of O2 to CO2 varies
between the surface and within tissue
Host immune system
• Innate immune system
• Pamps, prr
• Tlr dectin (segal)
Primary Route of Infection
(Ebel, F.)
Immune System and Fungal Infection
Segal (2009 )
Fungi That Cause Human Infection
• Yeast: Candida albicans, Cryptococcus neoformans
• Mould: Aspergillus, Penicillium, Fusarium,
Scedosporium
• Dimorphic: Histoplasma capsulatum
Associated with Fungi
•Sick building syndrome
•“Curse of the Pharaohs”
Classification of human fungal infections
• Superficial: ringworm (dermatophytes), thrush (Candida
species), dandruff (Pityrosporum)
• Subcutaneous: involve the dermis of the skin, deep tissues
or bone. Usually found in tropics/sub-tropics where caught
walking barefoot eg, mycetoma
• Systemic: due to pathogenic (Histoplasma) or opportunistic
(Aspergillus) fungi
Dermatophytosis
•Tinea – describes an infection caused by a dermatophyte
(ringworm fungi)
•Specialised pathogenic fungi
•Caused by – Trichophyton, Microsporum,
Epidermophyton
•Worldwide distribution
•Key feature is keratin degradation
•Don’t tend to grow at 37 °C
(Brasch 2008)
Dermatophytes
Onychomycosis
Tinea pedis
Tinea corporis
• Invasive Fungal Disease
Invasive Fungal Infection
•Invasive fungal infections are major causes for morbidity
and mortality in severely ill or immunocompromised patients
•Main causative agents are Candida spp, Aspergillus spp.
•Emerging infectious fungi – Fusarium spp. Scedosporium
spp, Zygomycetes, e.g. Mucor spp
•Invasive aspergillosis (IA) occurs in 10% of patients
undergoing haematopoietic stem cell transplantation (HSCT)
and 25% in autopsy of leukaemic patients
•IA has reported mortality rates of 86% and 66% for
pulmonary and sinus IA
Candida spp
• Is usually a harmless commensal organism
• It causes both superficial and invasive infections where
the host is immunocompromised or epithelial barriers
have been damaged
• Candida albicans – primary cause of candidiasis
Aspects of virulence
•Dimorphism
•Phenotypic and mating
type switching
•Biofilms – higher resistance
to antifungals
Candida spp can cause disseminated infection
Aspergillus fumigatus (Neosartorya fumigata)
Ubiquitous soil microbe
Decay of organic matter in
compost heaps
Dispersed by spores, conidia.
Opportunistic pathogen of
mammals and birds
Most important cause of
Invasive Fungal Disease in
Immunocompromised
individuals
Aspergillus fumigatus
In a tissue sample
conidia
germination
hyphae
mycelium
Mycetoma
http://www.doctorfungus.org
•Is most common in Africa and
South America
•Is a chronic destructive disease
affecting skin, underlying tissue
and sometimes adjacent bone
•Caused by various fungi including
Madurella spp., Scedosporium
spp., Leptosphaeria spp.
•Infection results from traumatic
implantation of spores into the skin,
e.g. thorns, splinters
• Legend:
Multiple draining sinuses, swollen tissue,
and sclerotia are present.
Genus/Species: Madurella mycetomatis
Image Type: Clinical Presentation
Histoplasmosis
http://www.doctorfungus.org
•The most common endemic
mycosis in North America, also
found in Central and South America
•A thermally dimorphic fungus,
found as a mould in the
environment but as budding yeast
in tissue
•Inhalation of spores is the primary
route of infection
•Prolonged exposure to aerosolised
spores is a major risk factor
•Fewer than 5% of individuals
exposed to the fungus develop
symptomatic disease
• Genus/Species: Histoplasma
capsulatum var. duboisii
• Image Type: Histopathology
Blastomycosis
•Occurs in North and South
America, also Africa
http://www.doctorfungus.org
•It is a mould in the environment but
forms large budding yeast in tissues
•Infection through inhalation
•Normally in individuals with
outdoor occupations
•The skin is the most common site
of disseminated disease
• Genus/Species: Blastomyces
dermatitidis
• Image Type: Histopathology
Diagnosis of Fungal Infection
•Microscopy – direct staining of fungi in sections can
distinguish between yeasts and molds
•Culture – can lead to diagnosis of the exact species.
Candida can be grown in blood cultures but Aspergillus
cannot
•Serology – direct detection of fungal antigens in serum
samples. ELISA to detect galactomannan (Platelia – BioRad)
or detection of ß-d-glucan, does not detect Cryptococcus spp
or zygomycetes
•Radiography – direct observation of patients to spot
characteristic signs of infection, e.g. halo signs, cavities
•PCR – assays target fungal ribosomal operon, nucleic acid
extraction from blood or BAL. Potentially very sensitive but
still no standardised tests
(Hope et al. 2005)
Antifungals
Azole Fungicides:
Itraconazole
Voriconzaole
Posaconazole
Fluconazole
Inhibit ergosterol
biosynthesis – affect
cell membranes
Echinocandins:
Caspofungin
Micafungin
•Inhibit 1,3-betaglucan synthase,
affects cell wall
•Broad spectrum, low
toxicity
Fluorinated
Pyrimidines:
Flucytosine (5FTC)
•Inhibit nucleic acid
synthesis
Polyenes:
Amphotericin B
Nystatin
•Forms Pores in
membranes by
interacting with
ergosterol
•Toxic
Antifungal Agents
Antifungal Drug Resistance
•Efflux pumps in fungi of the ABC and MFS superfamilies
and variation in target genes (e.g. cyp51A) are involved in
resistance to azoles
•Resistance to polyenes is uncommon. Resistant isolates
of rare Candida spp show altered levels of membrane
sterols
•Resistance to FTC is associated with reduce uptake and
reduced activity of genes responsible for conversion of
FTC to FUMP; FUMP disrupts RNA synthesis
•Mutations in the FKS gene (target) can lead to resistance
to echinocandins or resistance may be induced by cell wall
salvage mechanisms….
Reading
General Mycology Books
Introduction to Modern Mycology by J.W. Deacon
The Fungi by M.J. Carlile, S.C. Watkinson and G.W. Gooday
Review Articles
• Cooney, N. M. & Klein, B. S. (2008). Fungal adaptation to the
mammalian host: it is a new world, after all. Curr Opin Microbiol 11,
511-516.
• Hope, W. W., Walsh, T. J. & Denning, D. W. (2005). Laboratory
diagnosis of invasive aspergillosis. Lancet Infect Dis 5, 609-622.
• Segal, B. H. (2009). Aspergillosis. N Engl J Med 360, 1870-1884.
Websites
http://www.doctorfungus.org/
http://www.aspergillus.org.uk/