virulence factors of fungi
TRANSCRIPT
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INTRODUCTION There are 100,000 species of fungi . These fungi inhabit different niches, a
number of them are symbiotic and may live in commensalism, mutualism or parasitism with other organisms
Of all these species, only around 600 species are human pathogens.
This fact that has led to several studies providing a better understanding of the relationship among parasite, host and virulence factors.
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Data from the late 1950s and early 1960s indicate that invasive fungal infections were extremely rare, even in immunocompromised cancer patients.
Now, fungal infections have dramatically increased in the past two decades as a result of improved diagnostics, high frequency of catheterization, instrumentation and an increasing number of immunosuppressed patients
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Virulence and Pathogenicity Pathogenesis is the ability of a
microorganism to infect the host and produce disease resulting from interaction of pathogen with host via expression of certain factors on both sides.
Virulence refers to quantitative ability for pathogenesis of a species.
Determinants of pathogenicity are called virulence factors.
The symbiotic-parasitic relationship produces an infectious process leading to lesions of the host tissues and establishment of disease due to a direct imbalance in parasite-host interaction.
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VIRULENCE FACTORS FUNCTIONS FUNGAL PATHOGENS
A. SURFACE COMPONENTS a. Cell wall glycoproteins
Adherence to epithelial surfaces
Candida species
b. Melanin pigment Shield against immunologically active cells, hydrolyses
Cr. NeoformansW. dermatitidis
c. Capsules, glucans
Anti - phagocytic Cr. neoformans
B. Thermotolerance Survive and replicate at 370C
Human pathogens
C. Resistance to microbiocidal products of neutorophils e.g. H2O2, by dimorphic primary pathogen
Evasion of host defence mechanisms by tissue phase (yeast, spherule) of virulent dimorphic fungi
Primary pathogensBlastomyces, Coccidiodes, Histoplasma, Paracoccidiodes, Sporothrix schenkii
D. Epithelial cell and monocyte cytocidal activity
Evasion of host defences Candida albicansCandida tropicalis
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VIRULENCE FACTORS
FUNCTIONS FUNGAL PATHOGENS
E. Exoenzymes
a. Elastase Degrades elastin, scleroproteins, enhances invasion of elastin containing tissue ( lung, skin, blood vessels)
Aspergillus flavus, A. FumigatusDermatophytes
b. Alkaline protease Degrades collagen, elastin, enhances invasion of lung tissue
Aspergillus flavus, A. FumigatusRhizopus spp.
c. Keratinase, collagenases
Degrades scleroproteins in skin Dermatophytes
d. Acid protease Cleavage of IgA2 Candida spp., A. Fumigatus
F. Toxins
a. Aflatoxin Hepatotoxicity Aspergillus flavus
b. Endotoxin Tissue necrosis A. Flavus, A. Fumigatus
G. Dimorphism Evasion of host defencesEnvironmental and tissue forms present different and surface features, requiring different host response of mechanisms to contain each form
True pathogensOpportunistic pathogens
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Cell wall components and capsule Capsule
› Produced by Cryptococcus
› Polysaccharide in nature
Cell wall› Chitin › Mannans› Glucans
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Capsule Cryptococcus
neoformans. viscous
polysaccharide capsule
composed of glucuronoxyomannan and other components.
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down regulate cytokine secretion, inhibit leukocyte accumulation, induce suppressive T-cells, inhibit antigen presentation, and inhibit lymphoproliferations Hence, it serves as a barrier to host
defenses in a variety of way.
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Virulence factors associated with cell wall components
Component Fungus Activity
α – (1,3) – glucan
B. dermatitidis Antigenic masking of WR – 1 adhesin
P. brasiliensis Resistance to digestion by phagocytosis
H. Capsulatum Destruction of macrophage in vitro
Glucuronoxylomannan C. neoformans Resistance to phagocytosis
Melanin C. neoformans Interference with oxidative metabolism of phagocytes
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MORPHOLOGY MORPHOLOGICAL VERSATILITY Almost all pathogenic fungi can grow
in more than one form. The major exception is C.
neoformans , which apparently exists only in the yeast form in vivo. In vitro it also grows mostly as a yeast; however, it does form filaments during the mating process.
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Transition from yeast form to hyphal form is facilitated by › nutrients, › near-neutral pH, › CO2 concentration about 5.5%, › presence of N-acetyl-D-glucosamine,
serum, some amino acids, and biotin. Reverse transition from hyphal to yeast
form is provoked by › acidic pH, › absence of serum, and› higher concentration of glucose
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Many molds form conidia , or vegetative spores; scattered by wind or water, these small, resistant cells serve as a mode of dissemination. In the case of aspergillosis, conidia serve as
the propagule that infects debilitated patients. Hydrophobicity is thought to contribute to
the efficacy of Aspergillus conidia, already an ideal size for deposition into alveoli, to disperse in air.
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PHENOTYPIC SWITCHING Ability of organisms of single strain to
switch reversibly at high frequencies among different colony types.
Observed in C.albicans, allows Candida to adapt to a different
host environment during infection. Colonies of C. albicans show
morphological variation, including smooth, rough, star, stippled, hat, wrinkle, and fuzzy at high frequency.
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At present, of all the phenotypes described, the white-opaque system in strain WO-1 is the most studied.
Opaque cells switch to white cells within one generation at 37oC.
OPAQUE PHENOTYPE WHITE PHENOTYPEFlat,Grey, opaque colonies Smooth, white colonies
elongated or bean shaped round ovoid
more virulent in a systemic mouse model of infection.
Less virulent in a systemic mouse model of infection.
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•Ability to survive and replicate at 37°C.• Cryptococcus neoformans• Histoplasma capsulatum • Sporothrix schenckii
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Most isolates of C. neoformans var gattii that do not grow efficiently at
37oC are not able to produce fatal infection in mice
whereas isolates of var neoformans germinate and grow at 37°C producing lethal infection
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Low-virulence strains of H. capsulatum require more
time for mycelium-to-yeast-phase transition at 37ºC
whereas the more virulent strains are capable of withstanding
drastic temperature changes and of transforming more quickly
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Isolates of S. schenckii from systemic lesions can grow at
35ºC and at 37ºC, but isolates from fixed cutaneous lesions can
only grow at 35ºC It is believed that even small differences in temperature tolerance can influence the pathogenic potential of a microorganism as well as the form of disease presented by the host
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Biofilm formation A biofilm is an
assemblage of surface-associated microbial cells that is enclosed in an extracellular polymeric substance matrix.
Biofilm may form on a wide variety of surfaces, including living tissues, indwelling medical devices, industrial or potable water system piping.
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•Keratinases•Collagenases •Gelatinases•Phospholipases •Lipases•Acid proteinases•Acid phosphatases
nutrient uptake
tissue invasion
adherence Disseminatio
n inside the host.
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PHOSPHOLIPASES Phospholipases are enzymes that hydrolyze
ester linkages of glycophospholipids. Classified into phospholipases A,B,C and D. Produced by candida albicans, cryptococcus
neoformans and aspergillus fumigatus. Roles
› Adherence to host cell› Penetration› Host cell injury› Signal transduction› Stimulation of host cells to release cytokines.
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Extracellular proteinsases Proteolytic activities attributed to secretory
aspartic proteinases. Produced by candida spp.(C. Albicans, C.
Tropicalis, C. Parapsilosis), Aspergillus spp., Coccidiodes immitis.
Role in › adherence and survival of the pathogen on mucosal
surfaces ,› invasion of host tissues and › digestion of immunoglobulins.
C. immitis endospores produce proteinases with elastase and collagenase activity
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Mycotoxins (Myco = of fungal origin) Secondary metabolites (chemicals) of a fungus that
produce toxic results in another organism. Unlike bacterial toxins, fungal toxins (mycotoxins) are
not proteins and therefore are not usually detectable by the immune systems of humans and animals
Lack of visible appearance of fungus does not negate presence of mycotoxins. Toxins can remain in the organism after fungus has been removed.
Cytotoxic: disrupt cell structures such as membranes, and processes such as protein, DNA, and RNA synthesis.
Can be heat stable, not destroyed by canning or other processes.
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How many mycotoxins are there? Today 300 - 400 mycotoxins are known Mycotoxins of human concern based on
toxicity: Aflatoxin Deoxyniva-lenol (DON) or Vomitoxin Zearalenone Fumonisin T-2 toxin Ochratoxin A
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MYCOTOXINS
FUNGAL SPECIES SOURCE CLINICAL CONDITIONS
Aflatoxins A. Flavus,A. Parasiticus, A. Nominus, Penicillium puberulum
Nuts, maize Aflatoxicosis, Reye’s syndrome, hepatitis, hepatoma,
Fumonisins Fusarium moniliforme Maize Equine leukoencephalomalacia(ELEM), Porcine pulmonary edema(PPE)
Trichothecenes
Fusarium graminearum, F. Sporotrichoides
Maize, sorghum Human toxicosis, Alimentary toxic aleukia, Biological warfare
Ochratoxins Aspergillus ochraceus, A, niger, Penicillium verrucosum
Cereals, coffee beans, bread
Nephropathies i.e Balkan endemic nephropathy and Mycotoxic porcine nephropathy
Cyclopiazonic acid
A. Flavus, A. Versicolor, A. Oryzae, Penicillium cyclopium
Ground nut, corn, meat
Co – contaminant, kodua poisoning
Zearalenones Fusarium graminearum Wheat, maize, barley, sorghum
Genital disorders in animals i.e pigs
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When facing agressive conditions some fungi are able to use various and complex strategies involving
mechanisms such as
suppression of cytokine production
reduction of the fungicidal activity of macrophagesUtilization of the alternative complement pathway
Mechanisms of Evasion from Host Defenses
These mechanisms lead to immunoregulatory disturbances and
impairment of the host defenses
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Mechanisms FungusActivation of complement system C. Neoformans
Intracellular surviving and multiplication H. CapsulatumP. brasiliensis
Down regulation of antigen presentation by macrophages
C. Neoformans
Immunosuppresive effect of fungal antigen on the cytokine production by mononuclear phagocytes
C. Neoformans
Immunosuppression induced by antigenemia C. ImmitisH. CapsulatumP. brasiliensis
Stimulation of suppressor cells C. NeoformansP. brasiliensis
Interefence with fungicidal activity of phagocytes
H. Capsulatum
Escape mechanisms of host defences
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Conclusions: Understanding the pathogenicity
mechanisms that fungi use during infection is crucial for the development of new antifungal therapies and diagnostics.
Classically, antifungal drugs were designed to exert fungicidal activities.
Recently however, specifically targeting virulence factors has been proposed as a new and promising antifungal strategy.
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Several virulence factors, such as dimorphism, the secretion of proteases and the expression of adhesins and invasins, have been suggested as attractive targets.
As our detailed understanding of fungal pathogenicity mechanisms improves, the potential for developing novel therapeutic and diagnostic strategies expands.
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References Chander, J. Textbook of Medical Mycology, 3rd ed. Mehta
Publication: New Delhi, 2009. Hogan et al. Virulence factors of medically important fungi,
Clinical Microbiology Reviews, Oct. 1996, p. 469–488 Cilmery S K, Maria F S, Maria T S. Virulence factors in fungi
of systemic mycoses, Rev. Inst. Med. trop. S. Paulo vol. 40 n. 3 São Paulo May/June 1998
Ananthnarayan & Paniker. Textbook of Microbiology, 9th ed. Universities press: Hyderabad, 2014