Virulence factors of fungi

1. PRESENTED BY : DR. KUMAR VIKRAM (MD 3rd YR),IGIMS,Patna
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2.  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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3.  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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4.  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.
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5. 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.

6. 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. Neoformans
W. 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 pathogens
Blastomyces,
Coccidiodes,
Histoplasma,
Paracoccidiodes,
Sporothrix schenkii
D. Epithelial cell and
monocyte cytocidal activity
Evasion of host defences Candida albicans
Candida tropicalis
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7. 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.
Fumigatus
Dermatophytes
b. Alkaline protease Degrades collagen, elastin, enhances invasion of lung tissue Aspergillus flavus, A.
Fumigatus
Rhizopus 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 defences
Environmental and tissue forms present different and surface
features, requiring different host response of mechanisms to
contain each form
True pathogens
Opportunistic
pathogens
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8.  Capsule
› Produced by
Cryptococcus
› Polysaccharide in
nature
 Cell wall
› Chitin
› Mannans
› Glucans
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9.  Cryptococcus
neoformans.
 viscous polysaccharide
capsule
 composed
of glucuronoxyomanna
n and other
components.
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10.  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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11. 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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12.  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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14.  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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15.  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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16.  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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17.  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.
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OPAQUE PHENOTYPE WHITE PHENOTYPE
Flat,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.

18. •Ability to survive and replicate at 37°C.
•Cryptococcus neoformans
•Histoplasma capsulatum
•Sporothrix schenckii
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19. 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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20. 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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21. 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
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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

22.  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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24. •Keratinases
•Collagenases
•Gelatinases
•Phospholipases
•Lipases
•Acid proteinases
•Acid
phosphatases
nutrient uptake
tissue invasion
adherence
Dissemination
inside the
host.
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25.  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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26.  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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28.  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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29.  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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30. Aspergillus
Penicillium
Stachybotrys
Fusarium
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31. 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(EL
EM), 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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32. 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 macrophages
Utilization of the alternative complement pathway
These mechanisms lead to immunoregulatory
disturbances and impairment of the host defenses
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33. Mechanisms Fungus
Activation of complement system C. Neoformans
Intracellular surviving and multiplication H. Capsulatum
P. 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. Immitis
H. Capsulatum
P. brasiliensis
Stimulation of suppressor cells C. Neoformans
P. brasiliensis
Interefence with fungicidal activity of phagocytes H. Capsulatum
Escape mechanisms of host
defences
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35.  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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36.  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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37.  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
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