lecture 15) candida albicans fungal virulence factors case study Flashcards
what is the main species of candida that causes infection?
candida albicans
what is virulence?
ability to cause disease
continuous variable
what significantly impacts virulence?
host response
what is meant by the term fully virulent?
average virulence, what you would expect the virulence of that specific species
what does hypervirulent mean?
more virulent than what you would expect to see in that species
what does attenuated/avirulent mean?
less virulent than what you would expect to see in that species
why should you be careful with the term avirulent?
next to impossible to prove as may be virulent in a certain patient group
virulence factors/traits are considered to be something that the pathogen has that damages the host. why is this not strictly true?
the host’s response is important
normally the host response to the pathogen that causes the damage
what is the definition of disease from microbial attack “damage to the host”?
damage results from a range of host pathogen interactions
describe the 2 cases in which disease would arise on the basis of pathogen - host immune system interaction
when the host response is too weak (impariment in host eg immunocompromised, direct action of pathogen might be to suppress immune response, release of toxin = damage due to microbial factors)
damage due to overreaction of host immune response (could still be killing the pathogen but the immune overreaction is causing damage to the host)
what is the one thing to keep in mind about systemic infections of candida?
candida is always growing
what happens in systemic infections of candida?
host response too low (immunocompromised)
new route of infection into sterile sites eg wound
due to growth of candida through the host
what are superficial infections of candida due to?
low host immune response
what genetic techniques are used for the identification of candida?
reverse and forward genetics
what does reverse genetics involve?
sequence genome
look for proteins that may have a role in virulence (can compare proteins from other organisms that cause virulence)
expression based approach
infection model
genome wide expression analysis
see which ones are being switched on during virulence process
what does forward genetics involve?
randomly mutagenise strain
screen for mutants and identify which ones cause disease
why would forward genetics be tricky to carry out?
would have to make thousands of mutants (cant be tested in animals due to cost and ethics)
what is the alternative to forward genetics?
signature tag mutagenesis
random mutants, tag them, put through a model and see which ones come out the other end as virulent factors
how is the majority of work on candida carried out?
reverse genetics
why cant you do random mutagenesis on candida?
obligate diploid
2 copies of each gene
too much damage would be done to DNA so would cause severe problems elsewhere
how would you apply koch’s postulates to confirm a gene in an organism is important in causing disease?
mutate gene of interest
demonstrate the strain has attenuated virulence
complementation: reintroduce gene should cause virulence
name the 4 stages of infection of candida albicans
adherence and colonisation
epithelial penetration
vascular dissemination
endothelial colonisation and penetration
how do systemic disease of candida albicans spread?
spread through blood stream
has to grow and stay alive to cause disease
what nutrient does candida albicans need in order to survive?
iron
what makes candida albicans multifactorial?
continually growing
name 5 virulence factors of fungal pathogens as the result of host-pathogen interaction
adhesion to host surfaces invasion of epithelial penetration beyond epithelia obtaining nutrients opposing host defences
name 3 host defence mechanisms in virulent fungal pathogens as a result of host-pathogen interaction
flushing mechanisms
molecular recognition
phagocytosis and killing
immune response
describe the balancing acts between candida in the host for superficial infections
doesnt take much to tip the balance in favour of superficial infections
describe the balancing act between candida and host in systemic infections
balance needs tipped further in the benefit of candida to cause systemic infection
describe the asexual mating of candida
2 diploids mate forming a tetraploid
meiosis not identified, diploid forms due to chromosome loss
what does polymorphic mean?
can exist in more than one morphology
what does the CUG codon in candida mean?
they have a serine residue as opposed to a leucine residue
makes working with candida very difficult as previous tools cant be used on candida as they would be inactive
new tools had to be built from scratch to work with candida
what are the 5 most common morphologies of candida albicans?
yeast pseudohyphae hyphae opaque chlamydospore
how do yeast morphology of candida grow?
bud off each other
apical extension switching to isotropic growth
how do filamentous morphology of candida grow?
occurs in certain environments elongated yeast cells stuck in apical extension for longer no cell separation so cells stay covalently bonded together can see where septation occurs
how do hyphae morphology of candida grow?
grows out of apical extension branch forming network of filamentous
switch from yeast to hyphae is very impotant in virulence
how do opaque morphologies of candida grow?
budding
elongated cells
involved in candida mating (must grow as opaque)
epigenetic switch
requires chromosome to be lost to get heterozygosity at a locus so they can mate
how do chlamydospore morphologies of candida grow?
response to environment
branch off hyphal filament
suspensor cell at the end of it you get chlamydospore
what is hyphal growth promoted by?
temperature greater than 35 degrees C serum neutral pH high CO2/low O2 N or C starvation matrix embedded growth
what is the most interesting morhphology switch and why?
yeast to hyphal
controlled by environment (serum and temperature)
what factors are yeast growth promoted by?
acidic pH
temperature less than 35C
ammonium ion
what morphology does candida grow in and when would this change?
grow as yeast when not starved
starvation: hyphal growth to move itself around to find food
why does it make sense for candida to use hyphal morphology when starving?
isnt equal when it separates (apical cell gets most of cytoplasm, subapical extensively vacuolated)
what does quorum sensing involve?
secrete QS molecule sense it respond to density high cell densities: promote yeast growth low cell density: promote hyphal growth
what is the classic view of the role of hyphal morphogenesis in virulence?
hyphae: tissue penetration
yeast: vascular dissemination
what makes morphology important in virulence?
a certain morphology isnt superior to another
its the fact that candida can switch between morphologies that makes it virulent
how do hyphae know when to burst through?
topism: hyphae can sense topography around them
sense weak point and push through the weak point
what are the direct roles of hyphal morphogenesis in virulence?
invasion and tissue damage
thigmotropism
escape from phagocytes
what are the indirect roles of hyphal morphogenesis in virulence?
genes co-regulated with morphogenesis
what are the key components for the adhesion of candida and where are they found?
mannoproteins
outer layer of cell wall
what are the 3 layers of the cell wall/membrane of candida albicans?
outer layer
inner wall
cell membrane
what component is found in the inner wall of candida albicans that acts as a linking molecule?
beta-1,6-glucan
what component is found just above the cell membrane of candida albicans that forms the skeleton?
beta-1,3-glucan aka chitin
what is the ALS family?
agglutinin-like sequence family of cell wall proteins
how many ALS members are there in candida albicans?
8: ALS 1-7, 9
describe the structure of ALS family members
common structure signal peptide conserved domain tandem repeats anchor
which ALS family member is especially important in adhesion?
ALS3
what makes the ALS family adaptable?
if one is removed or isnt functioning anymore there are 7 others to take its place
this stresses the importance of ALS otherwise it wouldnt have made all these other copies
name the 2 main routes of invasion and penetration of candida albicans
induced endocytosis
active penetration
how does candida albicans invade and penetrate the host via induced endocytosis?
adhere to cell and get cell to engulf it
taken up into host
passive process; fungus expresses invasin that interacts with the host cell surface
interaction triggers fungal engulfment through endocytosis (zipper-like mechanism)
how does candida albicans invade and penetrate the host via active penetration?
hyphae can push itself down and penetrate the host
directly into cells or between epithelial and endothelial cells
although active penetration of invasion of candida albicans isnt well understood, what might it potentially involve?
physical force from apical growth
turgor pressure
secretes enzymes that breaks down proteins between cells holding them together
use figmatopism to find way through
what type of biofilm surfaces does candida albicans form?
biotic and abiotic surfaces
where are the structured assemblies encased in biolfim formation of candida albicans?
extracellular matrix
what do the biolfims formed by candida albicans display resistance to?
antifungals
host defences
what impacts the formation of candida albican biofilms?
quorum sensing
polymicrobial interactions
what are the 4 stages of biofilm formation of candida albicans?
attachment/adhesion of yeast cells to substrate
initiation: formation of micro-colony
maturation: hyphal development and ECM production
dispersal: release of non-adherent yeast cells
what are the biofilms of candida albicans surrounded by?
sticky mass
what does candida albicans need to do in the host in order to survive?
it needs to acquire nutrients throughout infection cycle so it can grow
what does candida albicans eat when in the host?
unfussy, gorbs itself on whatever it can find
what metabolite suppression system do most microbes have that is more relaxed in candida albicans?
when there is glucose eat that first
candida use carbon sources simultaneously
what makes it able to survive in phagocytic cells?
metabolically adaptable
what does secretion of hydrolytic enzymes play a combined role in?
nutrient acquisition
combat host defences
adhesion
what is meant by the term aspartic (proteinases)?
nutrient acquisition but degrades the host tissue
why is the acquisition of iron important?
every living thing needs iron to live
explain the reductive route candida albicans uses to exploit iron within the host
rip iron off host proteins
reduces it
oxidises it and takes up the oxidised form for use
what is a siderophore?
a compound that a microbe releases that sequesters iron and then the microbes can take in this iron
explain the siderophore route candida albicans uses to exploit iron from the host
candida doesnt produce siderophores but will use other microbes’ siderophores
why might the siderophore route of iron exploitation only be important in superficial infections of candida albicans?
in systemic infections there may not be any other microbes in the sterile environment that candida can make use of their siderophores
explain the haemoglobin route that candida albicans uses for iron exploitation frrom the host
candida bonds and lyses RBCs haem released degraded/taken up into cell and degraded haem released uses this haem to rip iron from another haem group of the host
what stress would be associated with phagocytosis?
change in pH
low amino acid availability
lack of glucose
lack of iron
what stress would be associated with respiratory burst?
reactive oxygen and nitrogen species
what stress would be associated with cytokine burst?
inflammatory response
elevated temperatre
how does candida avoid stressing agents in the phagolysosome?
candida has to acquire nutrients in the phagolysosome
what is the pathogen response to stresses in the immune response?
immune evasion or pathogen response
explain candida’s response to thermal stress
as candida lives in us it is able to live up to 37 degrees celsius and a few degrees higher when we have a fever.
therefore candidas response to thermal stress is slow and gradual
explain candida’s response to oxidative stress
expresses a range of things to detoxify the environment
located on their surface rather than the cytoplasm like in other microbes
if there is damage, it will recognise it and repair it
what is most likely to get damaged from oxidative stress and why?
cell wall
this is the first part that the oxidate stress will hit
