Flashcards in Fungi and Anti-Fungal drugs Deck (108):
How many kinds of fungi are there? What are 3 reasons why fungal infections are becoming more common?
Over a million kinds
1. people are living longer
2. more immunocompromised patients
3. Antibiotics remove normal flora and allow for fungal infection
What are 3 situations where fungal infections are secondary to another illness.
1. ventilator acquired Candida (yeast) is 4th leading cause of nosocomial bloodstream infections
2. Chemo and organ transplant hold a risk for Aspergillis and zygomycetes (mold) infections in solid organs and bone marrow
3. HIV/AIDS- P. jiroveci causes pneumonia
Do fungal infections tend to be chronic or acute?
Exogenous or endogenous?
They tend to be chronic, exogenous and non-communicable
What can give you a HUGE clue to what kind of fungal infection the patient has?
The environment the patient has been exposed to because fungi have characteristic ecological niches
How can you distinguish a fungi from a bacteria?
1. Fungi are eukaryotic with nuclei, mitochondria, etc
2. Chitin and Glucan cell wall (NOT peptidoglycan)
3. 80S ribosome
4. filamentous structure and spores
How are fungi distinguished from plants?
Fungi are saprobic meaning they get energy from dead or decaying material
Describe the cell envelope structure of fungi from cytoplasm outward.
Cell membrane consisting of:
2. B(1,3) D glucan synthase
Cell wall consisting of :
1. Chitin (N-acetylglucosamine)
2. B(1.3) D glucan and B(1.6) D glucan
Outermost layer is polypeptides
Some fungi contain a capsule while others do not.
What is the capsule composed of?
What is the purpose?
What is an example of an encapsulated fungi?
3. Cryptococcus neoformans
What part of the fungal cell wall has been targeted by anti-fungal agents?
What are the anti-fungal agents called?
Echinocandins target the B(1,3) D glucan of the cell wall and inhibit glycan synthase
What differentiates fungal cell membranes from mammalian cells?
How does this allow for antifungal agents to act? What agents act?
They are both lipid bilayers composed of phospholipids.
Mammalian cells have cholesterol while Fungal cells have ergosterol.
Amphotericin B and azoles can act on the ergosterol
What antifungal class interrupts synthesis of ergosterol?
What antifungal class binds ergosterol in the membrane and disrupt membrane function?
allylamines and azoles- stop synthesis
Polyenes - bind and disrupt membrane
What are the three main functions of fungal cell membranes?
1. protect cytoplasm
2. regulate intake and secretion of solutes
3. synthesize cell wall (b1,3d glucan synthase)
Describe the respiration, growth speed and metabolism of fungi.
Respiration = aerobic
Metabolism= heterotrophic via acquisition of nutrients and production of by-products like ethanol, antibiotics, etc
Growth speed = very slow. doubling time is hours
What are the three major growth patterns of fungi?
1. yeast- single cell reproduces via budding
2. mold- filamentous structures
3. dimorphic- alternate between yeast and mold
What are two examples of yeasts?
Candida and cryptococcus
How do molds grow?
What are examples of monomorphic molds?
They grow as filamentous structures
They ONLY grow as mold.
Ex. Hyaline molds, Aspergillus (opportunistic)
In dimorphic pathogens, what form are they in humans? the environment?
Yeast in mammalian host and mold in soil or the environment
What is the difference between a perfect fungi and an imperfect fungi?
What are 2 examples of perfect?
Perfect- have known sexual forms
ex. crytococcus neoformans, saccharomyces cervisiae (BEER)
Imperfect- no known sexual forms
How do yeast look microscopically?
How do they look macroscopically on an agar plate?
Microscopically- oval/round, reproduce via budding, if they don't separate = pseudohyphae
Macroscopically- pasty white colonies that separate and look like large bacteria
How do molds look microscopically?
How do they look macroscopically on agar?
Micro- multicellular, hyphae (strings) that have spores. Hyphae can be septate or aseptate
Macro- cottony, pigment observed on the reverse
What is a conidia?
Asexual reproductive element similar to a spore.
A single-celled condidium extends a germ tube by apical extension. Side branching results in hyphae filament network.
What is arthoconidia?
What fungi asexually reproduces this way?
It is a thallic conidia so it results from the conversion of an entire preexisting hyphal element
It then breaks loose and initiates another growth cycle. (looks like a necklace with square beads)
What is blastoconidia?
It is when portions of the hyphae enlarge and bud before separating from the hyphae.
What is is called when a budding portion does not fully separate from the hyphae but continues growing?
What is an example of fungi that grows this way?
When budding is incomplete it forms a pseudohyphae.
Candida albicans does this. It looks like leaves and a stem
When blastoconidia grow, they can have 2 outcomes. What are they?
1. continue to grow off the hyphae and form "sausage-shaped filaments" --> pseudohyphae
2. Develop into true hyphae and do not separate at maturity
Sometimes when fungi are going through asexual reproduction, they form conidia of 2 different sizes. What are they called?
What is an example of this fungi?
Microconidia and macroconidia
Macro and micro can produce blastically or thallically
Ringworm, athlete's foot
Sometimes fungi go through asexual replication and cytoplasmic cleavage within a structure. What is the structure called?
What type of bacteria does this?
Zygomycetes do this
What is diagnostic about the hyphae of a sporangiospore?
The hyphae of these fungi are aseptate
What are the four types of fungal asexual reproduction?
What is an example of a bacteria in each group?
1. Athroconidia -> coccidioides immitis
2. Blastoconidia--> Candida
3. Micro/Macroconidia-> ringworm/athlete's foot
4. Sporangium- zygomycetes
What is mycoses?
How are they classified?
What are the major classifications?
infections caused by medically relevant fungi.
They are classified based on anatomic site of infection/inflammation:
3. Deep Systemic mycoses
4. Deep opportunistic mycoses
Where specifically do superficial and cutaneous fungi colonize and grow? At what temperature is growth idea?
They colonize best at 25 degrees C so they can't go deeper into the body than the:
KERATIN LAYER of nails, hair, outer skin layer
What causes cutaneous fungal infections?
Dermatophytes (Trichophyton, Epidermophyton, Microsporum)
Malassezia furfur causes tinea versicolor
What are the two most common subcutaneous mycoses?
Where is their infection confined?
What do they cause?
How is the infection introduced?
1. sporotrichosis, mycetoma (madura foot)
2. confined to cutaneous and subcutaneous tissue and rarely become systemic
3. They cause deep, ulcerated skin masses on extremities
4. The infectious agent is soil saprophytes introduced via trauma to hands, feet, legs
What typically causes systemic mycoses?
primary fungi endemic to a geographical area
(primary infections are illness that occur in healthy hosts)
What are the five major examples of endemic mycoses?
What is the portal of entry for the systemic mycoses?
1. Histoplasma capsulatum - TX, SE
2. Coccidioides immitis- near mexico border
3. Blastomyces dermatitidis
4. Paracoccidioides braziliencsis
5. Penicillium marneffei
They all enter through the lung
Are most endemic pathogens yeast, mold or dimorphic?
Most endemic fungal infections are caused by dimorphic fungi
(yeast at 37, mold at 25)
What are the two forms of dimorphic fungi?
What temperatures do they grow best?
Mold grow in soil at 25 degrees
Yeast grow in tissue of infected host at 37 degrees.
(if you cultured yeast, you would need nutrient-rich media)
What are opportunistic mycoses?
What are three examples?
They are fungi that have low inherent virulence, but will be bad for immunocompromised patients
Ex. aspergillus, candida, cryptococcus
What are risk factors for opportunistic mycoses?
Host Gene Defects
What type of mycoses does cryptococcus neoformans cause?
How does it present?
It is an opportunistic mycoses
In humans it is an encapsulated yeast that causes:
2. meningitis in a person with AIDS CD4<100
What type of mycoses will Pneumocystis jiroveci cause?
What is the presentation?
It is opportunistic and will cause pneumonia in people with AIDS and a CD4<200
What two fungal pathogens are opportunistic and cause very serious pathologies in AIDS patients?
What does each cause?
1. cryptococcus neoformans- meningitis
2. p. jiroveci- pneuomonia
How does aspergillus infect human hosts?
What happens in an immunocompetent host?
What happens in an immunocompromised host?
Who is especially susceptible?
It is inhaled as a spore.
Nothing happens if the person is immunocompetent.
In immunocompromised it causes pneumonia and has extra-pulmonary spread.
bone marrow transplant, liver transplant, chemo patients, Chronic granulamatous disease
If a patient has CGD what activities should they avoid? What fungal pathogen are they especially susceptible to?
They should avoid soils so gardening etc
they are susceptible to aspergillus which can cause pneumonia
Most fungi are _________ and are _______ in nature. The two exceptions are Candida and dermatophytes which inhabit______________.
Most are soil saprophytes and are ubiquitous in nature.
Candida and dermatophytes are inhabitants of the mucousal membrane and skin
What two fungal species can occasionally be communicable?
Candida and dermatophytes.
The other fungi are non-communicable
What are the four major modes of acquisition for fungi?
1. Inhalation of airborne spores (Conidia)**most common
2. Penetration through breach in the skin (subcu)
3. Mucosal colonization/overgrowth (endogenous)
4. Contact (anthropophilic, zoophilic, geophilic)
What does the development of the mycoses depend on?
1. inoculum size
2. level of natural resistance to infection
Little is known about fungal virulence factors. However we do know that most fungi are thermotolerant.
The only real virulence factor discussed was __________________.
This allows fungi to adhere to host cells.
1. complement receptors and hydrophobins of Aspergillus
2. Surface glycoproteins of P. jiroveci that allow it colonize lower respiratory airways
What about the structure of P. jiroveci allows it to colonize in HIV patients to cause pneumonia?
They normally have glycoproteins that allow it to adhere and grow unactivated in alveolar macrophages.
When the patient has AIDS, the mannose receptor-mediated binding and phagocytosis of P. jiroveci is disrupted
What role do neutrophils play in the elimination of fungal pathogens? Which two specifically are eliminated?
What develops in patients with neutropenia?
Neutrophils are recruited in response to:
And they destroy the fungal hyphae
If neutrophils are not present, the patient can get invasive fungal disease. This puts patients on chemo at risk of candida and aspergillis infections
What physiological modifications do Cryptococcus and Histoplasma use during invasion?
1. increase metabolic rate
2. modify metabolic pathways
3. modified cell wall structure
What fungi normally grow in unactivated alveolar macrophages?
Pneumocystis and Histoplasma capsalatum
How does the immune system control the multiplication of intracellular yeast (like histoplasma) ?
1. IFNg activated macrophages kill the intracelluar yeast
If the intracellular pathogen is resistant to phagocytic killing (like histoplasma)
2. The intracellular fungi is controlled by granulamatous inflammation to "wall it off"
What cells are involved in the granulomatous inflammation that walls off the intracellular yeast?
What 3 infections will the patient potentially get if they lack inflammatory cells?
Phagocytes, giant cells, lymphocytes, principally CD8 T cell.
If they are deficient in CD8 T cell, they are more susceptible to :
What is the clinical manifestation of an intense granulomatous reaction?
Calcified fibrinous granulomas (especially in the lungs for endemic mycoses)
What are the clinical manifestations of the cytokine response to clearing the fungal pathogen?
3. Weight loss
What are the negative potential consequences of defective cell-mediated immunity when attempting to eradicate a fungal infection?
1. Fungi disseminate via blood (fungemia)
2. cross BBB
3. multiply in the brain
4. Cause gelatinous lesions or space occupying lesions in the brain
What fungi is associated with gelatinous lesions in the brain in immunocompromised patients?
What fungi is associated with a space-occupying lesion in the brain in immunocompromised patients (CMI deficient)?
What history and physical info does the diagnosis of a fungal infection depend on?
1. History- acuity, symptoms, exposure, travel, immunizations, lack of antibiotic response
2. Physical- lesions, lab abnormalities, X-ray
What determines the technique used to ID the fungi?
The site of infection
What lab test would be done for a cutaneous or superficial infection?
Skin/hair/nail scrapings and Direct microscopy
1. KOH stain dissolves host tissue and all fungal structures except the cell wall which allows them to be seen
2. Calcofluor white, GMS and PAS may also be used
3. Yeast like candida and cryptococcus can also be gram stained
What three staining techniques can be used on a hair/skin/nail lesion scraping to determine if it is a fungal infection?
1. KOH because it dissolves host tissue and most of the fungi except the cell wall
2. PAS, GMS and Calcofluor white
What tests would be done for subcutaneous fungal infections?
1 Biopsy the skin lesion/ lymph nodes
2. Giemsa stain the cell walls
2. GMS (silverstain) reveals the organisms when Giemsa didn't work
What is required for a definitive diagnosis of fungal infection?
Culture and ID
What agar is used to culture fungi? What does the medium have?
Sabouraud Dextrose Agar
The medium has:
Inhibitory agents: cycloheximide, penicillin, streptomycin
What is different about the culturing technique when you are suspicious of a dimorphic fungi?
Two sets of plates must be swabbed and they incubate at different temperatures (25 and 37)
How long must you wait to consider something "negative growth"
What factors do you look for when examining growth of cultures?
1. colony color
2. growth characteristics
3. morphology on the underside of the colony
What is the normal color and size of yeast on culture? What are the two most common yeast isolated?
They are large, moist and white/tan.
Candida and cryptococcus
What preparation is used to determine the genus and species of mould?
Lactophenol cotton blue which allows for the visualization of:
2. degree of septation
3. morphology of fruiting structures
What serological tests allow determination of IgG against fungi?
What type of fungal infection are IgG tests good at identifying?
1. Agar immunodiffusion
2. Complement Fixation ***
3. Enzyme Immunoassay (ELISA)
They identify endemic mycoses and aspergillus
What 3 properties of fungi allow direct antigen detection in blood and CSF?
1. Latex Agglutination which can show the presence of Cryptococcal capsular polysaccharide
2. Galactomannan (aspergillus cell wall)
3. B,1,3, D glucan
What dimorphic fungi can be identified by DNA probe? What are DNA probes used for?
DNA probes can allow for "culture confirmation".
DNA directly hybridizes to an extract of a fungal colony.
1. Histoplasma capsulatum
2. Coccidioides immitis
3. Blastomyces dermatititidis
How are skin tests for fungi performed?
Dermal Hypersensitivity (DTH response) like a TB test.
Extract of fungi as an antigen
Why is skin testing not really used for diagnosis anymore?
1. There is cross-reaction between endemic mycoses
2. It does not show acute infection, just past exposure
What are skin test still relevant for?
1. testing a patients immune status- anergy in Candida
2. population exposure index in epidemiology studies
What are the three categories of fungal drug?
What is the target of each?
Polyenes- ergosterol (bind to it)
Azoles- ergosterol (stop synthesis)
Echinocandins- inhibit fungal wall synthesis
What are the three classifications of chemotherapy against fungal infections?
1. oral and parenteral drugs for systemic infection
2. oral drugs for skin/ mucous membrane infection
3. Topical drugs for skin/mucous membrane infection
What are the two polyenes? What is their target?
Who uses these drugs?
Amphotericin B and flucytosine
They target and bind to ergosterol in the fungal cell membrane
These drugs are reserved for use by immunocompromised patients with systemic infections (AIDs, neutropenia)
What is the structure of amphotericin B and how does that determine its method of action?
It has two sides- one hydrophobic (polyene) and one hydrophilic.
The hydrophobic side binds to ergosterol and the polyhydroxyl side associates with each other to form an aqueous channel disrupting membrane permeability and causing cell death
How is amphotericin B administered? What is the distribution? What is the half life?
It is IV administered, distributes to "greater than body water" because it binds well to tissue and serum proteins.
It does not cross BBB.
Half life is 2-3 weeks
What are the adverse effects of amphotericin B?
What is believed to cause these side effects?
What is a way to reduce toxicity?
1. Immediate chills, fever, vomiting, headaches (lessened by corticosteroids) due to cytokine storm (TNFa and IL1)
2. Nephrotoxicity- high nitrogen levels (azotemia) due to accumulation in kidney. GFR decreases, prox tubular cells are damaged. DIALYSIS needed
These side effects are believed to be because of the detergent (deoxycholate micellar solution) that Amphotericin B is given with.
Reduce toxicity by giving saline dose prior to administration
What fungicide has the broadest spectrum of action?
By what mechanism are fungal cells resistant to amphotericin B?
They have lower concentrations of ergosterol in their membranes
What newer drugs are being formulated that have the same effect as amphotericin B but less toxicity?
Why are these drugs not the major ones for treatment?
Ones that do not require deoxycholate micellar solution (DMS) but instead use a drug-lipid combination.
This has the same bioavailability but is less nephrotoxic.
They are not widely used because they are expensive (up to 1000$ a day)_
Are azole drugs fungistatic or fungicidal?
What are the two broad categories of azole? Which is more potent and used systemically?
They are fungicidal.
Imidazoles are used topically and Triazoles are more potent and used for systemic infections
What is the major imidazole drug?
What are the two major triazoles?
What are the drugs used for systemic fungal infection? Which are typically used in immunocompetent patients? Which are used in immunocompromised patients?
Immunocompromised = amphoterin B and flucytosine
Immunocompetent= itraconazole, fluconazole
What is the mechanism of action of azole drugs?
They inhibit p450 enzyme lanosterol 14a demethylase which depletes ergosterol formation and alters
membrane permeability causing lysis. Two outcomes:
1. increased lanosterol which is toxic to fungi
2. methylated ergosterol destabilizes membrane
What is the pharmokinetic advantage of azoles over polyenes?
What is the one exception?
Azoles are hydrophobic and can be administered orally. They distribute well, but not to CSF and are hepatically excreted.
The exception is fluconazole which is administered IV and does cross the BBB and is renally excreted.
Polyenes are amphipathic, IV and distribute well but not to CSF. Renally exreted.
What is the most notable adverse effect of azoles?
Who would it NOT be adverse in?
It inhibits p450 enzymes (including the ones involved in steroid metabolism) so it lowers testosterone, estrogen ,cortisol.
It can also have drug-drug interactions with other hepatically metabolized drugs.
It can be used to reduce androgen in prostate cancer
What drug increases metabolism of azole drugs?
Rifampin because it increases metabolism by inducing p450.
Describe the uses of ketonazole.
They are imidazoles so used for topical infections specifically:
2. non-meningicoccocal coccidiomycosis
in NON-immunocompromised patients
What are the three most important properties of fluconazole? What does this allow it to treat?
1. IV administered and highly water soluble
2. low inhibition of p450 enzymes/renal excretion
3. High penetration in CNS
Treats: cryptococcal and coccidiodal meningitis, candida (mucousal and disseminated) and prophylaxis for bone marrow transplants
What anti-fungal drugs are able to cross BBB?
3. mininally echinocandins
What is the most potent triazole? What is it used to treat?
What are the drawbacks?
Itraconazole- oral drug for infections by dermatophytes, histoplasma, blastomyces, sporothix
POOR CSF and RENAL TOXICITY
What contributes to azole resistance?
Who is most resistant?
Production of demethylase in patients with AIDS and thrush will negate the effects of itraconazole blocking lanosterol 14a-demethylase
What fungal drug is a prodrug?
What is its action?
What is it used in conjunction with? Why?
Flucytosine is metabolized to 5FU by fungal cytosine deaminase (not in mammalian cells).
5FU makes 5-FdUMP which blocks thymidylate synthase (a converter of dUMP to dTMP).
This means the fungi can't make DNA because they lack thymidine
Why do flucytosine and amphtericin B have a synergistic relationship?
Amphotericin B disrupts the fungal cell membrane which allows access of flucytosine to be converted to 5FU->5FdUMP to inhibit thymidylate synthase.
What are the adverse effects of flucytosine?
It can have bone marrow toxicity and blood dyscrasias (anemia, leukopenia, thrombocytopenia) because it can be metabolized by microoraganisms and taken up into precursors of blood cells .
What is the newest, safest most effective drug on the market? What does it have particularly strong efficacy against?
Caspofungin- an echinocandin against Candida
What is the mechanism of action of echinocandins (caspofungin)?
They inhibit 1.3 B glucan synthase to inhibit the formation of the fungal cell wall.
Fungi are the only organisms that have this enzyme so the drug works VERY specifically
What are the major benefits of using an echinocandin?
1. low nephrotoxicity
2. Mild hepatotoxicity
3. few drug-drug interactions
What are the pharmacokinetics of echinocandins?
IV, wide but not to eyes/CSF, hepatically metabolized by p450