Fungal infection & anti-fungal resistance Flashcards
Describe fungi
Eukaryotes
Kingdom = fungi
Separate from plants, animals and bacteria
Ubiquitous (found everywhere), mainly soil
How are fungi classified?
- Yeasts
- Moulds
- Dimorphic
Describe yeasts
Unicellular
Reproduce by budding
eg. Cryptococci, Candida, Pnuemocystis
Describe moulds
Also called filamentous fungi
Grow as filaments (hyphae) which may intertwine to form a mass (mycelium)
eg. Aspergillus
Describe dimorphic fungi
Grow as moulds in the environment and as yeasts in the body
eg. Histoplasma, Coccidioides
What are the 3 types of fungal infection?
Superficial
Subcutaneous
Systemic
Describe superficial infection
Affect mucous membranes (yeasts)
Keratinised tissue - skin, nail, hair (dermatophytes)
Describe subcutaneous infection
Inoculation into deeper skin tissue (usually traumatic)
Local disease, tissue destruction and sinus formation
Describe systemic infection
Haematogenous spread (via bloodstream) to any organ (esp eyes, liver, kidneys, spleen, lungs)
Often severe infections with high mortality
Opportunistic
What is candida?
Yeast
Normal gut (vaginal) flora
Can cause thrush in mouth or vagina
At risk - ICU patients
What is aspergillus?
Mould
Ubiquitous
Present in environment
Causes infection by inhalation
Causes localised infection, semi-invasive, invasive aspergillosis
At risk - transplant patients, ICU, haematology patients
What is cryptococcus?
Yeast
Ubiquitous
Present in environment
Inhaled
Causes pneumonia, meningoencephalitis
At risk - HIV groups, patients with solid organ transplant e.g. lung
What is pneumocystis?
Yeast
Coloniser of human lung
Causes pneumonia in HIV CD4<200
What is the innate predisposing factors to fungi?
Neutropenia (low neutrophils)
Diabetes mellitus (phagocytosis)
What is the acquired immune impairments to fungi?
HIV/AIDS
Chemotherapy for leukaemia/lymphoma
Organ transplant - BMT, SOT
Immunosuppressant drugs - steroids, tacrolimus, biologics (anti-TNF Abs)
Immaturity - premature birth, neonates
What are some other risk factors of Candida?
Prosthetic material - e.g. intravascular device, catheters
Broad spec antibiotic use
Mucosal breach - burns, mucositis, abdominal surgery
How are fungi emerging as a cause of human disease?
Increasing host susceptibility - cancer, autoimmune disease
Widespread use of broad spectrum antibiotics
Maybe global warming
What are the classes of antifungals?
- Azoles
- Echinocandins
- Polyenes
Name some examples of azoles
Fluconazole
Voriconazole
Posaconazole
How are azoles administered?
Orally
Name some examples of polyenes
Amphotericin B (intravenous)
Nystatin
Name some examples of echinocandins
Caspofungin
Micafungin
Anidulafungin
How are echinocandins adminstered?
Intravenously
What is another antifungal not in any of the other 3 classes?
5-Flucytosine a pyrimidine analogue
What do azoles target?
Inhibit synthesis of ergosterol in fungi via binding to sterol 14 alpha demethylase (CYP51)
What do the polyenes target?
Bind to ergosterol in plasma membrane of fungi
Forming aqueous pores that promote leakage of intracellular ions and disrupt active transport mechanisms
What do echinocandins target?
Target fungal cell wall
Inhibit synthesis of beta(1, 3)-D-glucan synthase
What are the uses of antifungals?
Prophylaxis - azoles
Empirical - AmB, echinocandins, azoles
Pre-emptive treatment - AmB, echinocandins, azoles
Targeted treatment -
What are reasons for failure of antifungal therapy?
- Host immune system unable to clear infection (immunosuppression)
- Poor patient adherence to treatment
- Source control - failure to remove a device that infection is on eg. biofilm on catheters, lines, artificial heart valves
- Inadequate doses or penetration of drugs to site of infection
- Antifungal resistance - intrinsic or acquired
How is antifungal susceptibility assessed?
Using measurement of minimum inhibitory concentration (MIC) using E-test or broth micro-dilution
What are the types of antifungal resistance?
Primary (intrinsic)
Secondary (acquired)
What is primary resistance?
When all known isolates of a species are resistant to an antifungal
e.g. Candida krusei to azoles
What is secondary resistance?
Develops when susceptible strains are exposed to antifungal agents
Dependent on altered gene expression
e.g. Candida glabrata to echinocandins or Aspergillus fumigatus to azoles
What are the most common mechanisms of antifungal resistance?
Target alteration
Drug efflux
What are drivers of antifungal resistance?
Agricultural use of antifungals and widespread use in humans
What are azoles?
Ergosterol synthesis inhibitors - block cytochrome P450 enzyme 14alpha-demethylase (converts lanosterol to ergosterol
What is the 14 alpha demethylase enzyme encoded by?
ERG11 gene in Candida/Cryptococcus
Cyp51 in moulds
Define fungistatic
Inhibit the growth of fungi without destroying them
Define fungicidal
Destroys the fungi altogether
How do azoles work against fungi?
Fungistatic in yeasts
Fungicidal in moulds
What are azoles used to treat?
Candidiasis
Cryptococcosis
Aspergillosis
What are mechanisms of azole resistance?
Efflux pumps
Target site mutation
Increased expression of target
Genomic plasticity
How do efflux pumps act as a mechanism of azole resistance?
They can pump out antimicrobial agents from the cell which will prevent them from working
What are efflux pumps encoded by?
ABD and MFS genes
e.g. CDR1 in Candida, AFR1 in Cryptococcus
How do target site mutations act as a mechanism of azole resistance?
Point mutations in ERG11 (yeasts), CYP51A (moulds) and FKS in Candida
What is increased expression of target as a mechanism of azole resistance?
Gain of function mutation in transcription factor UPC2 for ERG11 in C. albicans
How does genome plasticity work as a mechanism of azole resistance?
Duplication of entire chromosomes or regions in response to drug pressure-increased expression of target/pump
e.g. C. albicans, C. neoformans
How do polyenes function?
Bind to sterols (ergosterol preferentially) in cell membrane, forming channels
Results in leakage and disruption
Primary resistance
What is the mechanism of polyenes?
Reduction in ergosterol content
ERG mutations e.g. ERG3 loss of function mutation
How do echinocandins work?
Bind to and inhibits beta(1, 3) glucan synthase, causing cell lysis
Enzyme encoded by FKS gene
How do echinocandins work?
Active against Candida - fungicidal
Aspergillus - fungistatic
What is the mechanism of echinocandins?
Target site modification - FKS1 and FKS2 (encode 1,3 B-D glucan synthase)
