Fungal infection & anti-fungal resistance

Question 1

Describe fungi

Answer 1

Eukaryotes
Kingdom = fungi
Separate from plants, animals and bacteria
Ubiquitous (found everywhere), mainly soil

Question 2

How are fungi classified?

Answer 2

• Yeasts
• Moulds
• Dimorphic

Question 3

Describe yeasts

Answer 3

Unicellular
Reproduce by budding
eg. Cryptococci, Candida, Pnuemocystis

Question 4

Describe moulds

Answer 4

Also called filamentous fungi
Grow as filaments (hyphae) which may intertwine to form a mass (mycelium)
eg. Aspergillus

Question 5

Describe dimorphic fungi

Answer 5

Grow as moulds in the environment and as yeasts in the body
eg. Histoplasma, Coccidioides

Question 6

What are the 3 types of fungal infection?

Answer 6

Superficial
Subcutaneous
Systemic

Question 7

Describe superficial infection

Answer 7

Affect mucous membranes (yeasts)
Keratinised tissue - skin, nail, hair (dermatophytes)

Question 8

Describe subcutaneous infection

Answer 8

Inoculation into deeper skin tissue (usually traumatic)
Local disease, tissue destruction and sinus formation

Question 9

Describe systemic infection

Answer 9

Haematogenous spread (via bloodstream) to any organ (esp eyes, liver, kidneys, spleen, lungs)
Often severe infections with high mortality
Opportunistic

Question 10

What is candida?

Answer 10

Yeast
Normal gut (vaginal) flora
Can cause thrush in mouth or vagina
At risk - ICU patients

Question 11

What is aspergillus?

Answer 11

Mould
Ubiquitous
Present in environment
Causes infection by inhalation
Causes localised infection, semi-invasive, invasive aspergillosis
At risk - transplant patients, ICU, haematology patients

Question 12

What is cryptococcus?

Answer 12

Yeast
Ubiquitous
Present in environment
Inhaled
Causes pneumonia, meningoencephalitis
At risk - HIV groups, patients with solid organ transplant e.g. lung

Question 13

What is pneumocystis?

Answer 13

Yeast
Coloniser of human lung
Causes pneumonia in HIV CD4<200

Question 14

What is the innate predisposing factors to fungi?

Answer 14

Neutropenia (low neutrophils)
Diabetes mellitus (phagocytosis)

Question 15

What is the acquired immune impairments to fungi?

Answer 15

HIV/AIDS
Chemotherapy for leukaemia/lymphoma
Organ transplant - BMT, SOT
Immunosuppressant drugs - steroids, tacrolimus, biologics (anti-TNF Abs)
Immaturity - premature birth, neonates

Question 16

What are some other risk factors of Candida?

Answer 16

Prosthetic material - e.g. intravascular device, catheters
Broad spec antibiotic use
Mucosal breach - burns, mucositis, abdominal surgery

Question 17

How are fungi emerging as a cause of human disease?

Answer 17

Increasing host susceptibility - cancer, autoimmune disease
Widespread use of broad spectrum antibiotics
Maybe global warming

Question 18

What are the classes of antifungals?

Answer 18

• Azoles
• Echinocandins
• Polyenes

Question 19

Name some examples of azoles

Answer 19

Fluconazole
Voriconazole
Posaconazole

Question 20

How are azoles administered?

Answer 20

Orally

Question 21

Name some examples of polyenes

Answer 21

Amphotericin B (intravenous)
Nystatin

Question 22

Name some examples of echinocandins

Answer 22

Caspofungin
Micafungin
Anidulafungin

Question 23

How are echinocandins adminstered?

Answer 23

Intravenously

Question 24

What is another antifungal not in any of the other 3 classes?

Answer 24

5-Flucytosine a pyrimidine analogue

Question 25

What do azoles target?

Answer 25

Inhibit synthesis of ergosterol in fungi via binding to sterol 14 alpha demethylase (CYP51)

Question 26

What do the polyenes target?

Answer 26

Bind to ergosterol in plasma membrane of fungi Forming aqueous pores that promote leakage of intracellular ions and disrupt active transport mechanisms

Question 27

What do echinocandins target?

Answer 27

Target fungal cell wall Inhibit synthesis of beta(1, 3)-D-glucan synthase

Question 28

What are the uses of antifungals?

Answer 28

Prophylaxis - azoles Empirical - AmB, echinocandins, azoles Pre-emptive treatment - AmB, echinocandins, azoles Targeted treatment -

Question 29

What are reasons for failure of antifungal therapy?

Answer 29

- 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

Question 30

How is antifungal susceptibility assessed?

Answer 30

Using measurement of minimum inhibitory concentration (MIC) using E-test or broth micro-dilution

Question 31

What are the types of antifungal resistance?

Answer 31

Primary (intrinsic) Secondary (acquired)

Question 32

What is primary resistance?

Answer 32

When all known isolates of a species are resistant to an antifungal e.g. Candida krusei to azoles

Question 33

What is secondary resistance?

Answer 33

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

Question 34

What are the most common mechanisms of antifungal resistance?

Answer 34

Target alteration Drug efflux

Question 35

What are drivers of antifungal resistance?

Answer 35

Agricultural use of antifungals and widespread use in humans

Question 36

What are azoles?

Answer 36

Ergosterol synthesis inhibitors - block cytochrome P450 enzyme 14alpha-demethylase (converts lanosterol to ergosterol

Question 37

What is the 14 alpha demethylase enzyme encoded by?

Answer 37

ERG11 gene in Candida/Cryptococcus Cyp51 in moulds

Question 38

Define fungistatic

Answer 38

Inhibit the growth of fungi without destroying them

Question 39

Define fungicidal

Answer 39

Destroys the fungi altogether

Question 40

How do azoles work against fungi?

Answer 40

Fungistatic in yeasts Fungicidal in moulds

Question 41

What are azoles used to treat?

Answer 41

Candidiasis Cryptococcosis Aspergillosis

Question 42

What are mechanisms of azole resistance?

Answer 42

Efflux pumps Target site mutation Increased expression of target Genomic plasticity

Question 43

How do efflux pumps act as a mechanism of azole resistance?

Answer 43

They can pump out antimicrobial agents from the cell which will prevent them from working

Question 44

What are efflux pumps encoded by?

Answer 44

ABD and MFS genes e.g. CDR1 in Candida, AFR1 in Cryptococcus

Question 45

How do target site mutations act as a mechanism of azole resistance?

Answer 45

Point mutations in ERG11 (yeasts), CYP51A (moulds) and FKS in Candida

Question 46

What is increased expression of target as a mechanism of azole resistance?

Answer 46

Gain of function mutation in transcription factor UPC2 for ERG11 in C. albicans

Question 47

How does genome plasticity work as a mechanism of azole resistance?

Answer 47

Duplication of entire chromosomes or regions in response to drug pressure-increased expression of target/pump e.g. C. albicans, C. neoformans

Question 48

How do polyenes function?

Answer 48

Bind to sterols (ergosterol preferentially) in cell membrane, forming channels Results in leakage and disruption Primary resistance

Question 49

What is the mechanism of polyenes?

Answer 49

Reduction in ergosterol content ERG mutations e.g. ERG3 loss of function mutation

Question 50

How do echinocandins work?

Answer 50

Bind to and inhibits beta(1, 3) glucan synthase, causing cell lysis Enzyme encoded by FKS gene

Question 51

How do echinocandins work?

Answer 51

Active against Candida - fungicidal Aspergillus - fungistatic

Question 52

What is the mechanism of echinocandins?

Answer 52

Target site modification - FKS1 and FKS2 (encode 1,3 B-D glucan synthase)