mycology and antifungals

Question 1

outline the main characteristics of fungi and how they are motile.

Answer 1

A bit of biology:
- Eukaryotic
- Chitinous cell wall
- Heterotrophic
- “Move” by means of growth or through the generation of spores (conidia), which are carried through air or water

Question 2

outline the difference between yeasts and moulds and outline the term for the fungi able to switch between the 2.

Answer 2

• Yeasts are small single celled organisms that divide by budding
• Account for <1% of fungal species but include several highly medically relevant ones
• Moulds form multicellular hyphae and spores
• Some fungi exist as both yeasts and moulds switching between the two when conditions suit – dimorphic fungi

Question 3

there are 5 million types of fungi with only a few hundred causing disease, why is ithis?

Answer 3

• Inability to grow at 37 degrees
• Innate and adaptive immune response

Question 4

outline why options for selective toxicity of fungi are difficult to use as a treatment.

Answer 4

• fungi are eukaryotic cells alike to human cells
• they share very similar cell structures and metabolic pathways meaning its difficukt to target just the fungi cell without also effecting the human cells

Question 4

Mucosal candidiasis (candida) : examples, risk factors, treatment, resistance

Answer 4

infection caused by the candida species
most likely candida albicans
part of normal flora in the body
but when it travels to diff parts of body where its not supposed to be - can cause issues
yeast

reasons why this normal flora can become imbalanced and cause issues:
- poor oral hygiene
- diabetes
- immunocomprimised
- antibiotic / steroidal use

• fungal toenail
• oral thrush

transmission:
- human - human - rare
- childbrith - thrush
- environmental - hospital exposure
- sex - thrush

risk factors:
- immunosuppression
- diabetes antibacterial therapy
- mucosal disruption

treatment:
- topical or oral azoles

resistant:
- Acquired resistance in normally susceptible species
- Selection for intrinsically resistant species

tests:
- lots of different tests required because lots of different species of candida, in many different places in the body
- direct microscopy test with KOH is used to see if it is in fact a fungus or not and to further differenciate if it is a mould or yeast as we can see hyphae or budding
- culture test - identifies species and candida - can assess resistance and treatment
- germ tube - used to differenciate Candida albicans from other species

Question 5

dermatophyte (tinea) infections transmission, infections species, treatment.

Answer 5

transmission:
- Human-human or animal-human transmission

dermatophyte infection species:
- Trichophyton - Causes a wide range of infections, including tinea pedis (athlete’s foot), tinea unguium (nail infections), and tinea capitis (scalp infections).
- Microsporum - Frequently causes tinea capitis (scalp) and tinea corporis (body infections).
- Epidermophyton floccosum - Primarily associated with tinea cruris (jock itch - groin) and tinea pedis (athletes foot).

These fungi infect keratinized tissues like the skin, hair, and nails because they produce keratinase, an enzyme that breaks down keratin.

treatment:
- topical or oral azoles
- terbinafine

tests:
- clinical examination to see ringworm looking things - red, windy appearance
- direct microscopy test with KOH - can see spores, haphae and septate hyphae which is very specific to tinea that can be visualized under the microscope.
- fungal culture - identify the species

Question 6

name the dermatophyte (tinea) infections and what they are as well as the fungal species causing it.

Answer 6

Tinea capitis: Scalp infection. - trichophyton and Microsporum

Tinea corporis: Infection of the body (e.g., trunk, arms, legs). - Microsporum

Tinea cruris: Infection of the groin (jock itch). - Epidermophyton floccosum

Tinea unguium: Nail infection (onychomycosis). - trichophyton

Tinea pedis: Infection of the feet (athlete’s foot). - trichophyton

Question 7

name the 2 superficial / cutaneous fungi.

Answer 7

• dermatophytes - tunica - cutneous
• mucosal candidiasis - normal flora but can cause issues - superficial

Question 8

What is the difference between dimorphic fungi and superficial fungi?

Answer 8

Dimorphic Fungi:
- Exist in two forms: mold (in the environment) and yeast (in the host).
- Cause systemic or deep tissue infections. (severe)
- not localised or superficial - deep in body tissues
- Transmission: Inhalation of spores.
- Treatment: Systemic antifungals (e.g., Amphotericin B, itraconazole).

Superficial Fungi:
- Cause localized infections of the skin, nails, or hair. - not severe and more of a nuisance
- very localised to superficiel area eg skin nails hair - not deep into body
- generally mild irritation to cosmic issues and not life-threatening
- transmission: direct contact or formites
- Treatment: Topical or oral antifungals (e.g., clotrimazole, terbinafine).

Question 9

give some examples of dimorphic fungi.

Answer 9

Histoplasma capsulatum

Blastomyces dermatitidis

Coccidioides immitis and posadasii

Paracoccidioides brasiliensis

Sporothrix schenkii

Talaromyces marneffei

Question 10

what are coccidioides?

Answer 10

Coccidioides is a genus of dimorphic fungi that causes coccidioidomycosis, also known as Valley Fever.
Most commonly causes pneumonia but can spread.

Two main species:
Coccidioides immitis
Coccidioides posadasii

Disease:
Primarily affects the lungs (pneumonia-like symptoms) but can spread to other organs (disseminated coccidioidomycosis).

Symptoms: Fever, cough, chest pain, fatigue, and sometimes rash.

geographical location:
- south west USA
- warm, arid conditions

severity:
- asymptomatic / subclinical
- community aquired pnewumonia 1-3 weeks postexposure
- severe disease - resp failure and septic shock - cell mediated immune defect

tests:
- clinical examination - do they have the symptoms described above and history of travel to SW USA
- direct microscopy - Spherules with endospores, characteristic of Coccidioides.
- serology - detects antibodies against coccidiodes - The IgM response suggests recent infection, while the IgG response may indicate past exposure or chronic infection.

Question 11

what are the late manifestations of coccidioides?

Answer 11

• cavitatory lung disease
• orthopaedic - asymmetrical chronic arthiritis with effusion or vertebral osteomyelitits
• cutaneous ulcers and absecesses
• cervical lymphadenopathy
• intracranial - chronic meningitis

Question 12

what is the cause of most invasive candadiasis?

Answer 12

mostly due to infection of prosthetic devices or intra-abdominal disease
- yeast

Question 13

Differential diagnosis of sub-acute/chronic meningitis

Answer 13

Infective:
- Tuberculosis
- Cryptococcus
- Dimorphic fungi – Histoplasma, Coccidioides, Blastomyces
- Lyme
- Brucella
- Syphilis

Non-infective:
- Sarcoidosis
- Behçets’s
- SLE
- Malignant
- Drug induced

Question 14

What is Cryptococcus and what disease does it cause?

Answer 14

Cryptococcus is a genus of encapsulated yeast that causes cryptococcosis, a fungal infection.

• in association with rotting wood and bird guano
• exposure is really common - post people are seropositive by 10 years old
• yeast

Common Species:
Cryptococcus neoformans (affects primarily immunocompromised individuals).

Cryptococcus gattii (affects both immunocompetent and immunocompromised individuals, often in tropical regions).

Disease:
Primarily causes meningitis (inflammation of the brain and spinal cord) but can affect lungs and other organs.

Symptoms: Fever, headache, neck stiffness, confusion, nausea, and vomiting (if CNS is involved).

tests:
- direct microscopy - identify it
- culture - identify species
- Cryptococcal Antigen (CrAg) Test - IgG and IgM

treatment:
- liposomal amphotericin B and fluctosine with fluconazole

Question 15

What is Aspergillosis and what causes it?

Answer 15

Aspergillosis is an infection caused by molds of the genus Aspergillus.
- inhalation of spores
- mould

Common species:
Aspergillus fumigatus (most common cause of disease).
Aspergillus flavus, Aspergillus niger, Aspergillus terreus.

Disease:
Can range from mild, localized infections (e.g., in the sinuses) to severe, systemic infections (e.g., in the lungs or brain).
Common in immunocompromised individuals, such as those with leukemia, HIV/AIDS, organ transplant recipients, or those on immunosuppressive drugs.

Invasive aspergillosis is normally associated with profound immunocompromise but is increasingly recognised in patients with severe viral infection

treatment:
viroconazole

Question 16

Mucoraceous moulds (zygomyctes)

Answer 16

• rare but cause devastatingly rapidly progressive infections that cross tissue planes.
• need aggressive antifungal therapy and surgery for optimal outcomes
• inhaling spores
• mould

Question 16

Fungal Classification with Salient Examples

Answer 16

• yeasts - candida spp
• molds - aspergillus spp
• dimorphic fungi - both
• dermatophytes

Question 17

pneumocystis jirovecii

Answer 17

• atypical - not a mould or yeast
• forms biofilm around alveolar epitglium
• pneumonia with severe hypoxia in immunocomprimised

Question 18

Describe fungal structure and how it is different from human and bacterial cells.

Answer 18

cell wall
fungi - chitin, mannans and B-glucans (more rigid)
humans - (no cell wall)
bacteria - peptidoglycan

nucleus
fungi and humans - nucleus and chromosomes
bacteria - no nucleus - free floating

fungi and human - cytoplasm and organelles eg golgi, ribosomes and atp
bacteria - no membrane bound organelles

Fungi have unique components like ergosterol in their plasma membranes, which makes them an ideal target for antifungal treatments.

The hyphal structures and spores of filamentous fungi are quite distinct from both bacterial and human cell functions.

Question 19

describe the Direct Microscopy (KOH Wet Mount) test and outline some of its limitations.

Answer 19

used for:
Mucosal candidiasis, dermatophytes (e.g., Tinea), cryptococcosis, aspergillosis, etc.

how it works:
A sample from the infected area (e.g., skin scraping, sputum, vaginal discharge) is treated with potassium hydroxide (KOH) to dissolve host tissue and reveal fungal structures under the microscope.

limitations:
- Non-specific: Fungal elements may look similar across different fungal species, making it difficult to identify the exact pathogen.
- Sensitivity issues: Fungal cells may not always be present in sufficient numbers, leading to false negatives.
- Lack of species identification: While it confirms fungal presence, it doesn’t provide information about the specific species involved.

Question 20

describe the fungal culture as a test and outline some of its limitations.

Answer 20

Used For:
Candida, aspergillosis, dermatophytes, cryptococcosis, Coccidioides.
Culture allows isolation and identification of the pathogen from clinical samples.
How It Works:
A sample from the infected site is cultured on a selective fungal medium (e.g., Sabouraud agar or chromagar). After incubation, the colony morphology, growth pattern, and microscopic examination help identify the fungus.
Limitations:
Time-consuming: Cultures can take several days to weeks for results, particularly for slower-growing fungi like Cryptococcus.
Non-quantitative: It may not differentiate between colonization and true infection.
Contamination risk: Other microorganisms (e.g., bacteria or normal flora) may contaminate the culture, complicating the interpretation.

Question 21

describe the germ tube as a test and outline some of its limitations.

Answer 21

Used For:
Candida albicans (for identifying this species in cases of mucosal candidiasis).
How It Works:
A sample of Candida is incubated in serum at 37°C for 2-3 hours. If the sample is Candida albicans, it will form germ tubes, which are short, tube-like extensions of the yeast cell.
Limitations:
Limited to Candida albicans: The test only identifies Candida albicans and does not differentiate other Candida species or other fungal infections.
Not useful for invasive infections: This test is typically used for superficial infections, such as oral thrush or vaginal candidiasis, rather than systemic infections.

Question 22

describe serology (antigen / antibody) as a test and outline some of its limitations.

Answer 22

Used For:
Cryptococcus (cryptococcal antigen test), Aspergillus (galactomannan test), Histoplasma, Coccidioides (serological tests).
How It Works:
This method detects fungal antigens (e.g., Cryptococcus capsular antigen) or antibodies produced by the host in response to fungal infection, using techniques like ELISA or latex agglutination.
Limitations:
Cross-reactivity: Some antigen tests may show false positives due to cross-reactivity with other pathogens or conditions.
Only supportive: These tests are not diagnostic on their own and should be used alongside clinical symptoms and other tests.
False negatives: In some cases, especially with chronic or non-invasive infections, antigen/antibody levels may be low, leading to false negatives.

Question 23

describe the PCR as a test and outline some of its limitations.

Answer 23

Used For:
Invasive fungal infections (e.g., aspergillosis, cryptococcosis, Coccidioides).
How It Works:
PCR amplifies fungal DNA from a clinical sample, allowing for species-specific identification of the pathogen.
Limitations:
False positives: PCR can sometimes detect colonization or contamination, leading to false positives.
Limited by sample quality: The success of PCR depends on the quality of the sample (e.g., sputum, blood, tissue).
Requires expertise: PCR requires specialized laboratory equipment and trained personnel, making it less accessible in some settings.
Not widely available: PCR testing for fungi is not as routine as it is for bacterial infections and may not be available in all clinical settings.

Question 24

azoles

Answer 24

Mechanism of Action:
- Azoles inhibit lanosterol 14α-demethylase, an enzyme involved in the synthesis of ergosterol
- Depletion of ergosterol disrupts membrane integrity, leading to cell death.

uses:
- dimprophic
- candida - (e.g., mucosal candidiasis, candidemia), cryptococcal meningitis
- invasive asperillosis

stregnths:
- oral and IV

limitations:
- can cause hepatotoxicity
- inhibit CYP enzymes
- increasing resistance

Question 25

Echinocandins

Answer 25

Echinocandins inhibit 1,3-β-D-glucan synthase, an enzyme required for the synthesis of β-glucans in the fungal cell wall.
Disruption of the cell wall leads to fungal cell lysis and death.

used for:
- candida
- invasive aspergillosis

stregnth:
- low toxicity
- minimal drug interactions

limitations:
- limited activity against non-aspergillus moulds
- not for dimorphic
- only available as an IV

Question 26

Amphotericin B

Answer 26

Amphotericin B binds to ergosterol in the fungal cell membrane, forming pores that increase membrane permeability.
This leads to leakage of intracellular contents and cell death.

Clinical Uses:
Broad-spectrum antifungal, effective against:
- Invasive Aspergillus, Cryptococcus, Candida, Histoplasma, Blastomyces, Coccidioides.
Mucormycosis and other rare mold infections.
- Often used as an initial treatment for life-threatening fungal infections before switching to less toxic alternatives.

stregnths:
- broad spectrum of activity
- no resistance

limitations:
- toxicity
- lipid based formulas are less toxic but very costly
- IV