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Medical Microbiology and Virology > 4 The fungi > Flashcards

4 The fungi Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

Mycology is study of fungi, and infections known as mycoses. 70 000 species, but only 300 pathogenic to animals/ humans

Eukaryotes.
Gain nutrients by releasing enzymes and digest food externally.

What structure of fungal cell membrane and wall

A

Mannoproteins
Beta-Glucans
Chitin - polysaccharides
Cell membrane - phospholipid bilayer which uses ergosterol, and has beta glucan synthase

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2
Q

What are two major ways of classifying mycoses

Fungal infection become more prominent given rise in immunocompromised patients, transplants, invasive procedures

A

By growth form -

  • Filamentous (moulds) - multinucleate thread-like filaments hyphae which grow by branching. A mass of hyphae is termed a mycellium. Grow extracellularly
  • Yeasts - unicellular, round or oval and reproduce by budding. Can survive within neutrophils/ macrophages

( some can show both growth forms during cycle called dimorphic fungi)

Or by type of infection -

  • superficial
  • deep mycoses

Other forms such as mushrooms can occur

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3
Q

How do filamentous forms replicate and spread

A

Form hyphae to spread

Asexual reproduction - form sporangia which are sacs containing spores, at end of hyphae.

These rupture and disperse spores to continue life cycle. Spores can be inhaled and cause disease

Spores can also independently divide in certain conditions

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4
Q

How do yeast like forms reproduce and spread

A

Single cell reproduces by budding. Bud remains attached, and further budding leading to formation of chains known as pseudohyphae

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5
Q

What are three main types of mycoses

A

Superficial - grows on body surfaces e.g skin, hair, nails, mouth, vagina

Subcutaneous mycoses - nails and deeper layers skin

Systemic/ deep mycoses - can be opportunistic or can infect with normal immunity.

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6
Q

How are three types of mycoses spread

A

Superficial - person to person or animal- person contact

Subcutaneous - following skin penetration e.g mycetoma

Systemic - usually via respiratory tract or IV lines

Occasionally free-living fungi can cause disease. Occurs indirectly when fungi present in food - e.g aflatoxin carcinogen produced by aspergillus. Or when inhaled spores can cause immune response hypersensitivity pneumonitis (allergic bronchopulmonary aspergillosis)

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7
Q

Candida can be part of normal flora, but only causes problems if immunocompromised or IV lines or diabetes

What are causes of -

  • superficial fungal disease (hair, dead skin layer), and which diseases do they cause
A
  • superficial fungal disease
    Pityriasis versicolor
    Tinea nigra
    Piedra

Trichosporon
Malassezia
Exophilia

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8
Q

What are causes of

  • cutaneous fungal disease ( epidermis, hair, nails), and what diseases do they cause
A
  • cutaneous fungal disease
    Tinea (ringworm

Trichophyton
Microsporum
Epidermophyton

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9
Q

What are causes of -

Systemic fungal disease (deep mycoses) and organism name

Immunocompetent host

A

Blastomycosis - blastomyces
Coccidiodomycosis - coccidioides
Histoplasmosis - histoplasma
Paracoccioidomycosis - paracoccidioides

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10
Q

What are causes of -

Systemic fungal disease (deep mycoses)
Yeasts/ moulds

Immunocompromised host

A

Yeasts -
Candida
Cryptococcosis

Moulds (filamentous) -
Aspergillosis 
Pneumocystis 
Mucormycosis
Dimorphic fungi - Blastomycosis/ Coccidioides/ Histoplasma/ Penicillium
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11
Q

What is structure of fungal plasma membrane

A

Below cell wall is cell membrane called plasmalemma. Unlike humans where dominant sterol is cholesterol, fungal membrane rich in ergosterol. Compounds that selectively bind to ergosterol can be used as anti-fungal agents

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12
Q

What is mechanism of action, and example of antifungal

Echinocandins

Polyenes

A

Echinocandins - inhibit beta-(1,3)-D-glucan synthase, preventing glucan synthesis in fungal cell wall. Anidulafungin, capsofungin, micafungin

Polyenes - bind to ergosterol and disrupt cell membrane. Nystatin, amphotericin B

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13
Q

What is mechanism of action, and example of antifungal

Allyamines

Pyrimidines

A

-Allyamines - inhibit ergosterol synthesis. Terbinafine

Pyrimidines - Flucytosine is deaminated 5-fluorouracil which inhibits nucelic acid synthesis. Resistance emerges quickly when given as single agent

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14
Q

What is mechanism of action, and example of antifungal

Heterocyclic benzofuran

Topical antifungals

A

Heterocyclic benzofuran - interferes with hyphae mitosis

Topical - ciclopirox - inhibit uptake of key nutrients, tolnaftate - distorts hyphae and prevent growht

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15
Q

What is mechanism of action of these antifungals:

Griseofulvin

A

inhibit nucleic acid synthesis

antimitotic activity inhibiting microtubule assembly

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16
Q

What is mechanism of action, and example of antifungal

Azoles

A

Azoles - inhibit C14-demethylase, important for ergosterol synthesis. Miconazole, ketoconazole, itraconazole

17
Q

Some fungi can exist in both yeast and mould forms, depending on environemntal conditions. What are examples of this?

A

Coccidioides

Histoplasma

18
Q

Fungi can utilise amino acids absorbed via cell wlal, but larger proteins must first be broken down by extracellular enzymes.

What are these methods of fungal energy production?

Biotrophs

Saprotrophs

Chemoheterotrophic

Photoheterotrophic

A

Biotrophs - nutrients from living host

Saprotrophs - nutrients from dead plants/ animals

Chemoheterotrophic - organic carbon in environment

Photoheterotrophic - use light for energy, cannot use carbon dioxide as sole carbon source

19
Q

Fungi life cycle can either be asexual or sexual.

Describe asexual life cycle

A

Mitosis -

  • Mycelium undergoes mitosis to produce spores
  • Spores germinate
  • Produce further mycellium

Fragmentation -
- ends of hyphae detach by fission/ budding or break down, to produce fragments (arthrospores) that can develop into new individuals.

Offspring are genetically similar to parents

20
Q

Fungi life cycle can either be asexual or sexual.

Describe sexual life cycle

A

Plasmogamy - haploid cells form two different mycelia fuse to form heterokaryotic cell with two or more nuclie

Heterkaryotic stage

Karyogamy - nuclei fuse to form diploid zygote

Zygote stage

Meiosis - haploid spores formed

Spores germinate - new mycellium formed

21
Q

what is structure of fungal spore?

A

Outer exosporium cell wall - resistant to heat/ cold/ dessication/ pH changes

Inner core contains glycogens/ lipids for nourishment. Although reduced metabolism when dormant

Spores capable of germinating to produce new hyphae when conditions improve

Spores provide challenge to hospital infection prevention/ control as require specific methods of sterilisation to ensure they are effectively killed

22
Q

HIV patient present confusion/ photophobia. Does not takes ART. CD4 10. India ink positive.

Which fungal kingdom does likely pathogen belong to?

Asomycota
Basidiomycota
Chytridiomycota
Glomeromycota
Neocallimastigomycota
A

Basidiomycota - cryptococcus belongs to this

23
Q

Mouth swab shows oral candida

Why can it be detected so quickly?

A

Produces germ tube within 2-4 hours

Germ tube is an early hyphae outgrowth

24
Q

HIV patient with two week history of fever, lymphadenopathy, hepatosplenomegaly. Born in Thailand, moved to UK recently

CD4 80
Blood culture - yeast

What is most likely organism?

Blastomyces dermatitidis
Candida albicans
Cryptococcus gattii
Histoplasma penicillum
Talaromyces marneffei (formerly penicillosis)
A

Talaromyces marneffei - Talaromycosis

CD4 low, so risk of yeast infection.

Histoplasma/ blastomycoses restricted to North America. Cryptococcus more likely to cause meningitis. Candida would rarely cause hepatosplenomegaly

Talarymyces seen in SEA

25
Q

What is treatment of talaromycosis? (penicilliosis)

A

Amphotericin B IV 2 weeks then

itraconazole for 10 weeks

26
Q

What are dimorphic fungi?

What are examples?

A

These are fungal infections of the body caused by fungal pathogens which can overcome the physiological and cellular defences of the normal human host by changing their morphological form. Can exist as mould and yeast

They are geographically restricted and the primary site of infection is usually pulmonary, following the inhalation of conidia

Blastomycosis
Coccidiomycosis
Paracoccidiomycosis
Histoplasmosis
Talaromycosis
Sporotrichosis
27
Q

What is geographical spread of these dimorphic fungi?

Blastomycosis
Coccidiomycosis
Paracoccidiomycosis
Histoplasmosis
Talaromycosis
A

Blastomycosis - NA

Coccidiomycosis - NA

Paracoccidiomycosis - SA

Histoplasmosis - NA/ SA/ Africa

Talaromycosis - SEA

28
Q

Which anti-fungals act at these sites?

Act on nucleus/ protein synthesis

Act on cell wall

Act on cell membrane

A

Act on nucleus/ protein synthesis -
griseofulvin
flucytosine

Act on cell wall -
echinocandins

Act on cell membrane -
azoles
polyenes
allyamines

Medical Microbiology and Virology (60 decks)

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