MICROBIOLOGY - Fungal and other Infections

Question 1

What is the mechanism of action of polyene groups (amphotericin and nystatin)

Answer 1

Binds to ergosterol and alter’s ionic transport and permeability of cell wall leading to cell lysis.

Question 2

What is the mechanism of action of imidazole?

Answer 2

Inhibition of ergosterol production by affecting the cytochrome p450 pathway

Question 3

Which imidazole is effective against aspergillus?

Answer 3

Itraconazole

Amphotericin (polyene group) is also effective.

Question 4

What is the main side effect of imidazole ?
What is the main side effect of polyene groups?

Answer 4

1. hepatotoxicity
2. nephrotoxicity and anaemia

Question 5

What is candida albicans cultured on?

Answer 5

Sabouraud’s glucose media in 24-48 hours.

Question 6

When is candida albicans commonly seen in ophthalmology (4)

Answer 6

1. Cataract surgery
2. Corneal transplant
3. Therapeutic contact lenses
4. Steroids/immunosuppresion

Question 7

What are the ocular manifestitations of candida albicans?

Answer 7

1. Candidal chorioretinitis
2. Candidal endophthalmitis (endogenous)

Question 8

What are the clincial findings in fungal infections? (2)

Answer 8

1. pseudophyphae in corneal stroma
2. feathery-edged infiltrate with satellite lesions

Question 9

What is the pathogenesis of aspergillus/fusarium keratitis?

Answer 9

1. Hyphae which contain proteases penetrate through stroma into the AC (hypopyon)
2. C-type lectins bind to cell wall components (beta-glucan and alpha-mannose) leading to chemotactic/cytokine release

Question 10

What are the different types of acanthomoeba?

Answer 10

Protozoa
1. A castellani
2. A. polyphaga

Question 11

What are the clinical features of acanthomoeba keratitis

Answer 11

1. Ring infiltrate
2. Perineural infiltrate

Question 12

What is the pathogenesis of acanthomoeba keratitis? Which is the most potent form?

Answer 12

1. Atachment of amoeba by increased expression of mannose-binding protein
2. Expression of proteases which destroy the epithelium and upregulate MMPs promoting invasion and ulceration
3. Breaches bowman’s layer and degrades stroma and causes radial neuritis

T4 is the most potent form

Question 13

What is the main mechanism of transmission of toxoplasma gondii?

Answer 13

1. Ingesting food contaminated with cat faeces
2. Undercooked meat
3. Maternal transmission

Question 14

What are the virulence factors / pathogenesis of toxoplasmosis?

Answer 14

Toxo proteins activate toll-like receptors on monocytes and dendiritic cells to produce IL-2 and an indirect T cell response

Within the cell, the toxoplasma resides in a vacuole resistant to lysosomal destruction –> survives best in brain and retina.

Question 15

Whats the main differences between toxocariasis and onchocerciasis?
CAUSE/PATHOGEN, VIRULENCE/PATHOGENESIS, TREATMENT

Answer 15

Cause
Toxocariasis - canine parasite transmitted by dogs/cats (helminth)
Onchocerciasis - Simulium blackfly acts as vector and breeds in rivers, transfers infected larvae into human host

Pathogenesis
Toxocariasis: IgE mediated response with eosinophilia
Onchocerciasis: simulium blackfly vector

Treatment
Toxocariasis: Tiabendazole
Onchocerciasis: Doxycycline, Ivermectin

Question 16

1. What is the cause of loa loa
2. What is the pathogenesis of loa loa
3. What is the diagnosis of loa loa
4. What is the treatment of loa loa

Answer 16

Cause: Chrysop fly vectors -
Pathogenesis: calabar swellings form by eosinophilia
Diagnosis: microfiariae in blood
Management: –

Question 17

What are the three forms of toxoplasmosis gondii?

Answer 17

1. Oocyst/sporozoites - between 10-12 microns, soil form
2. Tachyzoite - between 4-8 microns (smallest), infectious form
3. Bradyzoite -between 10-200 microns, latent form (tissue cyst)

Question 18

What is the different forms of acanthamoeba? Which is the most common genotype?

Answer 18

Biphasic lifecycle
1. Trophozoite - active stage
2. Cyst form - physiological inert

Most common: T4.

Question 19

What is the most common genotype of acanthamoeba keratitis?

Answer 19

T4 genotype

Question 20

When does congenital infection occur in toxoplasmosis? When mother is previously exposed to toxoplasma or becomes infected during first time in pregnancy?

Answer 20

Becomes infected during first time in pregnancy

Question 21

What does candida stain?
What does candida culture?

Answer 21

Periodic Acid Schiff and Grocott’s methenamine silver stain

Grows on Sabouraud’s dextrose agar (better than blood)

Question 22

Whats the difference between aspergillus and candida for corneal disease in terms of risk factors?

Answer 22

aspergillus - trauma involving plants
candida - immunosupressed/ocular surface disease

Question 23

What are the 3 classifications of fungi with examples?

Answer 23

1. molds (filamentous fungi) - Fusarium
2. Yeasts - Candida
3. Dimorphs (mold or yeast) - histoplasma

Question 24

What are the ocular manifestations of toxocara (3)

Answer 24

Only elicit response when they die.
1. Low grade fibrous retinal involvement
2. Rapid retinal inflammation with eosinophilia
3. Vitritis and pars planitis

Question 25

What type of parasite is
1. Toxocara
2. Acanthomoeba
3. Toxoplasmosis
4. Schistosoma

Answer 25

1. Toxocara - Nematode
2. Acanthamoeba - Protozoa
3. Toxoplasmosis - Protozoa
4. Schistosoma - Trematode

Question 26

In which trimester is congenital toxoplasmosis most severe?

In which trimester is highest risk of developing congenital toxoplasmosis

Answer 26

most severe: First

most common: Third.

Question 27

What is the most common ocular manifestation of lyme disease?

Answer 27

intermediate uveitis

Question 28

What are the different forms of candida?

Answer 28

Yeast: Ovoid-shaped, budding, infects cornea
Pseudohyphae: formed in corneal stroma
True hyphae:

Question 29

Parinaud’s oculoglandular syndrome is caused by which organism?

Answer 29

Bartonella (cat-scratch disease)

Granlumatous conjunctivitis and ipsilateral swallen pre-auricular lymph nodes