An Intro to Fungi

Question 1

Provide an overview of mycology characteristics

Answer 1

Eukaryote (posses membrane bound organelles

Widely distributed in the environment

It can grow at ca. 25°C (some at 37°C)

They are non-photosynthetic & aerobic

Cell walls contain chitin & other polysaccharides

Heterotrophic nutrition
Obtain nutrients from dead organic matter -decomposers
Obtain nutrients from other living organisms -parasites

Can reproduce sexually & asexually

Tolerate high osmolarity and low pH

Resistant to antibacterial drugs

Question 2

Describe the structural components of a fungal cell

Answer 2

Contain mebrane bound organelles

Endoplasmic reticulum
Nucleus
Vacuole
Mitochondria
Membrane
Chitin cell wall
Vesicles

Question 3

Describe the morphological features used to describe fungi

Answer 3

Yeast
Yeast form - oval/spherical appearance
i.e. Malassezia

Moulds
Filamentous fungi -hyphal forms
i.e. Microsporum canis, Aspergillus

Some are dimorphic fungi & will growth in yeast or hyphal forms depending on temp of growth
i.e. Histoplasma capsulatum

Question 4

Describe the reproductive features used to describe fungi

Answer 4

Reproduction is complex

Many can reproduce both sexually or asexually

Sporulation bodies can be used for identification in clinical samples.

Spores are relatively resistant and can survive and spread infection

Spores germinate where conditions are favourable.

Question 5

What are the two main types of asexual spores

Answer 5

Conidia
formed on conidiophores
In dermatophytes (skin infecting fungi) you get macroconidia and microconidia.

Sporangiospores
formed on sporangiophores

Question 6

Describe the culture features used to describe fungi

Answer 6

Sabouraud dextrose media

Slightly acidic pH (-5.6)

Cyclohexamide, penicillin, streptomycin or other inhibitory antibiotics are often 
     added to prevent bacterial contamination and overgrowth. 

The cultures are examined macroscopically and microscopically.

They are not considered negative for growth until after 4 weeks of incubation

Question 7

How would you identify whether a fungi is dimorphic ?

Answer 7

culture at both temp:
- 25°C Sabouraud dextrose media 1-4 weeks
- 37°C Brain heart infusion agar 1-4 weeks

Question 8

How would you differentiate fungal colonies?

Answer 8

Mould and yeast

	○ Size appearance after set time
	○ Colour both sides
	○ Surface elevation depressions 
            ○ Other patterns

Moulds

 Examination of spore structures from culture or clinical sample
 
  Features of vegetative hyphae
	○ Presence absence of septa
	○ Hyaline (colourless) or dematiaceous (pigmented)
	○ Hyphal structures (spiral, racket shape).

Question 9

List the processes in which you test for tissue invading fungi

Answer 9

Biopsy and histopathology.

Microscopy:

Serology:

Question 10

Describe how you would use a microscope to investigate tissue invading fungi

Answer 10

Infected tissue mounted in potassium hydroxide (KOH) on a slide and examined directly under the microscope.
Chitin in the cell wall is resistant to KOH cells are not.

Typical stain: Periodic acid-Schiff reaction (PAS).
Stains polysaccharides

Fluorescent microscopy may be used for identification, even on non-viable cultures or on fixed tissue sections. (The reagents for this test are difficult to obtain).

The common tissue H&E stain does not always stain the organism.

Question 11

CR: Describe Ringworm

Answer 11

Zoonotic

Circular skin lesions caused by dermatophytes, commonly
Microsporum canis (cats, dogs)
Trichophyton verrucosum (cattle)
Trichophyton equinum (horses)

Colonize & invade skin, hair & nails

Spores shed from infected animals remain infective for many month

Transmission:
Contact
Contaminated surface

Highly likely to get it at some stage (student or when in practice).

Very often occurs under watch straps presumably because of the rubbing

Question 12

CR: Describe Malassezia pachydermatis

Answer 12

Yeast: Unicellular, bottle shaped cells

Commensal on skin

Associated with canine skin and ear infections
 Opportunistic infection!

Question 13

CR: Describe Aspergillus Spp.

Answer 13

Filamentous mould with septated hyaline hyphae & conidia formed on conidiophores

Ubiquitous
i.e. soil, poor quality hay etc

Rapidly growing pigmented colonies

Respiratory pathogen, acquired by inhalation of spores, most common animal pathogen: Aspergillus fumigatus

Invasive

Dogs:
Sinonasal aspergillosis
 Invasion of nasal mucosa & turbinate bones