Week 8

Question 1

Introduction to mycology

Answer 1

 Mycology - study of fungi
 name derived from Greek words mykos for mushroom and logos for science
 fungi initially classified with plants
 Robert H. Whittaker in 1969 transferred fungi to a separate kingdom on basis of cell structure and way in which organisms obtain
nutrition from their food
 fungi are an important component in the energy cycle where they function as decomposers
 of an estimated 250,000 species, fewer than 150 are pathogenic to humans

Question 2

Modes of growth Fungi

Answer 2

Unicellular yeasts
Filamentous moulds
However, there are some dimorphic fungi which switch between these two forms

Question 3

Yeast

Answer 3

 produce moist, creamy, opaque, or pasty colonies
 morphology reflects unicellular growth of fungi
 cells usually spherical or ellipsoidal, vary in diameter from 3µm to 15µm
 most reproduce by budding - formation of blastoconidia
 differ very little in microscopic and colonial morphology

Question 4

Budding

Answer 4

 initiated by localised lysis of cell wall at specific point
 internal pressure on weakened cell wall causes wall to balloon outward
 swollen portion enlarges, nucleus divides by mitosis
 progeny nucleus migrates into newly formed bud
 nascent bud continues to enlarge
 cell wall grows together at constricted point of attachment
 bud breaks off from parent cell
 buds that fail to detach become elongated and produce chain of elongated yeast cells that resemble hyphae - pseudohyphae

Question 5

Filamentous (mould-like) fungi

Answer 5

Thallus (vegetative body) - mass of
threads with many branches
resembling cotton ball

Mass: mycelium

Threads: hyphae, tubular cells that in
some fungi are divided into segments
(septate), or in other fungi the
hyphae are uninterrupted by
crosswalls (nonseptate)

Grow by branching and tip elongation

Question 6

Fungi

Answer 6

 eukaryotic, aerobic, chemoheterotrophic organisms
 contain definite nucleus with surrounding nuclear membrane
 have tendency to be multinucleate
 cell wall rigid, primarily composed of glucose and mannose
polymers (chitin) and / or cellulose
 cell membrane contains sterols
 resistant to antibacterial antibiotics
 susceptible to polyene antifungal agents
 all reproduce asexually - imperfect state or anamorph
 some can also reproduce sexually - perfect state or teleomorph
 unicellular or multicellular

Question 7

Moulds - filamentous fungi characteristics

Answer 7

 produce fluffy, fuzzy, cottony, woolly, velvety, or powdery
colonies
 basic structural units - tubelike projections - hyphae hypha
− 5µm to 10µm in width
− may be hyaline (colourless) or dematiaceous (pigmented)
− may be coenocytic (aseptate) or septate
− grows by elongation at its tip

 intertwined hyphae form loose network - mycelium vegetative mycelium
− nutrient-absorbing and water-exchanging portion
 aerial mycelium / reproductive mycelium
− portion projecting above substrate surface
− gives rise to fruiting bodies from which asexual spores born
 generally identified by observation of their microscopic and colonial morphology

Question 8

Cell envelope Fungi

Answer 8

Cell wall contains chitin and glucan rather than peptidoglycan (bacteria) and is therefore unaffected by many antibiotics
Cell membrane contains ergosterol rather than cholesterol found in mammalian cells
Antifungal agents such as amphotericin which bind to
ergosterol creating pores that disrupt membrane function
The imidazole antifungal drugs (clotrimazole, ketoconazole, miconazole) and the triazole antifungal agents (fluconazole, itraconazole) block demethylation of lansterol to ergosterol

Question 9

Major groups of medically important fungi

Answer 9

 Class: Zygomycetes
 Class: Ascomycetes
 Order: Onygenales
 Class: Basidiomycetes
 Class: Deuteromycetes
 Class: Oomycetes

Question 10

Classification Fungi

Answer 10

 Clinicians find more value in grouping fungi into 4 mycoses categories
 Superficial / cutaneous mycoses (Epidermophyton sp. Microsporum sp. Trichophyton sp.)
− fungal infections that involve hair, skin, or nails without invasion of tissue
 Subcutaneous mycoses (Sporothrix schenckii)
− infections confined to subcutaneous tissue without dissemination to distant sites
 Systemic mycoses (Histoplasma capsulatum)
− group of fungal infections caused by agents that can invade deeply into tissues and organs
− have capability of spreading widely throughout body
 Opportunistic mycoses (Aspergillus fumigatus)
− infections caused by fungi that are inherently of limited virulence
− can cause local or disseminated disease in immunocompromised persons

 fungi cause human disease in 3 ways:
− fungi may actively grow on human host (mycoses)
− fungi may generate toxic substances (mycotoxicoses)
− fungi may generate allergens

Question 11

Fungal Infections (Mycoses)

Answer 11

Superficial mycoses (cutaneous):
e.g. Ringworm
Involve colonisation of skin, hair or nails, and
infect surface layers only. These infections
are common, relatively benign and selflimiting.
Subcutaneous mycoses:
e.g. Sporotrichosis
Involve colonisation of deeper layers of skin.
Lesions usually begin by fungal infection of a
small wound or abrasion.
Systemic mycoses:
e.g. Histoplasmosis
Classed as primary or secondary infections. Primary infections occur in otherwise normal individuals; secondary infections occur in hosts with a predisposing condition.

Question 12

Superficial Mycoses

Answer 12

Cutaneous – Tinea

Body (tinea corporis) – Ringworm
Scalp (tinea capitis) – Scalp Ringworm
Beard (tinea barbae)
Hands (tinea manuum)
Groin (tinea cruris) – Jock Itch
Feet (tinea pedis) – Athlete’s Foot
Nails (tinea unguium)

Question 13

Superficial Mycoses (Dermatophytoses)

Answer 13

Three genera of Dermatophytes:
- Epidermophyton
- Microsporum
- Trichophyton

they can grow on skin, hair, and/or nails
All 3 genera infect the skin, but Microsporum does not infect nails and Epidermophyton does not infect hair

Question 14

How do we diagnose tinea?

Answer 14

– Woods lamp
– Skin scrapings
– Microscopy and culture

Question 15

Dermatophytosis definition

Answer 15

− infection of skin, nails, or hair by any of keratinophilic,
filamentous fungi called dermatophytes

Question 16

Dermatophytes definition

Answer 16

− parasitise non-living, cornified integument − secrete keratinases - proteolytic enzymes that digest keratin− keratin - scleroprotein containing large amounts of sulphur, structural component of epidermis, nails, hair − species similar in their morphology, physiology,
biochemical composition

Question 17

3 Dermophyte Categories based on Natural Habitat / Host Preference

Answer 17

Geophilic: soil-inhabiting organisms, soil source of infection for humans and animals e.g.
* Microsporum gypseum

Zoophilic: essentially pathogens of animals, animal-to-human transmission not uncommon e.g.
* Microsporum canis

Anthropophilic: almost exclusively infect humans, animals rarely infected e.g.
* Trichophyton rubrum

Question 18

Natural habitat dermatophytes

Answer 18

 Infections
− anthropophilic species cause mild chronic infections in
humans
− zoophilic species cause acute inflammatory infections in humans

 Transmission
− geophilic species - from soil source to humans or animals (rare)
− zoophilic species - by animal-to-human contacts (cats, dogs, cattle, laboratory animals, etc.) or by indirect
transmission involving fomites
− anthropophilic species - by close human contact or indirectly by sharing of clothes, combs, towels, bedsheets

Question 19

Clinical significance dermatophytes

Answer 19

 dermatophytoses are among the most prevalent infections in world
 begin in cutaneous tissue after contact and trauma
 host susceptibility enhanced by moisture, warmth, youth,
heavy exposure
 incidence higher in hot, humid climates, under crowded living conditions
 termed ringworm or tinea because of characteristic raised
circular lesion
 traditionally named according to anatomic location involved

 Tinea barbae (beard, moustache)
− acute pustular folliculitis, can progress to suppurative
boggy lesions (kerion) in highly inflammatory forms
− dry, erythematous, scaly lesions in less severe forms
 Tinea capitis (scalp, eyebrows, eyelashes)
− vary in presentation
− highly erythematous, patchy, scaly areas with dull grey hair stumps
− highly inflammatory with folliculitis, kerion formation,
alopecia, scarring
 Tinea favosa - favus (scalp, glabrous skin)
− chronic infection with formation of cup-shaped crusts
resembling honey combs (scutula)
 Tinea corporis (face, trunk, major limbs)
− circular, erythematous lesions with scaly, raised, active,
often vesicular borders
 Tinea cruris - “jock itch” (groin, perineal, perianal areas)
− scaly, erythematous to tawny brown, bilateral, asymmetric lesions extending down to inner thigh
− sharply marginated border frequently studded with small vesicles
 Tinea manuum (palms)
− annular, scally, may be embellished with erythema, vesicles, or allergic reactions
 Tinea pedis - “athlete’s foot” (soles, toe webs)
− varies in appearance
− maceration, peeling, itching, painful fissuring between 4thand 5th toes
 Tinea unguium - onychomycosis (nails)
− thickened, deformed, friable, discoloured nails with accumulated subungual debris
 Dermatophytid
− allergic response to fungal antigens
− dermatophyte infection in one area (e.g., tinea pedis) elicits allergic reaction elsewhere

Question 20

Collection, transport, and storage of
specimens dermatophytes

Answer 20

 Wood’s lamp (filtered UV light peak of 365nm)
− patients with suspected tinea capitis should be examined in darkened room
− hairs infected by certain types of dermatophytes fluoresce -Wood’s light “+”

Question 21

dermatophytes Direct examination

Answer 21

 Skin, nail, hair infections diagnosed by dissolving skin
scrapings, nail clippings, hairs in 10% potassium hydroxide (KOH)
 Skin and nails
− small (2µm to 3µm), hyaline, branched, septate hyphae that break up into chains of rectangular arthroconidia
− among squamous epithelial cells
 Hairs
− appearance, size, location of arthroconidia depend on
infecting genera or species
− 3 types: ectothrix type, endothrix type, favic type
− terms ectothrix and endothrix refer to location of arthroconidia in relation to hair shaft
 Ectothrix hairs
− arthroconidia appear as mosaic sheath around hair or as chains on surface of hair shaft
 Endothrix hairs
− short hair stubs, thick, usually twisted, filled with chains of large arthroconidia
 Favic hairs
− hairs invaded throughout their length by hyphal elements
− empty areas (tunnels) where hyphae degenerated into fat droplets seen inside hair

Question 22

Isolation and identification dermatophytes

Answer 22

 Primary recovery media
− Sabouraud dextrose agar with antibiotics or Inhibitory
mould agar
 dermatophytes resistant to cycloheximide

Dermatophyte test medium
 screening for presence of dermatophytes
 dermatophytes change colour of medium (indicator
phenol red) from yellow to red within 14 days (produce rise in pH into alkaline range)

Cornmeal agar
 induces sporulation

Potato dextrose agar
 induces sporulation

 Incubation conditions − specimens should be cultured at 25oC to 30oC

Question 23

Identification - physiologic tests dermatophytes

Answer 23

 In vitro hair perforation test
− distinguishes atypical isolates of Trichophyton mentagrophytes, Trichophyton interdigitale, Trichophyton eriotrephon from Trichophyton rubrum
− Trichophyton rubrum does not form these perforating
structures (“-” test)

 Urease test
− aids in differentiation of Trichophyton mentagrophytes,
Trichophyton interdigitale, Trichophyton eriotrephon (urease “+”)
− from Trichophyton rubrum (usually urease “-”)

 Growth on Trichophyton agars 1 to 7
− aid in identification of Trichophyton species
− certain species have distinctive nutritional requirements, others do not
− method employs Casein basal medium vitamin free
(Trichophyton agar T1)
− to which various vitamins are added (Trichophytonagars T2 to T7)

Question 24

Genus Trichophyton

Answer 24

 characterised by presence of multiseptate, smooth, thin walled, pencil- to cigar-shaped macroconidia which are borne singly or in clusters
 in most instances microconidia predominant
 capable of invading skin, nails, hair
 infected hairs do not fluoresce under a Wood’s lamp, appear as ectothrix, endothrix, or favic type
 has 16 species

Question 25

Trichophyton rubrum

Answer 25

 world-wide in distribution
 anthropophilic
 causative agent of tinea corporis, tinea pedis, tinea unguium, rarely tinea capitis in humans
 species recognised in several forms
 main 2: “granular or parent SE Asian strain” and “downy
strain

Question 26

Characteristics of Trichophyton rubrum

Answer 26

 Growth rate
− slow, mature within 14 days
 Colony morphology
− surface granular or fluffy, white to buff
− reverse yellow when cultures young, turns wine red with age

 Microscopic morphology
− septate hyphae
− microconidia teardrop-shaped, formed directly on hyphae and macroconidia, producing “bird-on-the-fence”
appearance

Question 27

Therapy dermatophytes

Answer 27

 Skin and hair infections
− multiple daily applications of topical antifungal agents
(miconazole, ketoconazole, terbinafine)
 Nail infections
− oral administration of itraconazole or terbinafine