Week 8 Flashcards
Introduction to mycology
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
Modes of growth Fungi
Unicellular yeasts
Filamentous moulds
However, there are some dimorphic fungi which switch between these two forms
Yeast
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
Budding
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
Filamentous (mould-like) fungi
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
Fungi
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
Moulds - filamentous fungi characteristics
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
Cell envelope Fungi
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
Major groups of medically important fungi
Class: Zygomycetes
Class: Ascomycetes
Order: Onygenales
Class: Basidiomycetes
Class: Deuteromycetes
Class: Oomycetes
Classification Fungi
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
Fungal Infections (Mycoses)
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.
Superficial Mycoses
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)
Superficial Mycoses (Dermatophytoses)
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
How do we diagnose tinea?
– Woods lamp
– Skin scrapings
– Microscopy and culture
Dermatophytosis definition
− infection of skin, nails, or hair by any of keratinophilic,
filamentous fungi called dermatophytes
Dermatophytes definition
− 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
3 Dermophyte Categories based on Natural Habitat / Host Preference
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
Natural habitat dermatophytes
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
Clinical significance dermatophytes
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
Collection, transport, and storage of
specimens dermatophytes
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 “+”
dermatophytes Direct examination
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
Isolation and identification dermatophytes
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
Identification - physiologic tests dermatophytes
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)
Genus Trichophyton
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
