Fungus Part I

INTRODUCTION
• Eukaryotes
• Don’t possess chlorophyll
• Unicellular/multicellular
• Mostly soil saprophytes
• Role in degradation of organic compounds
• About 250000 fungal species are identified.
• Out of them only about 150-200 are known to cause
human infections.
• Fungi cause infections in debilitated patients, eg.
Immunocompromised patients like AIDS patients.
• Useful fungi: edible mushrooms, yeasts used in
fermentation, fungi producing antibiotics (eg penicillium).

STRUCTURE

• Fungal cells are eukaryotic cells containing cell wall, cell
membrane, true nuclei, nuclear
membrane, mitochondria, vaculoes, reticular
endothelium, ribosomes etc like other eukaryotic cells.

• Cell division – sexual/asexual

• Cell wall – made up of chitin, glucans, mannans and
complex polysaccharides.

• Cell membrane contains ergosterol in contrast to
mammalian cells which contain cholesterol.

Fungal cell wall
• Consists of chitin not peptidoglycan like
bacteria.
• Thus fungi are insensitive to antibiotics as
penicillins.
• Chitin is a polysaccharide composed of
long chain of n-acetyleglucasamine.
• Also the fungal cell wall contain other
polysaccharide, B-glucan, which is the site
of action of some antifungal drugs.

Fungal cell membrane
• Consist of ergosterol rather than cholesterol like
bacterial cell membrane.
• Ergosterol is the site of action of antifungal
drugs, amphtericin B & azole group

Atmospheric & carbon source
requirements
• Most fungi are obligatory aerobes, some
are facultative anaerobes, but none are
obligatory anaerobes.
• All fungi require a performed organic
source of carbon –association with
decaying matter.
• Natural habitat
• The environment.

Opportunistic Fungi
In addition to those species which are
generally recognized as pathogenic to
man it is firmly established that under
unusual circumstances of abnormal
susceptibility of patient, or the
traumatic implantation of the
fungus, other fungi are capable of
causing lesions. Those are called
(Opportunistic Fungi.)

These circumstances may be :

1. A debilitating condition of the host, as
Diabetes.
2. A concurrent disease such as leukaemia.
3. Prolonged treatment with corticosteroids.
4.Immunosuppressive drugs or an antibiotic
for long duration.

CLASSIFICATION
Taxonomical classification:
• Phylum class
• Zygomycota zygomycetes
• Ascomycota ascomycetes
• Basidiomycota basidiomycetes
• Deuteromycota deuteromycetes (fungi
imperfecti)

2. Zygomycota
• Zygote – fertilized egg
• Decomposers
• Rhizopus sp. – bread mold
• Mold… any fungi in its asexual stage

Bread Mold – a
Zygomycete Fungi

Zygomycota – common
molds

The fungal mass of
hyphae, known as the
MYCELIUM
penetrates the bread
and produces the
fruiting bodies on top
of the stalks

Mycelia = a mass of hyphae or
filaments

3. Ascomycota
• Cup shaped reproductive structures
• Yeast – unicellular exception

Morel Fruiting Bodies

Cup Fungi – Ascomycete Fungi

Note the cup shapes and
orange peel colour

4. Basidiomycota – club
fungi

• Club-shaped reproductive structure
• Many common examples –
mushrooms, puffballs, shelf fungi

5. Deuteromycota - (Imperfect
Fungi)
-Regarded as imperfect because no sexual stage has been observed
in their life cycle

-Members are not closely related and are not necessarily similar
in structure or appearance; do not share a common ancestry

 Morphological classification:
1. Yeasts
2. Yeast like fungi
3. Moulds/ filamentous fungi/ mycelial fungi
4. Dimorphic fungi

 Description:
 Yeasts are round to oval unicellular fungi which
reproduce by budding or binary fission eg cryptococcus.
 Yeasts like fungi – some yeasts grow partly as yeasts
and partly as chains of elongated budding cells joined
end to end forming pseudohyphae and
pseudomycelium eg candida.

 Filamentous fungi/moulds:
• They grow as multicellular branching filaments.
• Hypha = each filament is called hypha.
• Mycelia = tangled masses of hyphae are known as
mycelia.
• Thallus = body of fungus.
• Septa = transverse walls dividing hyphae at irregular
intervals.
• The septate hyphae are morphologically coenocytic
because septae have holes through which free flow of
nuclei and other cytoplasmic material can occur.
• Aerial mycelium = the part of mycelium projecting
above the culture medium.
• Vegetative mycelium = the part of mycelium growing in
the culture medium.

• There is abundant formation of spores on aerial
mycelium that affects their airborne transmission.
• Eg
dermatophytes, aspergillus, zygomycetes, penicillium.

 Dimorphic fungi:
• They exist as both yeast form and filamentous form
depending on conditions of growth.
• Yeast form (parasitic phase) occurs in host tissues and
in cultures at 37 C.
• Filamentous form (saprophytic phase) occurs in soil and
in cultures at 22-25 C.
• Eg fungi responsible for systemic mycoses like
histoplasma, blastomyces, coccidiodes, paracoccidiodes
.

mycelium: septate mycelium: non septate

Yeasts
1. These occur in the form of round or oval
bodies which reproduce by the formation of
buds known as blastospores.

2. Yeasts colonies resemble bacterial
colonies in appearance and in consistency.
3. The only pathogenic yeast in medical
mycology is Cryptococcus neoformans.

Yeast-Like
1.These are fungi which occur in the form of
budding yeast-like cells and as chains of
elongated unbranched filamentous cells
which present the appearance of broad
septate hyphae. these hyphae intertwine
to form a pseudomycelium.

2. The yeast like fungi are grouped together
in the genus Candida.

Pseudohyphae and pseudomycelium

Candida albicans
• A germ tube is an outgrowth produced by spores of spore-releasing fungi during
germination.
• The germ tube differentiates, grows, and develops by mitosis to create somatic
hyphae.
• A germ tube test is a diagnostic test in which a sample of fungal spores are
suspended in serum and examined by microscopy for the detection of any germ
tubes. It is particularly indicated for colonies of white or cream color on fungal
culture, where a positive germ tube test is strongly indicative of Candida albicans.

Thermally Dimorphic
Fungi
These are fungi which exhibit a
filamentous mycelial morphology
(saprophytic phase) when grown at
room temperature 27oC, but have a
typical yeast morphology (parasitic
phase) inside the body and when
grown at 37oC in the laboratory (e.g.
Histoplasmosis).

REPRODUCTION

• Reproduction in fungi can be of two types:
• Asexual
• Sexual

 Asexual spores:
• Sporangiospores = spores formed in sporangium by
formation of cleavage planes. Seen in zygomycetes eg
rhizopus, mucor.
• Conidiospores = borne externally on sides eg
aspergillus, penicillium, dermatophytes.
• Microconidia = small single spores.
• Macroconidia = large single or multicellular spores.

Sporangiospore, conidiospore, blastospore and
chlamydospore, arthrospore

• Sexual spores: four types have been identified
• Oospores
• Zygospores
• Ascospores
• Basidiospores

• Other vegetative spores:
• Blastospores – a fungal spore that arises by budding.
• Arthrospores - one of a number of spores of various fungi and
certain blue-green algae, united in the form of a string of
beads, formed by fission.
• Chlamydospores - a thick-walled intercalary or terminal asexual
fungal spore formed by the rounding-up of a cell; it is not shed.
Formed by differentiation of hyphae; seen in Candida and
Histoplasma spp.
• Phialospores - A type of conidium found, for example, in many of the
Eurotiales and Hypocreales. Phialospores develop at the tips of
specialized finger-like cells termed phialides.

FUNGI IMPERFECTI
(DEUTEROMYCETES)

• All those fungi whose sexual or perfect state is not
known.

• They form septate hyphae and asexual spores or no
spores.

• Most fungi causing human infections belong to this
group.

INFECTION
 Fungal infections are of 4 types based on target tissue:
• Superficial mycoses = surface infections limited to
outermost layers of skin and hair.
• Cutaneous mycoses = fungal infections extending
deeper into the epidermis and its integuments.
• Subcutaneous mycoses = infections involving
dermis, subcutaneous tissue, muscles and fascia.
• Systemic mycoses = infections originating primarily in
the lungs ( acquired by inhalation) and spreading to
other organs.

• Opportunistic mycoses = besides the above four
mentioned types, this category includes infections in
which fungi of no significance or low virulence infect
humans with compromised immune system.

LABORATORY DIAGNOSIS (main points)
 Specimens:
• Skin scrapings, nail clippings, hairs
• Scrapings from mucous membrane
• Scrapings, crusts, aspirated pus, tissue biopsy.
• Blood, CSF etc in systemic mycoses.

 Microscopy:
• KOH mount – KOH dissolves keratin and cellular
material but does not affect fungi. Specimen is placed
on a slide, a drop of 10-20% KOH is added and covered
with a coverslip, left for 20 min in incubator at 37°C to
digest keratin. Then examined microscopically.

LACTOPHENOL COTTON BLUE

• The lactophenol cotton blue (LPCB) wet mount
preparation is the most widely used method of staining
and observing fungi and is simple to prepare. The
preparation has three components: phenol, which will kill
any live organisms; lactic acid which preserves fungal
structures, and cotton blue which stains the chitin in the
fungal cell walls.

• Stains: gram stain, papanicoulau stain, periodic acid
schiff stain (PAS), methenamine sliver stain, giemsa
stain etc.

• Direct immunofluorescence test

• Histology

• Antigen detection tests eg cryptococcal antigen in CSF.

 Culture:
• Sabouraud’s dextrose agar is commonly used for fungal
culture.
• pH =5.6 does not allow bacterial growth.

• Drugs like chloramphenicol, cyclohexamide and other
antibiotics are added to prevent bacterial or saprophytic
fungal infection.

• Cultures are incubated at two temperatures:
• One tube at 25°C (room temperature)
• One tube at 37°C (incubator).
• This helps reveal fungal dimorphism.

• Cultures are incubated for atleast 2-3
weeks and in some cases upto 6 weeks.
• Cultures are examined macroscopically for
colony morphology, and microscopically
for fungal morphology.
• Czapek-Dox agar
• Cornmeal agar

Fungus Part I

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Fungus Part I