BY2012 Microbiology
Gallery of Dimorphic Fungi
Fungal Dimorphism
¾ Fungal dimorphism can be defined as
growth of a fungal species in either a
hyphal (mould) form or in a yeast
form
¾ Such morphological transformations
are important in the pathogenesis of
infections caused by some fungi,
including Candida species
Examples of Dimorphic
Fungal Pathogens
¾ Candida spp: Yeast cell is normal flora
commensal; hyphal or pseudohyphal
form is invasive form causing
infection
¾ Histoplasma capsulatum: Mycelial (or
hyphal) phase is saprophytic; yeast
form is parasitic and causes infection
¾ Paracoccidioides brasiliensis: Hyphal
or mycelial form is saprophytic; yeast
phase causes infections
Fungal Dimorphism
Mucor rouxii – A Model System
Filamentous mould
with hyphae and
sporangia (arrowed)
Budding yeast
of Mucor rouxii
Fungal Dimorphism
• Yeast (commensal) to hyphae or
pseudohyphae (invasive form)
transformation
¾
Candida albicans
¾
Candida dubliniensis
¾
Other Candida species
Candida albicans and Oral Thrush
Source: Atlas of Clinical Oral Pathology, 1999
Fungal Dimorphism
Candida albicans
Yeast phase – normal
flora [Gram stain]
Hyphal phase infection
Yeasts growing at 30°C on agar. Filamentous hyphae produced
when growing at 37°C in vivo causing invasion of epithelial tissue.
Fungal Dimorphism
Candida albicans
*
Fluorescent stain to identify C. albicans yeast cells
(asterisk) and hyphae (arrowed) in a tissue section
Fungal Dimorphism
• Hyphal mycelium (saprophytic form) to
yeast (invasive form) transformation
• Fungal spores are inhaled or infect the skin
and on germination form yeasts
• Thermally dimorphic fungi:
¾
Histoplasma capsulatum (lungs)
¾
Blastomyces dermatitidis (lungs)
¾
Sporothrix schenckii (skin)
¾
Paracoccidioides brasiliensis (mouth &
nose)
¾
Penicillium marneffei (skin and lungs)
Fungal Dimorphism
Histoplasma capsulatum
Mycelial growth
of hyphae on agar
as saprophyte
Yeast cells
Hyphae forming
budding in tissue
macroconidia
infection
(sporangia)
[arrowed]
Filamentous mould growing at 20-25°C. Yeasts produced when growing at 37°C in vivo.
Fungal Dimorphism
Blastomyces dermatitidis
Mycelial growth
of hyphae on agar
as saprophyte
Hyphae,
conidiophores and
conidia (arrowed)
Yeast cells in
tissue infection
Filamentous mould growing at 20-25°C. Yeasts produced when growing at 37°C in vivo.
Fungal Dimorphism
Sporothrix schenckii
Mycelial growth
of hyphae on agar
as saprophyte
Hyphae,
conidiophores and
conidia (arrowed)
Yeast cells in
tissue infection
Filamentous mould growing at 20-25°C. Yeasts produced when growing at 37°C in vivo.
Fungal Dimorphism
Paracoccidioides brasiliensis
Mycelial growth
Yeast cells
Hyphae,
of hyphae on agar conidiophores and budding in tissue
as saprophyte
infection
conidia (arrowed)
Filamentous mould growing at 20-25°C. Yeasts produced when growing at 37°C in vivo.
Fungal Dimorphism Penicillium
marneffei, a dimorphic fungus
infecting immunocomprised patients
Conidiophores and
conidia
Filamentous mould
growing at 20 - 25°C
Yeast cells are
infective in skin
and other tissues
Penicillium yeasts
divide by fission
not budding
Yeasts produced when growing at 37°C in vivo
Fungal Dimorphism
• Hyphal mycelium (saprophytic form)
to spherule [endospore] (invasive
form) conversion
¾ Coccidioides immitis
Fungal Dimorphism
Coccidioides immitis
Mycelial growth of
hyphae on agar as
saprophyte
Hyphae and formation Spherules (sporangia)
of arthroconidia from containing endospores
the hyphae (arrowed) in tissue infection
by segmentation
Filamentous mould growing at 20-25°C. Spherules containing
endospores are produced when growing at 37°C in vivo.
Fungal Dimorphism
Coccidioides immitis
Thick-walled spherule Numerous endospores
containing numerous (2–5 μm) in lymph node
endospores