DIMORPHISM
Dimorphic from Greek word:
“dis”: twice
“morphe”: producing to morphologically distinct
types of zoospores
Dimorphic fungi: a fungus able to grow either in yeast
form or in mycelial form which exhibiting dimorphism
→
including
Zygomycete,
Ascomycete,
Basidiomycete and Deuteromycete.
Dimorphic fungi have the ability to live in 2 forms:
1) Spherical
2) Tubular
Filamentous fungi → the spherical form is during spore
formation.
DIMORPHISM (CON’T)
Most of them are human and animal pathogens.
Some are saprophytes.
Reason for Dimorphism:
1) There exist stability between the filamentous and
spherical growths.
2) There are biochemical differences between the
two forms that allow for differences in the
morphology.
Ex: An example is Penicillium marneffei
 Mycelial saprotrophic form grows at 25° C
 Yeast-like pathogenic form at 37° C
Thermal dimorphism in P. marneffei.
A) The mould phase of P. marneffei depicting phialides bearing typical conidia
(slide culture incubated at 25°C).
B) Thin, multiply branched hyphae developing from conidia (arrows) incubated in
SDB for 24 hours at 25°C.
C) Short, broad hyphae generated from conidia (arrows) incubated in SDB for 24
hours at 37°C.
D) Yeast cells of P. marneffei produced from conidia incubated in SDB for 96 hours
at 37°C.
EFFECT OF DIMORPHISM
Hyphal growth allow the cells to move and penetrate
unsoluble barriers e.g. animal tissues.
Pseudohyphae cells of Saccharomyces cerevisiae
can penetrate agar but yeast colonies can only sit
on the surface of agar.
DIMORPHIC FUNGI
Human pathogens
Candida albicans, Histoplasma capsulatum,
Paracoccidiotes brasiliensis, Coccidioides
immitis, Wangiella dermatiditis and Sporothroix
schenckii.
Plant pathogens
Ophistoma ulmi and Ustilago maydis.
DIMORPHIC FUNGI (CON’T)
1) Candida albicans and Ustilago maydis → their
mycelia are more pathogenic than their yeast-like
cells.
2) Histoplasma capsulatum , Paracoccidiotes brasiliensis
and Blastomyces dermatiditis → pathogenic in the
yeast form but saprophytic in the mycelial form.
3) H. capsulatum, P. brasiliensis and Coccidioides
immitis → pathogenic in the yeast forms.
 These fungi do not infect the mucosal layers but
get into the lungs via spores.
 Spores get into the lungs when breathing in and
infection spreads especially when the immune
system
is
weak
(patients
undergoing
chemotherapy).
Systemic Mycosis: Histoplasmosis
Disseminated Histoplasma
capsulatum, lung infection.
Disseminated Histoplasma
capsulatum, skin infection.
Source: Microbiology Perspectives, 1999.
DIMORPHIC FUNGI (CON’T)
4) Trichophyton sp. infect skin of man (panau).
5) Ophistoma ulmi and Ustilago maydis
 use the tips of hyphae to penetrate host cells.
 The infection process is in the vascular tissues are
via smaller yeast cells.
 Ex: causing Dutch elm disease.
DIMORPHIC FUNGI (CON’T)
6) Candida albicans
the fungus Candida is in
a yeast form; but when
it enters tissues, it can
form what is referred to
as pseudohyphae.
Unlike molds, Candida
albicans cannot grow
hyphae (long filaments),
but the form that it has
while inside tissues is
long and looks like
hyphae; thus, it is called
pseudohyphae.
Gram stain of Candida albicans
showing germ tube production in
serum.
Media used contains N-acetylglucosamine or
serum or both to initiate growth of hyphae.
Changes in the environment can influence one of
the forms ex:
→ Rise in temperature, neutral pH and media with
depletion of nutrients encourage hyphal growth
compared to yeasts.
Other factors that influence dimorphism of
Candida albicans include:
1) Adhesin cell wall and ligand
2) Protease
3) Phenotypic switching
Candida albicans infection of the nails: Cutaneous
Mycosis
Source: Microbiology Perspectives, 1999.