DIMORPHISM

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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
• Spherical
• Tubular
• Filamentous fungi ? the spherical form is during
  spore formation.

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DIMORPHISM (CONT)

• Most of them are human and animal pathogens. Some
  are saprophytes.
• Reason for Dimorphism
• There exist stability between the filamentous and
  spherical growths.
• 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

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• Thermal dimorphism in P. marneffei.
• A) The mould phase of P. marneffei depicting
  phialides bearing typical conidia (slide culture
  incubated at 25C).
• B) Thin, multiply branched hyphae developing from
  conidia (arrows) incubated in SDB for 24 hours at
  25C.
• C) Short, broad hyphae generated from conidia
  (arrows) incubated in SDB for 24 hours at 37C.
• D) Yeast cells of P. marneffei produced from
  conidia incubated in SDB for 96 hours at 37C.

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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.

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DIMORPHIC FUNGI

• Human pathogens
• Candida albicans, Histoplasma capsulatum,
  Paracoccidiotes brasiliensis, Coccidioides
  immitis, Wangiella dermatiditis and Sporothroix
  schenckii.
• Plant pathogens
• Ophistoma ulmi and Ustilago maydis.

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DIMORPHIC FUNGI (CONT)

• Candida albicans and Ustilago maydis ? their
  mycelia are more pathogenic than their yeast-like
  cells.
• Histoplasma capsulatum , Paracoccidiotes
  brasiliensis and Blastomyces dermatiditis ?
  pathogenic in the yeast form but saprophytic in
  the mycelial form.
• 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).

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Systemic Mycosis Histoplasmosis
Disseminated Histoplasma capsulatum, lung
infection.
Disseminated Histoplasma capsulatum, skin
infection.
Source Microbiology Perspectives, 1999.
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DIMORPHIC FUNGI (CONT)

• 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.

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DIMORPHIC FUNGI (CONT)

• 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.
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• 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

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Candida albicans infection of the nails
Cutaneous Mycosis
Source Microbiology Perspectives, 1999.