DIMORPHISM

MYCOLOGY (MIC 206)

DIMORPHISM

Dimorphic from Greek word: “dis”: twice “morphe”: producing to morphologically distinct types of zoosporesDimorphic 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) Spherical2) TubularFilamentous 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 DIMORPHISMHyphal 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 FUNGIHuman pathogensCandida albicans, Histoplasma capsulatum, Paracoccidiotes brasiliensis, Coccidioides immitis, Wangiella dermatiditis and Sporothroix schenckii.

Plant pathogensOphistoma 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.

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 albicansthe 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.

DIMORPHIC FUNGI (CON’T)

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 ligand2) Protease

3) Phenotypic switching

Candida albicans infection of the nails: Cutaneous Mycosis

Source: Microbiology Perspectives, 1999.