Dimorphism

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.