Morphogenetic Programmes Cell Types Suited for Pathogenesis Fungal Dimorphism and the Environmental Cues

The environmental signals triggering dimorphic switching are species-specific, and the different cell types serve different purposes. For example, the ability of C. albicans to switch from yeast cells to the hyphal state is thought to be important for penetrating tissues and for escaping from phagocytic cells of the host innate immunity system. In addition, concomitant with the morphological changes are differences in the physiological state of the C. albicans cells including such things as cell wall composition. These changes can contribute significantly to the variety of host immune responses observed and the ability of C. albicans to escape the immune system (Whiteway & Oberholzer, 2004 and references therein).

Of the dimorphic fungi, a group which are known to respond to the simple cue of temperature, switching from the vegetative hyphal phase at ambient temperature (25°C) to the pathogenic yeast phase at body temperature (37°C). These thermally dimorphic fungi include P. marneffei, H. capsulatum, Blastomyces dermatitidis, C. immitis, C. posadasii, Paracoccidioides brasiliensis, and Sporothrix schenkii. For P. marneffei, the transition from hyphal to yeast grow occurs over an extended temperature range beginning at 32°C where few yeast cells are produced to 37°C where yeast cells are the predominant or exclusive growth form. Other dimorphic pathogens respond to more complex sets of signals such as C. albicans which switches from yeast to hyphal growth in response to a number of environmental signals such as temperature, pH, N-acetylglucosamine, and serum (Lengeler et al., 2000). Ustilago maydis, a dimorphic plant pathogen, switches from the non-pathogenic yeast form to the infective dikaryotic hyphal form after mating on the plant surface (Bolker, 2001). Saccharomyces cerevisiae is not capable of undergoing dimorphic switching, however diploid cells can undergo a morphogenetic programme leading to pseudohyphal growth in response to nitrogen limitation (for review see Lengeler et al., 2000).

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