Talaromyces marneffei (Penicillium marneffei) is an important fungal pathogen of humans, in particular those who are immunocompromised. T. marneffei has the capacity to alternate between a hyphal and a yeast growth form, a process known as dimorphic switching. The strongest extrinsic trigger for dimorphic switching is in response to temperature. T. marneffei grows in the hyphal form at 25°C and in the yeast form at 37°C. The hyphal form produces conidia, which are likely to be the infectious agent, and believed to establish infection after inhalation. The yeast growth form is the pathogenic form found in infected patients. These yeast cells exist intracellularly in the mononuclear phagocyte system of the host.

T. marneffei is the only true pathogen in a genus comprising a large number of species and is also the only dimorphic fungus in this group. Yet there are a number of other fungi in more distantly related orders which also exhibit the capacity to alternate between hyphal and yeast growth forms. Many of these are also pathogens of animal or plants. As an intracellular pathogen, T. marneffei must be able to utilise the available nutrient sources in order to grow while evading or tolerating the host’s defence systems. The results from a number of lines of investigation into the molecular control of the dimorphic switch and the events which establish and maintain the morphological states in T. marneffei, both of which are central to understanding pathogenicity, will be presented.