ABSTRACT
While virulence factors of several pathogenic fungi have been proposed, little is known about the regulation of their expression or the environmental factors (signals) in the host, be it animal or human, that trigger their expression. In Candida albicans. for example, extracellular enzymes (secretory aspartyl proteinases [~APS] and phospholipase B), morphogenesis (yeast to hyphae transition), host recognition by cell surface molecules (adhesins), and phenotypic switching have been considered among the virulence attributes of this organism [I]. However, of all of these phenotypic properties. only signal pathways that regulate morphogenesis have been partially defined [2]. Recent literature indicates that deletions in genes of the morphogenesis signal transduction pathways in Candida albicans result in altered morphogenesis patterns and a reduction in the virulence of this organism [2-13]. At least two morphogenesis signal pathways have been proposed, but it is likely that other pathways exist. Like C. albicans. Cryptococcus neoformans has multiple virulence determinants, including capsule and melanin synthesis [14], and there also is an association of mating type and virulence [15,16]. However, until recently, little was known about the regulation of these virulence attributes, although an association of a nutrient-deprivation induced signal pathway and virulence has been suggested [17,18]. These and other examples of signal transduction as a means by which pathogenic fungi regulate their virulence and growth will be the focus of this chapter.
