Quorum Sensing in Dimorphic Fungi: Farnesol and Beyond

Production of farnesol by Candida albicans is the first quorum- sensing system discovered in a eukaryote (29). In C. albicans, accumulated farnesol affects both dimorphism (29, 50) and biofilm formation (62). Fungal dimorphism is defined (64) as an environmentally controlled reversible interconversion of morphology, particularly yeast and mycelial morphologies. Interest in this shift derives from the dimorphic character of many fungi that are pathogenic toward plants and animals (64). Numerous chemical and environmental parameters can shift the yeast-mycelium dimorphism, including temperature, pH, glucose levels, nitrogen source, carbon dioxide levels, transition metals, chelating agents, and inoculum size or initial cell density (64). Of these, the inoculum size effect is probably the least well studied. For fungi such as Ceratocystis ulmi (28, 42) and C. albicans (29), cells develop as budding yeasts when inoculated at ≥106 cells per ml and as mycelia when inoculated at (Table 1). In keeping with the precedent established by homoserine lactone-based signaling in gram-negative bacteria (22), the inoculum size effect in fungi is also called quorum sensing (29) and the extracellular cell density-dependent signals are called quorumsensing molecules (QSMs). Thus, the chemical identity of the respective QSMs is of interest. Apart from C. ulmi (28) and C. albicans (29), it is a “leap of faith” on our part that the other cell density phenomena listed in Table 1 are mediated by QSMs