P91: Examining yeast-hyphal transition in opaque Candida albicans.

Candida albicans is a dimorphic yeast that is normally found as a commensal in the lower bowel, the vagina and the skin. Perhaps the most interesting aspect of C. albicans biology is its ability to grow in multiple phenotypic states including alternative yeast and filamentous forms . The plasticity of yeast forms is exemplified by the “white-opaque switch”, in which cells can transit between a white state, where cells are round and give rise to dome-shaped colonies, and an opaque state, where cells are elongated and produce flatter, darker colonies.  White and opaque forms differ in multiple aspects including their interaction with immune cells, their virulence, and their mating competency. White cells can also form hyphal filaments when grown at 37°C, neutral pH or in the presence of serum.  However, opaque cells do not form filaments under these conditions, and it is generally thought that opaque cells can only grow as yeast cells. In this study, we evaluated whether opaque cells can undergo the yeast-hypha transition in response to certain environmental cues. Several novel growth conditions were identified that activate the program of filamentous growth in opaque cells, but not white cells.  Transcription factors that are critical for the program of white filamentation were also tested and pathways regulating opaque filamentation defined.  Overall, these studies indicate that white and opaque cells can both undergo filamentation but do so in response to different environmental signals, further reflecting intrinsic differences in their programming.