N-Acetylglucosamine Induces White to Opaque Switching, a Mating Prerequisite in Candida albicans

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Abstract
To mate, the fungal pathogen Candida albicans must undergo homozygosis at the mating-type locus and then switch from the white to opaque phenotype. Paradoxically, opaque cells were found to be unstable at physiological temperature, suggesting that mating had little chance of occurring in the host, the main niche of C. albicans. Recently, however, it was demonstrated that high levels of CO2, equivalent to those found in the host gastrointestinal tract and select tissues, induced the white to opaque switch at physiological temperature, providing a possible resolution to the paradox. Here, we demonstrate that a second signal, N-acetylglucosamine (GlcNAc), a monosaccharide produced primarily by gastrointestinal tract bacteria, also serves as a potent inducer of white to opaque switching and functions primarily through the Ras1/cAMP pathway and phosphorylated Wor1, the gene product of the master switch locus. Our results therefore suggest that signals produced by bacterial co-members of the gastrointestinal tract microbiota regulate switching and therefore mating of C. albicans. To mate, the human fungal pathogen Candida albicans must undergo a complex phenotypic change from a round “white” to large, elongated “opaque” cell. This involves the regulation of approximately 5% of the organism's genes. Surprisingly, this complex transition is not required for mating in other related yeast. Even more surprisingly, it was found that in vitro the mating-competent opaque phenotype was unstable at 37°C, the temperature of the host body. This observation led to a paradox. If C. albicans lives primarily in an animal host, physiological temperature would thwart mating, so where does C. albicans mate? This led to the suggestion that some physiological condition in the host niche stabilizes the opaque phenotype or even induces switching from white to opaque, so cells can mate. Recently, we demonstrated that the high concentrations of CO2 found in tissue and the gastrointestinal tract induced switching from white to opaque and then stabilized the opaque phenotype. Here, we demonstrate that a second factor, N-acetylglucosamine (GlcNAc), a sugar released primarily by bacteria in the gastrointestinal tract, also induces the switch from white to opaque and stabilizes the opaque phenotype. We demonstrate by mutational analysis that GlcNAc induction is regulated primarily by the Ras1/cAMP pathway, which also regulates filamentation of C. albicans. This is perhaps not surprising given that white-opaque switching shares with filamentation several phenotypic characteristics. Finally, we show that induction by GlcNAc requires the phosphorylated master switch gene that regulates spontaneous switching, suggesting that it induces the switch from white to opaque by activating the gene product of the master switch gene. Together, our results suggest that multiple signals from bacterial co-members of the gastrointestinal tract microbiota regulate switching and, therefore, mating of C. albicans in the colonized host.

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