Candida albicans, a commensal organism and a pathogen of humans, can switch stochastically between a white phase and an opaque phase without an intermediate phase. The white and opaque phases have distinct cell shapes and gene expression programs. Once switched, each phase is stable for many cell divisions. White-opaque switching is under a1-␣2 repression and therefore only happens in a or ␣ cells. Mechanisms that control the switching are unknown. Here, we identify Wor1 (white-opaque regulator 1) as a master regulator of white-opaque switching. The deletion of WOR1 blocks opaque cell formation. The ectopic expression of WOR1 converts all cells to stable opaque cells in a or ␣ cells. In addition, the ectopic expression of WOR1 in a͞␣ cells is sufficient to induce opaque cell formation. Importantly, WOR1 expression displays an all-or-none pattern. It is undetectable in white cells, and it is highly expressed in opaque cells. The ectopic expression of Wor1 induces the transcription of WOR1 from the WOR1 locus, which correlates with the switch to opaque phase. We present genetic evidence for feedback regulation of WOR1 transcription. The feedback regulation explains the bistable and stochastic nature of white-opaque switching.feedback regulation ͉ a1-␣2 repression C andida albicans is associated with humans both as a harmless commensal organism and a pathogen. In most humans, it is a normal part of the microflora in the gastrointestinal tract and in other parts of the body. However, when the host immune system is compromised, it can cause mucosal infections and life-threatening disseminated infections. The ability of C. albicans to undergo a yeast-to-hypha transition and high-frequency phenotypic switching provides the organism with a high degree of phenotypic diversity and the adaptability that is necessary to survive in different host environments (1). One of the high-frequency switching systems is white-opaque switching. White-opaque switching was first identified in a clinical isolate, WO-1, in which cells switched spontaneously between white and opaque phases (2). White phase cells appear round and form hemispherical, white colonies on solid agar, and opaque phase cells are elongated in cell shape with pimples on the surface and form flat, opaque colonies. Several features of white-opaque switching are particularly interesting (1, 3): (i) the transition involves only two phases, and there is not an intermediate phase, (ii) the transition is stochastic, generating heterogeneity in a cell population, and (iii) the phase is stable for many cell divisions, with daughter cells inheriting the phase of their mother cells. White-opaque switching occurs at a frequency of 10 Ϫ4 to 10 Ϫ5 per cell division, and opaque-white switching happens at a frequency of 5 ϫ 10 Ϫ4 per cell generation (4). Opaque cells are stable at 25°C. Upon shifting the temperature to 37°C, they switch en masse to white cells (5).White-opaque switching is under a1-␣2 repression (6). In C. albicans, cell type is determined by the MTL (mating type-li...

First described in 2009 in Japan, the emerging multidrug-resistant fungal pathogen Candida auris is becoming a worldwide public health threat that has been attracting considerable attention due to its rapid and widespread emergence over the past decade. The reasons behind the recent emergence of this fungus remain a mystery to date. Genetic analyses indicate that this fungal pathogen emerged simultaneously in several different continents, where 5 genetically distinct clades of C . auris were isolated from distinct geographical locations. Although C . auris belongs to the CTG clade (its constituent species translate the CTG codon as serine instead of leucine, as in the standard code), C . auris is a haploid fungal species that is more closely related to the haploid and often multidrug-resistant species Candida haemulonii and Candida lusitaniae and is distantly related to the diploid and clinically common fungal pathogens Candida albicans and Candida tropicalis . Infections and outbreaks caused by C . auris in hospitals settings have been rising over the past several years. Difficulty in its identification, multidrug resistance properties, evolution of virulence factors, associated high mortality rates in patients, and long-term survival on surfaces in the environment make C . auris particularly problematic in clinical settings. Here, we review progress made over the past decade on the biological and clinical aspects of C . auris . Future efforts should be directed toward understanding the mechanistic details of its biology, epidemiology, antifungal resistance, and pathogenesis with a goal of developing novel tools and methods for the prevention, diagnosis, and treatment of C . auris infections.

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.

Summary To mate, Candida albicans, must undergo homozygosis at the mating type like locus, MTL [1, 2], then switch from the white to opaque phenotype [3, 4]. Paradoxically, when opaque cells are transferred in vitro to 37 °C, the temperature of their animal host, they switch en masse to white [5–7], suggesting that their major niche may not be conducive to mating. It has been suggested that pheromones secreted by opaque cells of opposite mating type [8] or the hypoxic condition of host niches [9, 10] stabilize opaque cells. There is, however, an additional possibility, namely that CO2, which achieves levels in the host 100 times higher than in air [11–14], stabilizes the opaque phenotype. CO2 has been demonstrated to regulate the bud-hypha transition in C. albicans [15], expression of virulence genes in bacteria [16] and mating events in Cryptococcus neoformans [17]. Here we have tested the possibility that CO2 stabilizes the opaque phenotype. It is demonstrated that physiological levels of CO2 induce white-to-opaque switching and stabilize the opaque phenotype at 37 °C. It exerts this control equally under anaerobic and aerobic conditions. These results suggest that the high levels of CO2 in the host induce and stabilize the opaque phenotype, thus facilitating mating.

The emerging human fungal pathogen Candida auris has been recognized as a multidrug resistant species and is associated with high mortality. This fungus was first described in Japan in 2009 and has been reported in at least 18 countries on five continents. In this study, we report the first isolate of C. auris from the bronchoalveolar lavage fluid (BALF) of a hospitalized woman in China. Interestingly, this isolate is susceptible to all tested antifungals including amphotericin B, fluconazole, and caspofungin. Copper sulfate (CuSO4) also has a potent inhibitory effect on the growth of this fungus. Under different culture conditions, C. auris exhibits multiple morphological phenotypes including round-to-ovoid, elongated, and pseudohyphal-like forms. High concentrations of sodium chloride induce the formation of a pseudohyphal-like form. We further demonstrate that C. auris is much less virulent than Candida albicans in both mouse systemic and invertebrate Galleria mellonella models.

This study describes a novel “white-gray-opaque” tristable phenotypic switching system in the human fungal pathogen Candida albicans, revealing additional complexity in this organism's ability to adapt to changing environments.

All Mating Type Locus strain types of Candida albicans show white-opaque switching competency, not just MTL homozygotes, which allows them to adapt better to environmental changes.

Similar multicellular structures can evolve within the same organism that may have different evolutionary histories, be controlled by different regulatory pathways, and play similar but nonidentical roles. In the human fungal pathogen Candida albicans, a quite extraordinary example of this has occurred. Depending upon the configuration of the mating type locus (a/α versus a/a or α/α), C. albicans forms alternative biofilms that appear similar morphologically, but exhibit dramatically different characteristics and are regulated by distinctly different signal transduction pathways. Biofilms formed by a/α cells are impermeable to molecules in the size range of 300 Da to 140 kDa, are poorly penetrated by human polymorphonuclear leukocytes (PMNs), and are resistant to antifungals. In contrast, a/a or α/α biofilms are permeable to molecules in this size range, are readily penetrated by PMNs, and are susceptible to antifungals. By mutational analyses, a/α biofilms are demonstrated to be regulated by the Ras1/cAMP pathway that includes Ras1→Cdc35→cAMP(Pde2—|)→Tpk2(Tpk1)→Efg1→Tec1→Bcr1, and a/a biofilms by the MAP kinase pathway that includes Mfα→Ste2→ (Ste4, Ste18, Cag1)→Ste11→Hst7→Cek2(Cek1)→Tec1. These observations suggest the hypothesis that while the upstream portion of the newly evolved pathway regulating a/a and α/α cell biofilms was derived intact from the upstream portion of the conserved pheromone-regulated pathway for mating, the downstream portion was derived through modification of the downstream portion of the conserved pathway for a/α biofilm formation. C. albicans therefore forms two alternative biofilms depending upon mating configuration.