Candida albicans Forms a Specialized “Sexual” as Well as “Pathogenic” Biofilm

Abstract

Candida albicans forms two types of biofilm in RPMI 1640 medium, depending upon the configuration of the mating type locus. In the prevalent a/␣ configuration, cells form a biofilm that is impermeable, impenetrable by leukocytes, and fluconazole resistant. It is regulated by the Ras1/cyclic AMP (cAMP) pathway. In the a/a or ␣/␣ configuration, white cells form a biofilm that is architecturally similar to an a/␣ biofilm but, in contrast, is permeable, penetrable, and fluconazole susceptible. It is regulated by the mitogen-activated protein (MAP) kinase pathway. The MTL-homozygous biofilm has been shown to facilitate chemotropism, a step in the mating process. This has led to the hypothesis that specialized MTL-homozygous biofilms facilitate mating. If true, then MTL-homozygous biofilms should have an advantage over MTL-heterozygous biofilms in supporting mating. We have tested this prediction using a complementation strategy and show that minority opaque a/a and ␣/␣ cells seeded in MTLhomozygous biofilms mate at frequencies 1 to 2 orders of magnitude higher than in MTL-heterozygous biofilms. No difference in mating frequencies was observed between seeded patches of MTL-heterozygous and MTL-homozygous cells grown on agar at 28°C in air or 20% CO 2 and at 37°C. Mating frequencies are negligible in seeded patches of both a/␣ and a/a cells, in contrast to seeded biofilms. Together, these results support the hypothesis that MTL-homozygous (a/a or ␣/␣) white cells form a specialized "sexual biofilm."

Plasticity of Candida albicans Biofilms

“…Interestingly, white cells, which themselves cannot normally mate, play a role in mating. White cells form robust biofilms that are architecturally similar to the biofilms formed by a/␣ cells but differ functionally in that they support mating between minority opaque cells (24,38,330). Opaque cells, which themselves cannot form a biofilm, play a stimulatory role in the formation of MTL-homozygous white cell biofilms (39).…”

“…The characteristics of MTL-heterozygous biofilms, including impermeability to high-and low-molecular-weight molecules, including the azole fluconazole, and impenetrability by white blood cells (39,183,184,251), are compatible with a pathogenic role in the life history of this commensal but incompatible with the generation of pheromone gradients that direct chemotropism in the mating process and the directed extension of conjugation tubes during chemotropism (33,38,52,82,330). In contrast, the characteristics of MTL-homozygous biofilms, including permeability to low-and high-molecular weight molecules, including fluconazole, and penetrability by white blood cells, are incompatible with a pathogenic role but compatible with chemotropism, and hence a sexual role.…”

“…Second, opaque cells signal white cells, through the release of pheromone, to form a white cell biofilm, employing the same pheromone receptors, trimeric G protein complex, and MAP kinase pathway as those that regulate mating between opaque cells, but with different target transcription factors (243,319,332). Third, and most importantly, when minority a/a and ␣/␣ opaque cells are seeded in majority a/a and/or ␣/␣ white cell biofilms, they mate 50 to 100 times more frequently than minority a/a and ␣/␣ opaque cells seeded in majority a/␣ biofilms (330). Furthermore, Daniels et al (38) showed, by vitally staining a/a and ␣/␣ cells with tags of different colors, that an MTL-homozygous white cell biofilm supports chemotropism by conjugation tubes, which leads to fusion (Fig.…”

Role of Tec1 in the Development, Architecture, and Integrity of Sexual Biofilms of Candida albicans

“…Under exactly the same conditions, a/a and ␣/␣ cells in the white phase of the whiteopaque transition (6) form biofilms that have the same architecture and integrity as those of a/␣ biofilms but differ in that they are readily penetrated by human phagocytic white blood cells, susceptible to fluconazole, and permeable to low-and high-molecularweight molecules (1,5). MTL-heterozygous and MTL-homozygous biofilms also differ in the capacity to support mating (2,7). MTL-homozygous biofilms support mating of seeded minority opaque cells at 10 to over 100 times the frequency of that of MTLheterozygous biofilms (7).…”

Budding off: bringing functional genomics toCandida albicans

“…Biofilms are three-dimensional communities of cells surrounded by an extracellular matrix, and play important roles in device-associated infections and drug resistance by C. albicans strains [81,82]. EFG1 is a central regulator of both the white-opaque circuit and the biofilm circuit, indicating a mechanistic connection between the two programs [83,84]. Investigation of regulatory circuits such as these is providing new insights into the evolution of genetic networks, and reveals that complex networks are not necessarily a consequence of optimization by natural selection, but can instead arise via mostly nonadaptive mutations [85].…”