Candida albicans Forms a Specialized Sexual as Well as Pathogenic Biofilm

Candida albicans forms two types of biofilm in RPMI 1640 medium, depending upon the configuration of the mating type locus. In the prevalent a/alpha 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 alpha/alpha configuration, white cells form a biofilm that is architecturally similar to an a/alpha 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 alpha/alpha cells seeded in MTL-homozygous 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 degrees C in air or 20% CO2 and at 37 degrees C. Mating frequencies are negligible in seeded patches of both a/alpha and a/a cells, in contrast to seeded biofilms. Together, these results support the hypothesis that MTL-homozygous (a/a or alpha/alpha) white cells form a specialized sexual biofilm.