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 prevalenta /α 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 thea/a or α/α configuration, white cells form a biofilm that is architecturally similar to ana /α biofilm but, in contrast, is permeable, penetrable, and fluconazole susceptible. It is regulated by the mitogen-activated protein (MAP) kinase pathway. TheMTL -homozygous biofilm has been shown to facilitate chemotropism, a step in the mating process. This has led to the hypothesis that specializedMTL -homozygous biofilms facilitate mating. If true, thenMTL -homozygous biofilms should have an advantage overMTL -heterozygous biofilms in supporting mating. We have tested this prediction using a complementation strategy and show that minority opaquea/a and α/α cells seeded inMTL -homozygous biofilms mate at frequencies 1 to 2 orders of magnitude higher than inMTL -heterozygous biofilms. No difference in mating frequencies was observed between seeded patches ofMTL -heterozygous andMTL -homozygous cells grown on agar at 28°C in air or 20% CO2 and at 37°C. Mating frequencies are negligible in seeded patches of botha /α anda/a cells, in contrast to seeded biofilms. Together, these results support the hypothesis thatMTL -homozygous (a/a or α/α) white cells form a specialized “sexual biofilm.”