Role of dimorphism in the development of Candida albicans biofilms

Two model biofilm systems, involving growth on disks of catheter material o r on cylindrical cellulose filters, were used to investigate the structure of Candida albicans biofilms. To assess the importance of dimorphism in bio film development, biofilms produced by two wild-type strains were compared with those formed by two morphological mutants, incapable of yeast and hyph al growth, respectively. Scanning electron microscopy and thin sections of biofilms examined by light microscopy revealed that biofilms of the wild-ty pe strains formed on catheter disks consisted of two distinct layers: a thi n, basal yeast layer and a thicker, but more open, hyphal layer. The hypha( -) mutant produced only the basal layer, whereas the yeast(-) mutant formed a thicker, hyphal biofilm equivalent to the outer zone of the wild-type st ructures. Biofilms of the yeast- mutant were more easily detached from the catheter surface than the others, suggesting that the basal yeast layer has an important role in anchoring the biofilm to the surface. Biofilms formed on cylindrical cellulose filters were quite different in appearance. The h ypha(-) mutant and both wild types produced exclusively yeast-form biofilms whereas the yeast(-) mutant generated a dense hyphal mat on the top of the filter. All these biofilms, irrespective of morphological form, were resis tant to the antifungal agent, amphotericin B. Overall, these results indica te that the structure of a C. albicans biofilm depends on the nature of the contact surface, but that some surfaces produce biofilms with a layered ar chitecture resembling to that described for bacterial systems.