Interface of Candida albicans Biofilm Matrix-Associated Drug Resistance and Cell Wall Integrity Regulation

Candida albicans frequently infects medical devices by growing as a biofilm, i.e., a community of adherent organisms entrenched in an extracellular matrix. During biofilm growth, Candida spp. acquire the ability to resist high concentrations of antifungal drugs. One recently recognized biofilm resistance mechanism involves drug sequestration by matrix beta-1,3 glucan. Using a candidate gene approach, we investigated potential C. albicans beta-1,3-glucan regulators, based on their homology to Saccharomyces cerevisiae, including SMI1 and protein kinase C (PKC) pathway components. We identified a role for the SMI1 in biofilm matrix glucan production and development of the associated drug resistance phenotype. This pathway appears to act through transcription factor Rlmp and glucan synthase Fks1p. The phenotypes of these mutant biofilms mimicked those of the smi1 Delta/smi1 Delta biofilm, and overexpression of FKS1 in the smi1 Delta/smi1 Delta mutant restored the biofilm resistant phenotype. However, control of this pathway is distinct from that of the upstream PKC pathway because the pkc1 Delta/pkc1 Delta, bck1 Delta/bck1 Delta, mkk2 Delta/mkk2 Delta, and mkc1 Delta/mkc1 Delta biofilms retained the resistant phenotype of the parent strain. In addition, resistance to cell-perturbing agents and gene expression data do not support a significant role for the cell wall integrity pathway during the biofilm formation. Here we show that Smi1p functions in conjunction with Rlm1p and Fks1p to produce drug-sequestering biofilm beta-glucan. Our work provides new insight into how the C. albicans biofilm matrix production and drug resistance pathways intersect with the planktonic cell wall integrity pathway. This novel connection helps explain how pathogens in a multicellular biofilm community are protected from anti-infective therapy.