Abstract
Candida albicans is a major human fungal pathogen. One of the important features of C. albicans pathogenicity is the ability to form biofilms on mucosal surfaces and indwelling medical devices. Biofilm formation involves complex processes in C. albicans, including cell adhesion, filamentous growth, extracellular matrix secretion and cell dispersion. In this work, we characterized the role of the transcription factor Sfp1, particularly with respect to its function in the regulation of biofilm formation. The deletion of the SFP1 gene enhanced cell adhesion and biofilm formation in comparison to the wild type strain. Interestingly, the sfp1-deleted mutant also exhibited an increase in the expression of the ALS1, ALS3 and HWP1 genes, which encode adhesin proteins. In addition, Sfp1 was demonstrated to function downstream of the Rhb1-TOR signaling pathway. Bcr1 and Efg1 are transcription factors that are critical for controlling biofilm formation, and Efg1 is also required for hyphal growth. Deleting either the BCR1 or EFG1 gene in the sfp1-null background led to reduced adhesin gene expression. As a result, the bcr1/sfp1 or efg1/sfp1 double deletion mutants exhibited dramatically reduced biofilm formation. The results indicated that Sfp1 negatively regulates the ALS1, ALS3 and HWP1 adhesin genes and that the repression of these genes is mediated by the inhibition of Bcr1 and Efg1. In the second part of this study, we focused on the relationship among Sfp1, cell wall integrity and resistance to caspofungin, a cell wall inhibiting drug. Because the adhesion ability of the sfp1△/sfp1△ strain is better than wild type, the cell wall properties were investigated. Our studies revealed that the polysaccharide contents and thickness of the cell wall in the sfp1△/sfp1△ mutant are increased compared to wild type. Moreover, the sfp1△/sfp1△ strain exhibited up-regulation of the expression of FKS1, which encodes β-1, 3-glucan synthase. The results also showed that the sfp1△/sfp1△ strain increases cell wall integrity to reduce the susceptibility of caspofungin. Furthermore, the sfp1△/sfp1△ strain had a higher CAS5 gene expression to protect cells from damage. Deletion of the CAS5 gene in the sfp1△/sfp1△ background caused the cells to be hypersensitive to caspofungin. Although caspofungin had a high candidacidal activity against C. albicans planktonic and biofilm cells, the sfp1△/sfp1△ mutant not only enhanced its biofilm formation but also increased caspofungin resistance. Finally, several C. albicans clinical isolates reduced caspofungin susceptibility compared to the laboratory wild type strain. Interestingly, these clinical isolates became more susceptibility to caspofungin when SFP1 was overexpressed. These data suggest that Sfp1 may also play a role in caspofungin drug resistance in the clinics.