Abstract
Candida albicans biofilm formation is governed by a regulatory circuit comprising nine transcription factors which control a network of target genes. However, there are still unknown genes contributing to biofilm features. Thus, the GRACE library was screened to identify genes involved in mature biofilm development. Twenty-nine conditional mutants were selected for a second screening revealing three groups of genes: twenty- two conditional mutants were defective for normal growth and unable to form biofilms; six strains, conditionally defective in genes ARC40, ARC35, ORF19.2438, SKP1, ERG6, and ADE5,7 that are likely essential or involved in general cell processes, grew normally as free-floating cells but produced less biofilm; finally, the conditional strain for a putative essential isoleucyl- tRNA synthetase gene, ILS1, was unable to grow as yeast-phase cells but was capable of producing a tridimensional biofilm structure in spite of reduced metabolic activity. This unique biofilm still relied on the classical biofilm genes, while it differentially induced groups of genes involved in adhesion, protein synthesis, cell wall organization, and protein folding. Although the conditional mutant repressed genes annotated for morphology and homeostasis processes affecting morphology and metabolism, the dynamic cell growth enabled the formation of a complex biofilm community independent of ILS1.
Highlights
Candida albicans is a commensal fungus and a common component of the human microbiota of mucosal surfaces
One-third of the C. albicans genome was submitted to a large-scale genetic analysis of biofilm formation
Biofilm suppressor genes might have been under-represented in our study because the medium chosen to promote biofilm growth in this study supports the formation of robust biofilms which can reach a saturation threshold and hide overgrowth phenotypes[24,25]
Summary
Candida albicans is a commensal fungus and a common component of the human microbiota of mucosal surfaces. On the host surface or on medical devices such as catheters and prostheses, C. albicans can form a monospecies biofilm composed of yeast, pseudohyphae, and hyphal cells embedded in a self-produced extracellular matrix consisting of proteins, carbohydrates, lipids, and nucleic acids[2,3]. A regulatory transcriptional network involves six transcription factors (Efg[1], Tec[1], Bcr[1], Ndt[80], Brg[1], and Rob1) that positively regulate each other’s expression and globally control the expression of about 1,000 target genes[8] These target genes are implicated in important characteristics of biofilm formation such as hyphal formation, adhesion, drug resistance, and the production of extracellular matrix as well as processes unrelated to biofilms[3,8,10]. The expression level of a gene is not predictive of a given phenotype for a strain possessing mutations in the same environmental condition[18]
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