Relating assembly and function of the Candida albicans biofilm extracellular matrix.

摘要

As a biofilm, microbes produce an extracellular matrix that confers protection from the surrounding environment. For pathogenic microorganisms, this protection is manifested as high levels of drug resistance, making biofilm-related infections incredibly difficult to eradicate. Elucidation of matrix biogenesis mechanisms thus addresses an interesting biological question as well as an urgent medical need.;The most common hospital-acquired fungal pathogen, Candida albicans, frequently forms biofilms on implanted medical devices, often leading to lethal disseminated disease. The intrinsic resistance of biofilms is multi-factorial, but is due largely to the extracellular matrix encasing the biofilm cells. Prior studies indicate that the matrix is complex, with major polysaccharide constituents alpha-mannan, beta-1,6 glucan, and beta-1,3 glucan.;This work implemented multiple approaches to unravel the contributions of the three polysaccharides to matrix structure and function. Inhibiting synthesis of any one polysaccharide altered quantities of the others. Each polysaccharide was also required for matrix function, as assessed by assays for antifungal sequestration. The role of these matrix polysaccharides in drug resistance is similar for other clinically-relevant Candida species, suggesting the possibility of conserved matrix structure and function.;These results indicate that matrix biogenesis entails coordinated delivery and assembly of the individual polysaccharides. To ask whether this occurs inside or outside the biofilm cells, matrix-defective mutant strains were evaluated for functional matrix production in biofilm co-culture. These mixed biofilms, inoculated with mutants containing a disruption in each polysaccharide pathway, had restored mature matrix structure, composition, and biofilm drug resistance. These results argue that functional matrix biogenesis is coordinated extracellularly, and thus reflects the cooperative actions of the biofilm community.;A mechanism for the delivery of matrix materials by extracellular vesicles (EVs) is also newly defined here. Variants of EVs have been identified in cell types across biological domains, with diverse functions including unconventional secretion of macromolecules and intercellular signaling. EVs were isolated from Candida albicans biofilms, and found to contain carbohydrates as well as proteins involved in polysaccharide remodeling. Mutant biofilms deficient in EV production had increased drug susceptibility, supporting the notion that these structures are important for the accumulation and extracellular assembly of functional matrix.