Structure, cell wall elasticity and polysaccharide properties of living yeast cells, as probedby AFM

Abstract

Although the chemical composition of yeast cell walls is known, the organization, assembly,and interactions of the various macromolecules remain poorly understood. Here, we used in situ atomic force microscopy (AFM) in three different modes to probe theultrastructure, cell wall elasticity and polymer properties of two brewing yeast strains, i.e. Saccharomyces carlsbergensis and S. cerevisiae. Topographic images of the two strainsrevealed smooth and homogeneous cell surfaces, and the presence of circular budscars on dividing cells. Nanomechanical measurements demonstrated that thecell wall elasticity of S. carlsbergensis is homogeneous. By contrast, the budscar of S. cerevisiae was found to be stiffer than the cell wall, presumably dueto the accumulation of chitin. Notably, single molecule force spectroscopy withlectin-modified tips revealed major differences in polysaccharide properties of the twostrains. Polysaccharides were clearly more extended on S. cerevisiae, suggestingthat not only oligosaccharides, but also polypeptide chains of the mannoproteinswere stretched. Consistent with earlier cell surface analyses, these findings mayexplain the very different aggregation properties of the two organisms. This studydemonstrates the power of using multiple complementary AFM modalities for probing theorganization and interactions of the various macromolecules of microbial cell walls.