Nanoscale Mapping of the Elasticity of Microbial Cells by Atomic Force Microscopy

Abstract

Single microbial cells can show important local variations of elasticity due to the complex, anisotropic composition of their walls. An example of this is the yeast during cell division, where chitin is known to accumulate in the localized region of the cell wall involved in budding. We used atomic force microscopy (AFM) to measure quantitatively the local mechanical properties of hydrated yeast cells. Topographic images and spatially resolved force maps revealed significant lateral variations of elasticity across the cell surface, the bud scar region being significantly stiffer than the surrounding cell wall. To get quantitative information on sample elasticity, force curves were converted into force vs indentation curves. The curves were then fitted with the Hertz model, yielding Young's modulus values of 6.1 ± 2.4 and 0.6 ± 0.4 MPa for the bud scar and surrounding cell surface, respectively. These data lead us to conclude that in yeast, the bud scar is 10 times stiffer than the surrounding cell wall, a finding which is consistent with the accumulation of chitin in the bud scar region. This is the first report in which spatially resolved AFM force curves are used to distinguish regions of different elasticity at the surface of single microbial cells in relation with function (i.e., cell division). In future research, this approach will provide fundamental insights into the spatial distribution of physical properties at heterogeneous microbial cell surfaces.