Investigating the mechanical properties of yeast cells

To predict cell breakage in bioprocessing it is essential to have an understanding of the cell wall mechanical properties. This project involved a study of the wall mechanical properties of individual Baker’s yeast cells (Saccharomyces cerevisiae) using compression testing by micromanipulation. An analytical model has been developed to describe the compression of a single yeast cell between flat parallel surfaces. Such cells were considered to be thin walled, liquid filled, spheres. Because yeast cells can be compressed at high deformation rates, time dependent effects such as water loss during compression and visco-elasticity of the cell wall could be and were neglected in the model. As in previously published work, a linear elastic constitutive equation was assumed for the material of the cell walls. However, yeast compression to failure requires large deformations, leading to high wall strains, and new model equations appropriate to such high strains were developed. It was shown that the preferred model, based on work-conjugate Kirchhoff stresses and Hencky strains, fitted Baker’s yeast compression data well up to cell failure. This agreement validated the modelling approach, which might also be useful in characterising the material properties of the walls of other cells and microcapsules. Using the analytical model, the effects of compression speed on the elastic modulus obtained by fitting numerical simulations to experimental compression data was investigated. It was found that above a compression speed of approximately 45 µms\(^{-1}\) the estimated elastic modulus was essentially unchanged. By using a compression speed of 68 µms\(^{-1}\) it could be assumed that water loss during compression was negligible. It was then possible to treat the initial stretch ratio and elastic modulus as adjustable parameters within the numerical simulation. In addition to this, as the numerical simulations fitted experimental data well up to the point of cell rupture, it was possible to extract cell wall failure criteria. This study has given mean cell wall properties for late stationary phase Baker’s yeast of: elastic modulus 185 ± 15 MPa, initial stretch ratio 1.039 ± 0.006, circumferential stress at failure 115 ± 5 MPa, circumferential strain at failure of 0.46 ± 0.03, and strain energy per unit volume at failure of 30 ± 3 MPa. Following this, the effect on the intrinsic material properties of treating Baker’s yeast with dithiothreitol (DTT) was investigated. DTT has the effect on Baker’s yeast cells of breaking the disulphide bonds in the cell wall releasing invertase into the suspending solution. It was found that this did not affect the intrinsic material properties or failure criteria. In addition to this, Baker’s yeast cells were mechanically perturbed by sonication and the resulting intrinsic material properties investigated. The surface modulus was found to decrease with increased sonication time while the surface strain energy at failure remained constant. However, it was not possible to determine the extent of damage to each individual cell, preventing explicit conclusions from being reached.