Wall material properties of yeast cells. Part II. Analysis

In the preceding paper (Part I) force-deformation data were measured with t he compression experiment in conjunction with the initial radial stretch ra tio and the initial wall-thickness to cell-radius ratio for baker's yeast ( Saccharomyces cerevisiae). In this paper, these data have been analysed wit h the mechanical model of Smith et al. (Smith, Moxham & Middelberg (1998) C hemical Engineering Science, 53, 3913-3922) with the wall constitutive beha viour defined a priori as incompressible and linear-elastic. This analysis determined the mean Young's modulus ((E) over bar), mean maximum von Mises stress-at-failure (<(sigma)over bar>(VM,f)) and mean maximum von Mises stra in-at failure (<(epsilon)over bar>(VM,f)) to be (E) over bar = 150 +/- 15 M Pa, <(sigma)over bar>(VM,f) = 70 +/- 4 MPa and <(epsilon)over bar>(VM,f) = 0.75 +/- 0.08, respectively. The mean Young's modulus was not dependent (P greater than or equal to 0.05) on external osmotic pressure (0-0.8 MPa) nor compression rate (1.03-7.68 mu m/s) suggesting the incompressible linear-e lastic relationship is representative of the actual cell-wall constitutive behaviour. Hydraulic conductivities were also determined and were comparabl e to other similar cell types (0-2.5 mu m/MPa s). The hydraulic conductivit y distribution was not dependent on external osmotic pressure (0-0.8 MPa) n or compression rate (1.03-7.68 mu m/s) suggesting inclusion of cell-wall pe rmeability in the mechanical model is justified. <(epsilon)over bar>(VM,f) was independent of cell diameter and to a first-approximation unaffected (P greater than or equal to 0.01) by external osmotic pressure and compressio n rate, thus providing a reasonable failure criterion. This criterion state s that the cell-wall material will break when the strain exceeds <(epsilon) over bar>(VM,f) = 0.75 +/- 0.08. Variability in overall cell strength durin g compression was shown to be primarily due to biological variability in th e maximum von Mises strain-at-failure. These data represent the first estim ates of cell-wall material properties for yeast and the first fundamental a nalysis of cell-compression data. They are essential for describing cell-di sruption at the fundamental level of fluid-cell interactions in general bio processes. They also provide valuable new measurements for yeast-cell physi ologists. (C) 2000 Elsevier Science Ltd. All rights reserved.