Observation of Oxidative Stress on Yeast Cells

Before the advent of the atomic force microscope (AFM), scanning electron microscopy (SEM) was used to obtain high-resolution visualizations of the surface of biological samples. Normally, to scan samples of yeast cells, each preparation was coated with a film of evaporated gold approx 20 nm in thickness (1 ,2 ). Although necessary for scanning, the application of gold to the sample resulted in distortions in its surface. In addition, the application of a conductive coating to the surface effectively masked all the information that can exist below the gold film. The AFM apparatus permits the observation of samples without the use of this mask (samples are uncoated and nonfixed). If we compare the thickness of the gold coating to the thickness of the yeast cell wall (Saccharomyces cerevisiae cell wall is about 25 nm; ref. 3 ), we find that they have approximately the same dimensions, which results in loss of resolution from the surface of the cells, including any changes that might occur on the cell wall. With improvements in AFM technology, it became possible to examine the surface of many preparations at much greater resolutions than previously described (4 ). Recently, it has become possible to observe, with AFM, that the surface of the cell wall of S. cerevisiae contains natural undulations (rugosities) never described when SEM was used (4 ) and that these cell walls contain pores along the surface that vary from strain to strain (4 ). With AFM is also possible to observe pores on membrane of others eukaryotic cells (5 ).