Assessment of Membrane Fluidity in Individual Yeast Cells by Laurdan Generalised Polarisation and Multi-Photon Scanning Fluorescence Microscopy

Here we describe techniques that we developed for monitoring membrane fluidity of individual yeast cells during environmental adaptation and physiological changes. Multiphoton scanning fluorescence microscopy using laurdan as a membrane probe enables determination whether fluidity changes seen by spectroscopy reflect universal responses or changes only of sub-populations. Yeast membranes are a primary site of environmental response and adaptation. Using fluorescence spectroscopy with DPH polarization and laurdan Generalized Polarization (GP), we previously found rapid “average” membrane fluidity modulation in yeast populations during growth and in response to nutrients or environmental stresses. To determine whether such responses reflect all cells we conducted the first multi-photon scanning fluorescence microscopy study of yeasts, measuring laurdan GP. We assessed membrane fluidity responses of individual yeasts related to growth phase, heat stress and ethanol stress. Average fluidity decreased as cultures aged, however the decreased fluidity was due in some cases to an increasing proportion of uniformly low fluidity (high GP) cells, which were shown by vital dye to be dead. When yeasts were heat stressed, the mean laurdan GP increased in all cells, thus the entire population evidenced damage (viz. decreased membrane fluidity) to the same degree. On the other hand, with ethanol stress fluidity increased (GP decreased) on exposure of cells. All cells were affected although not to the same degree, and with variable recovery. The recovery assessed from GP microscopy was highly variable, and greater by that seen by spectroscopy.