Correlation between thermal death and membrane fluidity in Bacillus stearothermophilus.

Paramagnetic resonance spectra of spin labels partitioned into spheroplast membranes of Bacillus stearothermophilus indicate lateral lipid phase separations. Cells adjust their lipid composition in response to temperature changes so that the same change of state in membrane phospholipids is achieved at the respective growth temperature. A temperature-sensitive mutant that fails to change its lipid composition above a certain temperature can survive only up to the higher temperature boundary for lateral phase separation. These data are interpreted to indicate that the maximal and minimal growth temperatures of thermophiles are regulated by the onset and conclusion of phase separations of the particular lipid composition they synthesize. It is suggested that isolated lipid domains are required for functional membrane assembly.