The lipid membrane properties of a moderately halophilic gram-negative bacterium, Pseudomonas halosaccharolytica ATCC 29423, were studied by the use of stearate spin labels, 5 NS, 12 NS, and 16 NS, changing the temperature of ESR measurement from 15 to 50°C. The order parameter and the rotational correlation time of the spin labels incorporated into intact cell membranes of this bacterium grown at various temperatures in media containing different NaCl concentrations were calculated. The activation energy of rotational microviscosity was obtained from Andrade plots.
At low growth temperature and low NaCl concentration in the medium, extractable lipids of this bacterium contained comparatively large amounts of unsaturated fatty acids, but as the growth temperature and NaCl concentration in the medium increased, the contents of saturated and cyclopropanoic fatty acids increased to more than half of the total fatty acids. 5 NS gave the highest order parameters for the intact cells of this bacterium, while 12 NS gave lower and 16 NS gave the lowest results. The order parameters of 5 NS, 12 NS, and 16 NS were completely separated, and all order parameters decreased gradually as the measuring temperature was increased. In contrast, the rotational correlation times of the intact cells with 12 NS were as large as those with 5 NS, while those with 16 NS were distinctly smaller. Increasing NaCl concentrations in the growth medium caused an increase of the rotational correlation times, that is, stiffened the lipid bilayers. The Andrade plot for 16 NS was approximately a straight line, whereas 5 NS and 12 NS gave two straight lines crossing at a temperature near the growth temperature, indicating phase transition from solid to liquid. The microviscosity activation energies were 5-10 kcal/mol in the liquid phase and 15-25 kcal/mol in the solid phase.