Membrane fluidity in Bacillus subtilis. Physical change and biological adaptation.

The thermotropic behaviour of membrane phospholipids was estimated in intact cells of Bacillus subtilis. Membrane fluidity (microviscosity) of intact cells depended markedly on the ambient temperature - increase in cultivation temperature led to an increase in membrane fluidity. Estimated as anisotropy of 1,6-diphenyl-1,3,5-hexatriene fluorescence, a 30% difference was observed when cells cultivated at 20 and 40 degrees C were compared. This lack of rigorous homeostatic control of bulk-phase lipid fluidity prompted the reevaluation of the physiological significance of the "homeoviscous adaptation" in B. subtilis.