Membrane fluidity in Bacillus subtilis. Validity of homeoviscous adaptation.

The validity of the principle of homeoviscous adaptation for Bacillus subtilis was tested by comparing fluorescence anisotropy (1,6-diphenyl-1,3,5-hexatriene) and electron-spin resonance (16-doxylstearate) measurements carried out in isolated plasma membranes and in phospholipid fractions. The physical measurements were supplemented by fatty-acid analysis. The results support our previous findings on intact cells. The thermoadaptive mechanism of B. subtilis manifested as an increase in relative proportion of branched anteiso-C15 and anteiso-C17 fatty acids, are not strong enough to compensate for the marked physical change of membrane fluidity induced by temperature decrease.