Relationship of growth temperature and thermotropic lipid phase changes in cytoplasmic and outer membranes from Escherichia coli K12

Purified cytoplasmic and outer membranes isolated from cells of wild type Escherichia coli grown at 12, 20, 37 and 43°C were labelled with the fatty acid spin probe 5-doxyl stearate. Electron spin resonance spectroscopy revealed broad thermotropic phase changes. The inherent viscosity of both membranes was found to increase as a function of elevated growth temperature. The lipid order to disorder transition in the outer membrane but not the cytoplasmic membrane was dramatically affected by the temperature of growth. As a result, the cytoplasmic membrane presumably existed in a gel + liquid crystalline state during cellular growth at 12 and 20°C, but in a liquid crystalline state when cells were grown at 37 and 43°C. In contrast, the outer membrane apparently existed in a gel + liquid crystalline state at all incubation temperatures. Data presented here indicate that the temperature range over which the cell can maintain the outer membrane phospholipids in a mixed (presumedly gel + liquid crystalline) state correlates with the temperature range over which growth occurs.