When cells from cultures of strain FA-1 grown at 37 °C were exposed to incubation temperatures of 26 °C or less for 5 min or more, an extensive aggregation of particles was observed on the convex fracture faces of their freeze-cleaved membranes. Aggregation of particles was accompanied by a parallel increase in the activation energy for growth. By shifting the growth temperature from 37 to 24 °C for one doubling of culture mass, the transition temperature for membrane particle aggregation could be lowered from about 26 to 0 °C. Although membrane lipids became enriched with unsaturated fatty acids during this period of growth at 24 °C, this enrichment was not accompanied by an increased growth rate of the culture. However, the period of growth at 24 °C did result in bacteria that could grow more rapidly at 10 °C than could bacteria directly transferred from cultures grown at 37 °C. These observations suggest that the increase in membrane fluidity that occurs when bacteria are grown at 24 °C does not allow bacteria to grow faster at 24 °C, but rather allows them to adapt more readily to further decreases in growth temperature.


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