Question

4. (6 pts) Membrane composition E. coli bacteria can be cultured at different temperatures. How does the fatty acid composition in membranes of E. coli grown at 20 °C compare with the composition in membranes of bacteria grown at 37 °C? Explain.

Biochemistry: Membranes/metabolism

Answer 1

Variations in the temperature of growth and in the composition of the medium alter the proportions of individual fatty acids in the lipids of Escherichia coli. As the temperature of growth is 20 degree celsius, the proportion of unsaturated fatty acids (hexadecenoic and octadecenoic acids) increases. The increase in content of unsaturated acids with a decrease in temperature of growth occurs in both minimal and complex media. Cells harvested in the stationary phase contained large amounts of cyclopropane fatty acids (methylenehexadecanoic and methylene octadecanoic acids) in comparison with cells harvested during exponential growth. Cells grown in a chemostat, limited by the concentration of ammonium salts, show a much higher content of saturated fatty acids (principally palmitic acid) than do cells harvested from an exponentially-growing batch culture in the same medium. Cells grown in a chemostat, limited by the concentration of glucose, show a slightly higher content of unsaturated fatty acids than cells from the corresponding batch culture. The results do not indicate a direct relation between fatty acid composition and minimal growth temperature. As growth temperature is twenty degree celsius, the proportion of unsaturated acid increases. The percentage of the most abundant unsaturated fatty acid, octadecenoic acid, decreases continuously with growth temperature at 37 degree Celsius over the entire growth range. The most abundant saturated fatty acid, palmitic acid, increases continuously with i growth temperature at 37 degee celsius. The highest rate of change of these fatty acids occurs at high temperature. The percentage of the cyclopropane fatty acid, methylene hexadecanoic acid, also decreases with decreasing growth temperature, and ,3-hydroxymyristic acid is almost constant. The percentage of hexadecenoic acid increases only slightly at low temperature, but decreases dramatically above 40 C. Ratios of unsaturated to saturated fatty acids (Table 1) serve as useful indices of the degree of shift in fatty acid composition of the cells; e.g., the hexadecenoicpalmitic and octadecenoic-palmitic acid ratios both decrease approximately eightfold over the temperature range of 10 to 43 C.