There is an increase in unsaturated fatty acid residues. The usual lipid content of most bacteria is between 2% and 5%, most or all of which is in the cell membrane. Bacterial fats are glycerol esters of two types: neutral lipids, in which all three or only one or two of the –OH groups of glycerol are esterified with long-chain fatty acids; and phospholipids, in which one of the –OH groups is linked through a phosphodiester bond to choline, ethanolamine, glycerol, inositol, or serine.
The other two –OH groups are esterified with long-chain fatty acids.58 Many psychrotrophs synthesize neutral lipids and phospholipids containing an increased proportion of unsaturated fatty acids when grown at low temperatures compared with growth at higher temperatures. As much as a 50% increase in the content of unsaturated bonds of fatty acids from mesophilic and psychrotrophic
Candida spp. was found in cells grown at 10◦C compared to 25◦C.36 The phospholipid composition of these yeasts was unchanged. The increase in unsaturated fatty acids in Candida utilis as growth temperatures were lowered from 30◦C to 5◦C is shown in Table 16–6; linolenic acid increased at the expense of oleic acid at the lower temperatures.
The widespread occurrence of low-temperature-induced changes in fatty acid composition suggests that they are associated with physiological mechanisms of the cell. It is known that an increase in the degree of unsaturation of fatty acids in lipids leads to a decrease in the lipid melting point.
It has been suggested that increased synthesis of unsaturated fatty acids at low temperatures has the function of maintaining the lipid in a liquid and mobile state, thereby allowing membrane activity to continue to function. This concept, referred to as the lipid solidification theory, was first proposed by Gaughran20 and Allen.3 It has been shown by Byrne and Chapman10 that the melting point of fatty-acid side chains in lipids is more important than the entire lipid structure