Bacterial membranes and lipid packing theory

Recent physical studies on the lipids of biological membranes have emphasized the potential instability of the lamellar phase of mixtures of lipids containing unsaturated species of phosphatidylethanolamine, plasmenylethanolamine, or monoglycosyldiacylglycerols, all of which are important constituents of the membranes of different groups of prokaryotes. The polar lipid compositions of bacteria are examined in terms of lipid packing theory. This survey reveals that gram-negative species with high proportions of unsaturated fatty acids (greater than 65%), often have phosphatidylcholine (PC), in addition to the more common phosphatidylethanolamine (PE), phosphatidylglycerol, and cardiolipin. Physical studies have shown that PC is capable of inducing the bilayer phase when added to unsaturated PE. Many bacteria that are rich in unsaturated fatty acids and contain PC, have intracytoplasmic membrane systems (ICM), and the potential role of bilayer instability in the formation of ICM is discussed. Two groups of bacteria that are either natural fatty acid auxotrophs or utilize exogenous fatty acids when endogenous synthesis is inhibited, Acholeplasma laidlawii and the butyric acid-producing clostridia, are capable of adjusting their lipid class compositions according to the degree of unsaturation of their lipid aliphatic chains. Lipid class composition is also affected by growth temperature in both groups of organisms, and by incorporation of cholesterol in A. laidlawii. As the content of cis-unsaturated fatty acids or temperature is increased, lipids that form an unstable lamellar phase at physiological temperatures are replaced with lipids that have larger effective polar head groups, and can therefore form more stable bilayers.