The Regulation of Membrane Fluidity in Bacteria by Acyl Chain Length Changes

It has been appreciated for many years that certain organisms, notably poikilotherms including bacteria, alter the fatty acyl composition of their lipids in response to changes in environmental temperature. Publication of the “fluid mosaic model” of membrane structure (Singer and Nicolson, 1972) signaled a decade in which our comprehension of the physiological significance of these fatty acyl changes has grown very considerably. The application of physical techniques, such as electron spin resonance (ESR) and nuclear magnetic resonance, to membranes and their lipids has in particular shown how membrane fluidity is modulated by lipid acyl composition. These findings are embodied in the widely accepted “mosaic” model of membrane structure in which intrinsic proteins are embedded in or traverse a lipid bilayer; thus, lipid acyl chains are in contact with polypeptide chains and may regulate the activity of enzyme active sites directly or indirectly. The nature of the phospholipid (or glycolipid) head group may also be important in lipid-protein interactions. But much less is known about the role of head groups in membrane lipid fluidity, and this aspect will not be dealt with in this review (see Keough and Davis, this volume).