Bacterial cell membrane models: choosing the lipid composition

The reasons for the wide diversity of lipids found in natural cell membranes are still not fully understood but could potentially be exploited in treating disease and infection. This study aims to establish whether charge alone or specific chemical structure of an anionic lipid headgroup determines the structure and properties of model bacterial cell membranes. We compare different compositions of a zwitterionic lipid di-myristoyl phosphatidylethanolamine (DMPE) and two anionic lipids, di-myristoyl phosphatidylglycerol (DMPG) and tetra-myristoyl cardiolipin (TMCL). TMCL has a distinct condensing effect, increasing packing and decreasing the pressures of the phase transitions. Although relatively well solvated itself, TMCL does not substantially alter the solvation of mixed monolayers or bilayers. DMPE:TMCL mixtures have very similar electrochemical behaviour to mixtures of DMPE with di-myristoyl phosphatidylserine (DMPS) but DMPE:DMPG bilayers have greater surface charges. A ternary mixture representing an Escherichia coli membrane has similar electrochemical response to but is more tightly packed than DMPE:DMPG. These results establish the importance of the anionic lipid in modelling different types of cell membranes: DMPG will be required in model bacterial membranes and should not be replaced with DMPS. Even very small amounts of CL will have a measurable effect on structure, so its inclusion is important. Our results also highlight the importance of diverse techniques in understanding membrane behaviour: reflectivity measurements of monolayers over a range of surface pressure provide excellent insight into the electrochemical responses of lipid bilayers, while surface diffraction and infrared spectroscopy are much more sensitive to differences in packing between lipids.