Comprehending Physical Properties of Bacterial Mimic Model Membrane System
Cell membrane is a complex, heterogeneous and dynamic layer primarily composed of lipids and proteins. It is selectively permeable facilitating active and passive transport of molecules and it performs vital functions like molecular recognition and enzymatic catalysis. Bacteria are unicellular autonomous prokaryotes. Based on structural differences in the cell wall, bacteria are categorized as gram-positive and gram-negative. Gram-positive bacteria have one cytoplasmic membrane surrounding the cell with thick peptidoglycan whereas gram negative bacteria have an additional outer membrane composed mainly of lipopolysaccharide. Phosphatidyl-ethanolamine (PE) lipids are present in significant amounts in a bacterial membrane compared to a mammalian membrane. Other anionic lipids present in bacterial membranes include phophatidyl-glycerol (PG) and cardiolipin (CL). Comparatively, the plasma membrane in a mammalian cell contains a higher percentage of phosphatidyl-choline (PC) and sphingomyelin whereas phosphatidyl-serine (PS) and PE are found on inner leaflet in trivial amounts. Simpler models of biological membrane can be prepared with tailored size and composition to study the structure and functions of lipids and interactions of lipids with molecules such as proteins, drugs, surfactants etc. The well known mammalian membrane model systems include monolayer, vesicles and supported bilayers. Though studies have been carried out on trying to mimic bacterial membranes, their dynamical nature is not well established.
During the course of my summer research fellowship programme, I carried out an extensive study related to cell membranes and their dynamics along with experiments to elucidate the E.coli lipid monolayer prepared in LB trough and comprehending its physical properties such as compressibility by obtaining isotherms at different temperatures with water and NaCl as the subphase. Dynamics of E.coli lipid bilayer were planned to be extracted. Though the bilayer was prepared and imaged in the confocal microscope, fluorescence correlation spectroscopy (FCS) could not be carried out because it required some more optimization and has been planned for a future study. However, a basic dynamic study on DOPC lipid bilayer was carried out using FCS, and the diffusion coefficient was calculated which can be compared with the E.coli lipid dynamics in future.
Keywords: Gram positive, Gram negative, Isotherm, Compressibility, FCS.