High-Resolution Atomic Force Microscopy Studies of the Escherichia coli Outer Membrane:  Structural Basis for Permeability

The structural basis of the outer membrane permeability for the bacterium Escherichia coli is studied by atomic force microscopy (AFM) in conjunction with biochemical treatment and analysis. The surface of the bacterium is visualized with unprecedented detail at 50 and 5 Å lateral and vertical resolutions, respectively. The AFM images reveal that the outer membrane of native E . coli exhibits protrusions that correspond to patches of lipopolysaccharide (LPS) containing hundreds to thousands of LPS molecules. The packing of the nearest neighbor patches is tight, and as such the LPS layer provides an effective permeability barrier for the Gram-negative bacteria. Treatment with 50 mM EDTA results in the release of LPS molecules from the boundaries of some patches. Further metal depletion produces many irregularly shaped pits at the outer membrane, which is the consequence of progressive release of LPS molecules and membrane proteins. The EDTA-treated cells were analyzed for metal content and for their reactivities toward lysozyme and antibodies specific for LPS. The experiments collectively indicate that the metal depletion procedure did not remove all the LPS molecules despite a dramatic decrease in the metal content. The remaining LPS molecules are present outside the pits, whereas the bottom of the pits is devoid of these molecules. This new structure for the outer membrane exhibits higher permeability than that for the native cells.