Reconstruction of the Gram-Negative Bacterial Outer-Membrane Bilayer

The outer membrane (OM) of gram-negative bacteria is highly asymmetric. The outer leaflet comprises lipopolysaccharides (LPS) and the inner leaflet phospholipids. Here, it is shown that the outer membrane lipid bilayer (OMLB) of Escherichia coli can be reconstructed as a droplet interface bilayer (DIB), which separates two aqueous droplets in oil. The trimeric porin OmpF is inserted into the model OMLB and the translocation of the bacteriocin colicin E9 (colE9) through it is monitored. By contrast with LPS-free bilayers, it is found that colE9 made multiple failed attempts to engage with OmpF in an OMLB before successful translocation occurred. In addition, the observed rate for the second step of colE9 translocation is 3-times smaller than that in LPS-free bilayers, and further, the colE9 dissociates when the membrane potential is reversed. The findings demonstrate the utility of the DIB approach for constructing model OMLBs from physiologically realistic lipids and that the properties of the model OMLBs differ from those of a simple lipid bilayer. The model OMLB offers a credible platform for screening the properties of antibiotics.