Outer Membrane, Gram-Negative Bacteria

Abstract

The cell envelope of gram-negative bacteria contains, in addition to an inner, cytoplasmic membrane, an outer membrane, a structure unique to organisms of this group. The space between the two membranes is the periplasm, where we find not only the peptidoglycan (murein) cell wall but also many unique proteins. The outer membrane also contains many unique proteins, which often traverse the membrane as β-barrels and function as narrow (and sometimes specific) diffusion channels for small nutrient molecules. Some proteins act as a high-affinity transporter for larger molecules that exist in very low concentrations in the environment, such as ferric siderophore complexes and vitamin B12. The bilayer domain of the outer membrane is composed of an asymmetric bilayer, with lipopolysaccharides comprising the sole component of the outer leaflet, at least in enteric bacteria. The influx of lipophilic compounds across this asymmetric bilayer is severely retarded in comparison with that across the conventional membrane bilayer. The outer membrane thus acts as a protective barrier, limiting the access of noxious compounds – hydrophilic ones by the limited diameter of diffusion channels and hydrophobic ones by the low permeability asymmetric bilayer. The enteric bacteria, especially Escherichia coli and Salmonella, served as a paradigm for the structure and function of the outer membrane. Finally, although gram-positive, bacteria of Corynebacterium-Nocardia-Mycobacterium group are surrounded by a cell envelope that is quite similar to the gram-negative outer membrane and that serves as an effective permeability barrier.