The Structural and Physiological Roles of the Layers of the Gram-Negative Bacterial Envelope

Abstract

Although the envelope of an organism like Escherichia coli is usually thought to consist of three layers, it is functionally composed of five or six layers which together serve to accommodate the organism’s needs in obtaining medium constituents, preventing entry of toxic material, maintaining cell shape, and resisting the osmotic forces that otherwise would tear the cell apart (Beveridge, 1981; Cota-Robles, 1963; Donachie, Begg, and Sullivan, 1984; Graham, Beveridge, and Nanninga, 1991; Koch, 1988b; Ledúc et al., 1989). Conventionally, the envelope is divided into three structural components: outer membrane (OM); murein (M); and cytoplasmic membrane (CM). The six physiological layers (Figure 13.1) however, are the hydrophilic lipopolysaccharide portion of the outer leaflet of the OM, the hydrophobic inner leaflet, the 100,000 molecules of Braun’s lipoprotein connecting the OM to the M layer, the monolayered stress-bearing peptidoglycan M layer, and the two hydrophobic leaflets of the CM. The two leaflets of the CM are different because certain proteins interact with one, the other, or both layers. These are the intrinsic and extrinsic proteins of the CM; there are intrinsic proteins in the OM, such as porins. In addition there is a periplasmic space contained between the OM and the CM. The proteins in the periplasmic space are very important; these are needed for enzymatic cleavage of those nutrients entering from the environment that cannot be taken up into the cell directly. Also present in the periplasmic space are binding proteins that ferry nutrients from the periplasmic space to the transport sites (permeases) on the CM so that nutrients can enter the cell by carrier mediated transport (see Chapter 5). Although many of the activities that take place in the periplasmic space are understood, the size of the periplasmic space is debatable (Ferguson, 1990, 1992; Hiemstra, et al., 1987; Hobot, et al., 1984; Wielink and Duine, 1990; Schwarz and Koch, 1995). Also debatable is the role of the membrane derived oligosaccharides (MDOs) that are formed into the periplasm when the cells are grown in low osmotic strength medium (Kennedy, 1982).