Animal Cell Membranes as Substrata for Bacterial Adhesion

Abstract

The purpose of this chapter is to review briefly the composition and organization of animal cell surface structures that may be potential receptors for adhesins of bacteria. The understanding of the specificity of animal cell—bacteria interactions requires a basic knowledge of the molecular structure of the animal cell surface, especially of those molecules that serve as receptors for ligands in general and for bacterial adhesins in particular. All animal cell membranes share common compositional and organizational features (Figure 3-1): (1) The major membrane lipids are arranged in a planar bilayer configuration that is predominantly in a “fluid” state under physiological conditions. The membrane lipids are commonly composed of glycerolphospholipids, sphingolipids, and sterols. (2) The bilayer membrane contains integral membrane constituents composed of both glycolipids and glycoproteins that are inserted or “intercalated” into the bilayer structure. (3) Other glycoproteins and proteins are bound to the surface of the plasma membrane by weak ionic interactions, hydrogen bonding, or the hydrophobic effect. These surface-associated glycoproteins and proteins bound to integral membrane structures are referred to as peripheral or extrinsic components. (4) In many animal cells there is a substantial layer of carbohydrate-containing materials of variable thicknesses outside the plasma membrane but in close or intimate association with the membrane. This layer is known as the cell coat or extracellular matrix. The distinction between membrane constituents as being integral, peripheral, or belonging to the cell coat is based on the method required to dissociate the constituent in question from the cell membrane. The integral constituents may be released only after disruption or perturbation of the phospholipid bilayer, usually by detergents (Lichtenberg et al., 1983). Nonintegral surface constituents are commonly released by washing the cells with buffers of different pH or ionic strength, or by using chelating agents, such as ethylenediaminetetraacetic acid (EDTA). There is no general method, however, to release selectively either peripheral or extracellular matrix constituents. As a result, the distinction between the two classes of membrane constituents is sometimes difficult to resolve and very often they are referred to as nonintegral membrane constituents. One of the key features of the membrane is its asymmetry. For nonglycosylated lipids the asymmetry is only partial, in that every phospholipid is present on both sides of the bilayer but in different amounts. In human erythrocytes, for example, lipids with positively charged head groups (e.g., phosphatidylethanolamine and phosphatidylserine) are predominant in the internal leaflet facing the cytoplasm (Marinetti and Crain, 1978). The asymmetry with respect to proteins, glycoproteins, and glycolipids is absolute: every molecule of a given membrane constituent has the same orientation across the lipid bilayer, with the carbohydrate moieties of the glycosylated compounds always exposed on the outer surface. For further information on the organization of the animal cell membrane, the reader is referred to reviews (Lodish et al., 1981; Lotan and Nicolson, 1981; Singer, 1981; Aplin and Hughes, 1982) and a book (Sim, 1982).