Matrix Proteins

Albumen, fibronectin, collagen, vitronectin, and other milieu proteins play an important role in specifically augmenting or blunting the adhesive tendencies of bacterial and tissue cells, depending on the concentration and the presence of integrin receptors for matrix proteins on tissue cells and a similar class of receptors on bacterial cells. The exact role of each intermediate matrix protein is assumed to be specific to the cell involved, surface denaturization, or folding interactions, as well as pH and cation concentration.

Implanted biomaterials are rapidly coated by constituents of the serum and surrounding matrix, including fibronectin, osteonectin, vitronectin, albumin, fibrinogen, laminin, collagen, and covalently bound, short-chain oligosaccharides (Gristina et al., 1987; Baier et al., 1984). Bacterial and tissue cells may then adhere to constituents of this film. Eukaryocytes have been shown to adhere to surfaces coated with fibronectin and collagen (Proctor, 1987; Van Wachem et al., 1987). Certain strains of S. aureus, S. epidermidis, and E. coli have receptors or sets of receptors for fibronectin and collagen epitopes (Hermann et al., 1988; Vaudaux et al., 1984). Studies have shown that pathogenic bacteria have greater numbers of conditioning film protein receptors than do similar strains of nonpathogenic bacteria.

The interaction between conditioning film proteins and tissue and bacterial cells also depends on the physical state and quantity of bound proteins on the biomaterial surface. In vitro studies of fibronectin-coated biomaterials demonstrate that a low concentration of bound fibronectin enhances bacterial adherence, while a high concentration of bound fibronectin inhibits binding by the same bacteria (Hermann et al., 1988). Bacterial adhesion may also be decreased by the presence of albumin. Thus, conditioning protein molecules may play a variety of roles in bacterial adhesion, depending on their concentration and environmental conditions (Baier et al., 1984).

BIOMATERIALS—THE NATURE OF SURFACES

This section considers the general characteristics of biomaterials as substrata with special regard for their interactions with bacteria and tissue. Many implants consist of one or more metals or polymers. Biomaterials, foreign bodies, and devital ized tissue and bone in a biological environment are passive and susceptible substrata because they are inanimate and do not resist infection. In fact, regardless of "inertness," they are physiocochemically active and may directly modulate adhesion or interact with host defenses.

Surface atoms and molecules of metal alloys and polymers are not bound on all sides by bulk phase atoms or molecules. This imbalance causes changes in the distribution of constituents of the bulk phase (segregation) and rearrangement of outer atomic layers (relaxation), depending on the ambient, nonbulk, liquid, or gas environment. Even the most inert alloys and polymers have oxide defects and reactive zones that will differentially influence adsorption and chemical interactions at their surfaces. In a like manner, surfaces of tissue cells or acellular biologic surfaces such as articular cartilage or damaged bone present specific ligands or crystalline faces for receptor binding.

Most important for each material is the surface interaction of the outer atomic layers with environmental moieties, glycoproteins, elemental constituents, or prokaryotic and eukaryotic cells. Ideally, we would like to influence interactions to promote compatibility and/or integration and resistance to infections, as well as formal host defense responses.

Titanium alloys used in dental implants have been suggested to form a direct bone (osteocyte)-implant contact (osseointe-gration) at the ultrastructural level (Albrektsson, 1989; Gristina et al., 1985). Titanium and titanium alloy surfaces directly colonized by osteoblasts or tissue cells may be protected in part against colonization by bacterial pathogens. Porous coated surfaces used without methyl methacrylates have been reported recently as having a lower rate of infection. There has been no reported difference in infection rates between porous and smooth surfaces of the same material or alloy. The increase in surface area in these applications is less than one order of measure and not likely to increase colonization.

Studies have indicated greater bacterial adhesion to polymers than to metals for S. epidermidis, both in vitro and in vivo (Barth et al., 1989; Gristina et al., 1987). Interestingly, antibiotic resistance and host defense inhibition may be greater for infections centered on polymers (Barth et al., 1989).

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