Surface display of heterologous proteins on bacterial cells is an important objective for many applications in microbiology and molecular biology. In gram-negative bacteria, different types of surface proteins have been exploited for this purpose. These include the following: (i) outer membrane proteins LamB (4), PhoE (1), and OmpA (35); (ii) lipoproteins TraT (19) and the peptidoglycan-associated lipoprotein (10); (iii) the fimbria protein fimbrillin (20, 21); and (iv) the flagellar protein flagellin (32). The different surface display systems have been used extensively to express heterologous antigenic determinants on the bacterial cells for the purpose of developing live bacterial vaccine vehicles. Enteric bacteria, such as Escherichia coli and Salmonella typhimurium, have been studied in this context, and the cell surface presentation has been considered advantageous to induce an antibody response to the exposed antigens with live cells for immunization (11,28,41).The expression of functional single-chain antibodies on the surface of E. coli cells (7, 10) has opened the discussion of whether this strategy would be an alternative to the rapidly developing phage technology for the selection of peptides or recombinant antibody fragments from large libraries (31). An interesting application for bacterial cells exposing heterologous proteins might be the development of whole-cell adsorbents by the surface expression of suitable protein ligands. Immobilized recombinant bacteria cannot be considered for separation processes in pharmaceutical industries but might because of their low cost constitute competitive adsorbents for certain separations (3, 11). The use of enzyme-coated bacteria as novel biocatalysts has also been envisioned, because enzymes with retained activity have been surface displayed on E. coli cells (8, 9).Investigations with gram-positive bacteria for cell surface display of heterologous proteins have recently been initiated. Expression systems for the mouth commensal bacterium Streptococcus gordinii (37) and the nonpathogenic bacterium Staphylococcus xylosus (18) have been developed on the basis of the fibrillar M6 protein from Streptococcus pyogenes and protein A from Staphylococcus aureus, respectively. These cell surface display systems have been used for the surface expression of several antigenic determinants, and immunization with live recombinant bacteria induces both local and systemic antibody responses to the hybrid receptors (33, 37), suggesting that gram-positive bacteria might constitute potential live bacterial vaccine delivery systems. The surface receptors of gram-positive bacteria seem to be more permissive for the insertion of extended sequences of foreign proteins (6) than the different gram-negative systems, in which both translocation through the cytoplasmic membrane and correct integration into the outer membrane are required for proper surface exposure of the heterologous polypeptide. Considering the development of whole-cell adsorbents or bacterial biocatalysts by surface ...