Spore Surface Display

Display systems that present biologically active molecules on the surface of microorganisms have become increasingly used to address environmental and biomedical issues (1–3). Strategies using environmentally relevant proteins or peptides for display on the surface of phages or bacterial cells have been extensively reviewed by Wu et al. (4). Examples include proteins able to bind metal ions that can be used as bioadsorbents or biocatalysts, including cysteine-rich metallothioneins (MTs) or Cys-His rich synthetic peptides, known to bind Cd2+ and Hg2+ with a very high affinity. Eukaryotic MTs have been expressed on the surface of Escherichia coli cells through fusion to the porin LamB, with a 20-fold increased ability of Cd2+ accumulation of the recombinant cell with respect to its parental strain (5, 6). In addition, metal-binding peptides have also been expressed on the surface of soil bacteria known to survive in contaminated environments. The mouse MT was displayed on the surface of Pseudomonas putida (7) and Ralstonia metallidurans CH34 (8), resulting in a 3-fold increase in binding and removal of Cd2+, sufficient to improve plant growth in a contaminated soil (8). Synthetic phytochelatins (ECn) with the repetitive metal-binding motif (Glu-Cys)nGly were displayed on the surface of Moraxella sp. cells causing a 10-fold improvement in Hg2+ intracellular accumulation (9–11). In addition to heavy metals, organic contaminants can be removed from the environment by the use of microbial cells displaying heterologous enzymes. Examples include organophosphorus hydrolases (OPHs). These bacterial enzymes are able to degrade organophosphates, which are toxic compounds widely used as pesticides. E. coli cells expressing OPH on their surface via the Lpp-OmpA fusion system were able to degrade parathion and paraoxon 7-fold faster than cells expressing OPH intracellularly (12). Surface display approaches have also been used to develop whole-cell diagnostic tools and vaccine delivery systems. Functional single-chain antibody fragments have been expressed on bacterial cells and used as diagnostic devices in immunological tests. In the first report of an antibody fragment expressed in an active form on a bacterial surface, the murine anti-human-IgE scFv antibody fragment was exposed on the surface of Staphylococcus xylosus and S. carnosus cells (13). More recently, the oral commensal bacterium Streptococcus gordonii was engineered to display a single-chain Fv (scFv) antibody fragment, derived from a monoclonal antibody raised against the major adhesin of the dental caries-producing bacterium Streptococcus mutans (streptococcal antigen I/II or SA I/II). Recombinant S. gordonii was found to specifically bind to immobilized SA I/II and represents the first step toward the development of a stable system for the delivery of recombinant antibodies (14).