Characterization of adhesive epitopes with the OmpS display system

Identification of Vibrio harveyi proteins involved in the specific immune response of Senegalese sole (Solea senegalensis, Kaup)

Senegalese sole cultures are frequently affected by Vibrio harveyi disease outbreaks. Vaccines in aquaculture are one of the most successful methods of preventing fish pathologies; however, these vaccines are usually composed of inactivated whole cells containing a wide pool of antigens, and some do not induce any protection against pathogens. Thus, the aim of this study was to identify immunogenic proteins of V. harveyi involved in the specific antibody production by Senegalese sole. S. senegalensis specimens were immunized, by intraperitoneal injection, with V. harveyi bacterin supplemented with inactivated extracellular polymeric substances (ECP) and Freund incomplete adjuvant to obtain polyclonal antiserum. One month later, specimens were re-inoculated with the same antigens. Sera from immunized fish were collected two months post first immunization. Strong specific immune response to V. harveyi antigens was detected by ELISA using bacterin (limit dilutions of sera were 1:64000), ECP (1:4000) and outer membrane proteins (OMP) (1:4000) as antigens. Presence of immunogenic proteins in V. harveyi ECP and OMP were determined by 2D-PAGE. For Western Blot analysis some gels were transferred onto nitrocellulose membranes and incubated with sera from S. senegalensis specimens immunized against V. harveyi. 2D-PAGE and Western Blot showed at least five reactive proteins in the ECP and two in the OMP fraction. The spots that clearly reacted with the sole antiserum were excised from stained gel, and analyzed by mass spectrometry (MALDI/TOFTOF). A database search was then performed, using MASCOT as the search method. According to the results, the five ECP spots were identified as Maltoporine, protein homologous to Metal dependent phosphohydrolase, two porins isoforms of V. harveyi and a protein homologous to the cell division protein FtsH. Reactive proteins in the OMP fraction were identified as the protein 3-hydroxyisobutyrate dehydrogenase and a protein homologous to acid phosphatase.

A novel fibronectin-binding protein of Streptococcus suis serotype 2 contributes to epithelial cell invasion and in vivo dissemination

Streptococcus suis serotype 2 (SS2) is an important pathogen with zoonotic potential. In this study, a novel in vivo induced protein Ssa, encoded by the functionally unknown gene SSU05_1311, was identified as a surface anchored fibronectin-binding protein. The recombinant Ssa as well as its truncated proteins harboring the N-terminal domain (residues 33-153) could bind to human fibronectin with high-affinity. Isogenic knockout of ssa in SS2 led to decrease of bacterial binding to immobilized fibronectin. SS2 Δssa mutant showed reduced adherence and invasion of human pharyngeal epithelial (HEp-2) cells compared to the wild type strain. Heterologous surface display of Ssa fibronectin-binding domain in Escherichia coli enhanced the bacterial attachment and entry to HEp-2 cells. Less SS2 Δssa mutants than the wild type strain were recovered from the blood and brains of the Balb/c mice infected intranasally. But there was no significant difference between the wild type and the mutant on phagocytosis by macrophages (RAW264.7) and bacterial killing by murine PMNs under opsonizing condition. Our data suggest that Ssa is an important virulence factor for SS2 crossing the mucosal epithelia to disseminate in vivo.

Microbial cell-surface display

Cell-surface display allows peptides and proteins to be displayed on the surface of microbial cells by fusing them with the anchoring motifs. The protein to be displayed - the passenger protein - can be fused to an anchoring motif - the carrier protein - by N-terminal fusion, C-terminal fusion or sandwich fusion. The characteristics of carrier protein, passenger protein and host cell, and fusion method all affect the efficiency of surface display of proteins. Microbial cell-surface display has many potential applications, including live vaccine development, peptide library screening, bioconversion using whole cell biocatalyst and bioadsorption.

Selection of peptide entry motifs by bacterial surface display

Surface display technologies have been established previously to select peptides and polypeptides that interact with purified immobilized ligands. In the present study, we designed and implemented a surface display-based technique to identify novel peptide motifs that mediate entry into eukaryotic cells. An Escherichia coli library expressing surface-displayed peptides was combined with eukaryotic cells and the gentamicin protection assay was performed to select recombinant E. coli, which were internalized into eukaryotic cells by virtue of the displayed peptides. To establish the proof of principle of this approach, the fibronectin-binding motifs of the fibronectin-binding protein A of Staphylococcus aureus were inserted into the E. coli FhuA protein. Surface expression of the fusion proteins was demonstrated by functional assays and by FACS analysis. The fibronectin-binding motifs were shown to mediate entry of the bacteria into non-phagocytic eukaryotic cells and brought about the preferential selection of these bacteria over E. coli expressing parental FhuA, with an enrichment of 100000-fold. Four entry sequences were selected and identified using an S. aureus library of peptides displayed in the FhuA protein on the surface of E. coli. These sequences included novel entry motifs as well as integrin-binding Arg-Gly-Asp (RGD) motifs and promoted a high degree of bacterial entry. Bacterial surface display is thus a powerful tool to effectively select and identify entry peptide motifs.

Cell-Surface display of heterologous proteins: From high-throughput screening to environmental applications

A variety of expression systems for the display of either short peptides or fully folded proteins on E.coli and, to a lesser extent, on Gram-positive bacteria have been developed. The expression of proteins on the surface of microbial cells has proved extremely important for numerous applications ranging from combinatorial library screening and protein engineering, to whole cell biocatalysts and adsorbants for bioremediation purposes.

Display of proteins on bacteria

Display of heterologous proteins on the surface of microorganisms, enabled by means of recombinant DNA technology, has become an increasingly used strategy in various applications in microbiology, biotechnology and vaccinology. Gram-negative, Gram-positive bacteria, viruses and phages are all being investigated in such applications. This review will focus on the bacterial display systems and applications. Live bacterial vaccine delivery vehicles are being developed through the surface display of foreign antigens on the bacterial surfaces. In this field, 'second generation' vaccine delivery vehicles are at present being generated by the addition of mucosal targeting signals, through co-display of adhesins, in order to achieve targeting of the live bacteria to immunoreactive sites to thereby increase immune responses. Engineered bacteria are further being evaluated as novel microbial biocatalysts with heterologous enzymes immobilized as surface exposed on the bacterial cell surface. A discussion has started whether bacteria can find use as new types of whole-cell diagnostic devices since single-chain antibodies and other type of tailor-made binding proteins can be displayed on bacteria. Bacteria with increased binding capacity for certain metal ions can be created and potential environmental or biosensor applications for such recombinant bacteria as biosorbents are being discussed. Certain bacteria have also been employed for display of various poly-peptide libraries for use as devices in in vitro selection applications. Through various selection principles, individual clones with desired properties can be selected from such libraries. This article explains the basic principles of the different bacterial display systems, and discusses current uses and possible future trends of these emerging technologies.

Novel bacterial membrane surface display system using cell wall-less L-forms of Proteus mirabilis and Escherichia coli

We describe a novel membrane surface display system that allows the anchoring of foreign proteins in the cytoplasmic membrane (CM) of stable, cell wall-less L-form cells of Escherichia coli and Proteus mirabilis. The reporter protein, staphylokinase (Sak), was fused to transmembrane domains of integral membrane proteins from E. coli (lactose permease LacY, preprotein translocase SecY) and P. mirabilis (curved cell morphology protein CcmA). Both L-form strains overexpressed fusion proteins in amounts of 1 to 100 microg ml(-1), with higher expression for those with homologous anchor motifs. Various experimental approaches, e.g., cell fractionation, Percoll gradient purification, and solubilization of the CM, demonstrated that the fusion proteins are tightly bound to the CM and do not form aggregates. Trypsin digestion, as well as electron microscopy of immunogold-labeled replicas, confirmed that the protein was localized on the outside surface. The displayed Sak showed functional activity, indicating correct folding. This membrane surface display system features endotoxin-poor organisms and can provide a novel platform for numerous applications.

Outer membrane proteins as surface display systems

Outer membrane proteins (OMPs) of gram-negative bacteria can be used as carrier proteins to present foreign peptide epitopes on the bacterial cell surface. They all have a common structural motif of a beta-barrel that is composed of a variable number of transmembrane beta-strands connected at the periplasmic side with short turns and at the outside with long surface-accessible loops. Outer membrane proteins occur as monomers like OmpA, or assemble into trimers like the porins. Foreign gene products have been fused to surface-accessible regions of several outer membrane proteins including the porins OmpC, PhoE and LamB, lipoproteins as well as the OmpA protein. Short epitopes that are inserted into outer membrane proteins induce epitope-specific antibody responses, and are thus appealing candidates for live recombinant vaccines. Also large insertions, of more than 100 amino acids, are in some cases tolerated and do not affect the overall conformation of the carrier protein. The possible applications for outer membrane display include recombinant vaccines, peptide library screening, development of biocatalysts or whole-cell adsorbents, and adhesin-receptor interaction studies. It is expected that in the near future, development of new display systems will still increase the utilization of this emerging exciting technology.

Expressing genes in different Escherichia coli compartments

Production of heterologous proteins or parts thereof in different extra-cytoplasmic compartments (in the periplasm, outer membrane or extracellularly) of Escherichia coli offers multiple applications, for example, in vaccine development, immobilised enzymes and bioremediation. Nowadays, not only surface display of short peptides, but also cell-surface anchoring or secretion of functional proteins is possible. Factors influencing folding, stability and export of extra-cytoplasmic proteins are also better understood.