Selective phage infection mediated by epitope expression on F pilus.

Proteins and peptides can be displayed on bacterial and bacteriophage surfaces as fusions to bacterial integral membrane proteins or phage coat proteins. We now report on the expression of peptide antigens on the surface of F pili, elaborated by F+ strains of Escherichia coli. The peptides were expressed as fusions to F pilin, the building block of the F pilus that is encoded by the traA gene on the F plasmid. Filamentous bacteriophage infection of E. coli is normally mediated by phage binding to pilin at the F pili tip. Expression of 13 to 15 amino acid long peptides on the F pilus completely blocked infection by derivatives of wild-type infectious M13 phage. However, when a phage displaying a specific recombinant antibody fragment was allowed to interact with F pili displaying an antigenic peptide a bacterial infection could be demonstrated. This infection, mediated by the antibody-antigen interaction, resulted in bacterial cells containing plasmids encoding both the protein and the ligand. In a model library, where a scFv antibody against the human cytomegalovirus AD-2 epitope was selected we achieved an enrichment of 2500 of phage carrying the specific antibody, indicating an efficient selective infection.