Reliable titration of filamentous bacteriophages independent of pIII fusion moiety and genome size by using trypsin to restore wild-type pIII phenotype.

Phage display holds a key position in the use of combinatorial library approaches for the purpose of protein engineering and discovery. However, modifying the pIII protein of the phage can severely and negatively influence the infectiousness of the phage particle. This concern is particularly relevant when large pIII fusions in combination with multivalent display systems are in use. We here describe the use of trypsin to restore wild-type pIII phenotype as a small modification to the standard titration protocol. The results show that the trypsin treatment has a very large but heterogeneous effect on the phage infection efficiency, depending on the pIII fusion domain and the valence of display.