Engineering the Modular Receptor-Binding Proteins of<i>Klebsiella</i>Phages Switches Their Capsule Serotype Specificity

Abstract

The high specificity of bacteriophages is driven by their receptor-binding proteins (RBPs). Many <i>Klebsiella</i> bacteriophages target the capsular exopolysaccharide as the receptor and encode RBPs with depolymerase activity. The modular structure of these RBPs with an N-terminal structural module to attach the RBP to the phage tail, and a C-terminal specificity module for exopolysaccharide degradation, supports horizontal transfer as a major evolutionary driver for <i>Klebsiella</i> phage RBPs. We mimicked this natural evolutionary process by the construction of modular RBP chimeras, exchanging N-terminal structural modules and C-terminal specificity modules. All chimeras strictly follow the capsular serotype specificity of the C-terminal module. Transplanting chimeras with a K11 N-terminal structural RBP module in a <i>Klebsiella</i> phage K11 scaffold results in a capsular serotype switch and corresponding host range modification of the synthetic phages, demonstrating that horizontal transfer of C-terminal specificity modules offers <i>Klebsiella</i> phages an evolutionary highway for rapid adaptation to new capsular serotypes.<b>IMPORTANCE</b> The antimicrobial resistance crisis has rekindled interest in bacteriophage therapy. Phages have been studied over a century as therapeutics to treat bacterial infections, but one of the biggest challenges for the use of phages in therapeutic interventions remains their high specificity. In particular, many <i>Klebsiella</i> phages have a narrow spectrum constrained by the high diversity of exopolysaccharide capsules that shield access to the cells. In this work, we have elaborated how <i>Klebsiella</i> phages deal with this high diversity by exchanging building blocks of their receptor-binding proteins.