Adhesive organelles of Gram‐negative pathogens assembled with the classical chaperone/usher machinery: structure and function from a clinical standpoint

This review summarizes current knowledge on the structure, function, assembly and biomedical applications of the superfamily of adhesive fimbrial organelles exposed on the surface of Gram‐negative pathogens with the classical chaperone/usher machinery. High‐resolution three‐dimensional (3D) structure studies of the minifibers assembling with the FGL (having a long F1–G1 loop) and FGS (having a short F1–G1 loop) chaperones show that they exploit the same principle of donor‐strand complementation for polymerization of subunits. The 3D structure of adhesive subunits bound to host‐cell receptors and the final architecture of adhesive fimbrial organelles reveal two functional families of the organelles, respectively, possessing polyadhesive and monoadhesive binding. The FGL and FGS chaperone‐assembled polyadhesins are encoded exclusively by the gene clusters of the γ3‐ and κ‐monophyletic groups, respectively, while gene clusters belonging to the γ1‐, γ2‐, γ4‐, and π‐fimbrial clades exclusively encode FGS chaperone‐assembled monoadhesins. Novel approaches are suggested for a rational design of antimicrobials inhibiting the organelle assembly or inhibiting their binding to host‐cell receptors. Vaccines are currently under development based on the recombinant subunits of adhesins.