A Multi-Rail Structure in the Cell Envelope for the Bacteroidetes Gliding Machinery

Abstract

Many bacteria belonging to the phylum Bacteroidetes move on solid surfaces, which is called gliding motility. In our previous study with the Bacteroidetes gliding bacterium Flavobacterium johnsoniae, we proposed a helical loop track model, where adhesive SprB filaments are propelled along a left-handed closed helical loop on the cell surface. Attachment of SprB to the substratum results in cell movement. In this study, we observed the gliding cell rotating counterclockwise about its axis when viewed from the rear to the advancing direction of the cell, which was consistent with the helical loop track model. Total internal reflection fluorescence microscopy of SprB on a gliding cell revealed that one labeled SprB focus sometimes overtook and passed another SprB focus that was moving in the same direction, suggesting the presence of multiple lanes in the helical loop track. Several electron microscopic analyses revealed the presence of a multi-rail structure underneath the outer membrane, which was associated with SprB filaments and contained GldJ protein. A similar structure was observed in the distantly related marine gliding Bacteroidetes Saprospira grandis. These results provide new insights into the mechanism of Bacteroidetes gliding motility, in which the SprB filaments are propelled along the multi-rails underneath the outer membrane.