Unforeseen swimming and gliding mode of an insect gut symbiont, Burkholderia sp. RPE64, with wrapping of the flagella around its cell body.

A bean bug symbiont, Burkholderia sp. RPE64, selectively colonizes the gut crypts by flagella-mediated motility: however, the mechanism for this colonization remains unclear. Here, to obtain clues to this mechanism, we characterized the swimming motility of the Burkholderia symbiont under an advanced optical microscope. High-speed imaging of cells enabled the detection of turn events with up to 5-ms temporal resolution, indicating that cells showed reversal motions (θ ~ 180°) with rapid changes in speed by a factor of 3.6. Remarkably, staining of the flagellar filaments with a fluorescent dye Cy3 revealed that the flagellar filaments wrap around the cell body with a motion like that of a ribbon streamer in rhythmic gymnastics. A motility assay with total internal reflection fluorescence microscopy revealed that the left-handed flagellum wound around the cell body and propelled it forward by its clockwise rotation. We also detected periodic-fluorescent signals of flagella on the glass surface, suggesting that flagella possibly contacted the solid surface directly and produced a gliding-like motion driven by flagellar rotation. Finally, the wrapping motion was also observed in a symbiotic bacterium of the bobtail squid, Aliivibrio fischeri, suggesting that this motility mode may contribute to migration on the mucus-filled narrow passage connecting to the symbiotic organ.