A new form of bacterial movement, dragging of multicellular aggregate structures over solid surfaces, is powered by macrofiber supercoiling.

Growing Bacillus subtilis macrofibers use twist and supercoiling to: power their own self-assembly, join fibers together into multiclonal aggregates, move themselves over solid surfaces, and to drag other structures (cargo) over solid surfaces. The dragging of multiclonal aggregates attached to the ends of growing macrofibers is analyzed here. The linkage between fibers and cargo arose naturally in macrofiber cultures. Dragging was triggered when growing macrofibers became linked to cargo at both of their ends. Such macrofibers supercoiled, reduced their length, and dragged the cargo toward one another. In parallel experiments immobile wire was used in place of cargo at one end of macrofibers that were linked to cargo at the other. The cargo was dragged toward the wire when these fibers supercoiled. To estimate the force required for dragging we determined the dimensions of the cargo, the buoyant density of macrofibers in the growth medium where dragging occurred, the rate and distance over which the aggregate structures were dragged, and the viscosity of the growth medium. Friction resulting from contact with the solid surface over which the structures were dragged was estimated using the measured parameters. The results indicate that the supercoiling tension required to overcome limiting friction must have been approximately 10 nN, while that needed to overcome fluid drag was of the order of 1 nN. These values suggest that only a small fraction of the total power available from macrofiber supercoiling was needed to drive this new form of multicellular bacterial movement.