Motions caused by the growth of Bacillus subtilis macrofibres in fluid medium result in new forms of movement of the multicellular structures over solid surfaces

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Abstract
Bacillus subtilis macrofibres, highly ordered multicellular structures, undergo twisting and writhing motions when they grow in fluid medium as a result of forces generated by the elongation of individual cells. Macrofibres are denser than the fluid medium in which they are cultured, consequently they settle to the bottom of the growth chamber and grow in contact with it. The ramifications of growth on plastic and glass surfaces were examined. Macrofibres were observed to rotate about a vertical axis near the centre of their length in a chiral-specific direction. Right-handed fibres rotated clockwise on plastic surfaces at approximately 4° min−1, left-handed structures of lower twist rotate anti-clockwise at about half that rate. Very large ball structures produced late in macrofibre formation perched on many small protruding fibres but rotated only when driven by large fibres attached to their periphery. Closer examination showed that fibres made contact with surfaces at only a few points along their length (between 1 and 6 on glass). The regions in contact with the surface changed periodically as a result of rotation of the fibre shaft caused by growth. Every time the weight of a fibre transferred from one contact point to another, each section of the fibre took a small step approximately proportional to its distance from the fibre mid-point. The net result was a rolling of each section over the surface so that the fibre rotation about a vertical axis was produced. Macrofibres also took large steps when part of the structure rose off the floor, swept through an arc in the fluid and then returned to the floor at a new location. The rate of movement during a large step, measured as the change of angle between the moving and stationary portions of the fibre, was 5° s−1. These observations reveal that the forces derived from helical growth that lead to macrofibre formation also cause characteristic macrofibre motion that differs from classical motility.