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

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 4S min N1 , 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 5S s N1 . 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.

The dynamic behavior of bacterial macrofibers growing with one end prevented from rotating: variation in shaft rotation along the fiber's length, and supercoil movement on a solid surface toward the constrained end

“…Earlier we have shown using a similar floating wire protocol that preventing rotation at both ends during growth leads to supercoiling [11]. In natural macrofiber populations blocked rotation usually results from contacts fibers make with the floor of the growth chamber [7]. We describe here an inadvertent discovery in which a fiber prevented from rotating at one end by tethering to a wire later became prevented from rotating at various positions along its length as a result of touching the floor.…”

“…The presumed gradient of rotation rate along the shaft of a macrofiber has never been observed directly although pivoting motions observed as fibers grow on solid surfaces clearly show a gradient of velocity of rotation in the horizontal plane which is the result of rolling over the surface that correspondes to the rate of rotation in the fiber [7,8]. Simple theory predicts that if rotation is prevented at only one end of a fiber the other should rotate at twice the rate it normally would, and that the rotation rate should vary linearly along the shaft from zero at the blocked end to maximum at the other.…”

Self-Elongation with Sequential Folding of a Filament of Bacterial Cells

“…The interesting motion of macrofibers was reported by Mendelson and coworkers. [23,24,25,26]. Kumada et al also observed the growth of filaments of cells without separation for B. subtilis, in which the dependence of the morphology on C a was systematically studied [27].…”

The mechanisms responsible for 2-dimensional pattern formation in bacterial macrofiber populations grown on solid surfaces: fiber joining and the creation of exclusion zones

“…Cell growth continues beyond this point however forcing the fiber shaft to supercoil into a free standing helix that contracts into a ball-like form [6]. Filaments that grow on the surface of a ball can buckle and initiate the outgrowth of fibers that remain anchored to the ball surface [7]. These too supercoil when they reach a critical length or when they encounter an external impediment to their twisting.…”

“…Growth with twisting is the predominant cause of motions that take place during macrofiber morphogenesis and the production of the two-dimensional patterns of larger structures described in this paper. Helix-hand specific pivoting motions of macrofibers and their walking over glass and plastic surfaces using forces generated by cell growth have been described previously [7,8]. These motions coupled with the joining of structures to one another upon contact govern the spacing distances between structures in a population situated on a solid surface.…”