Bending, Folding, and Buckling Processes During Bacterial Macrofiber Morphogenesis

Self-Assembly of Bacterial Macrofibers: A System Based Upon Hierarchies of Helices

Cylindrical-shaped cells of Bacillus subtilis (0.7 by 4 μm) are the building blocks of macrofibers, highly organized, helically twisted, multifilament structures millimeters to centimeters in length. The forces responsible for self-assembly and the cylinder-helix deformation trace to the assembly of cell wall polymers and restraint of the motions generated by cell growth. An hierarchical relationship exists involving: (i) molecular level events associated with cell surface assembly, which in turn govern, (ii) cellular level events concerned with motions that accompany cell growth, and these in turn drive, (iii) multicellular level events such as the folding and plying of cell filaments to form a mature macrofiber. Cell growth generates new material and engenders twisting of the cell cylinder along a screw axis as it elongates. The helix hand and degree of twist at the cellular level eventually dictate the hand and twist of the mature multifilament macrofiber. Although several different routes can lead to the initiation of macrofiber production, once initiated a repetitive cycle of folding and plying becomes established. The self-assembly proceeds until mechanical and geometrical factors preclude further folding cycles.

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

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 degrees 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 degrees 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.

Chiral self-propulsion of growing bacterial macrofibers on a solid surface

Supercoiling motions that accompany the growth of bacterial macrofibers (multicellular filamentous structures formed in B. subtilis by cell division without separation) are responsible for rolling, pivoting, and walking of fibers on a surface. Fibers possess a fulcrum about which they pivot and step in a chiral manner; forces and torques associated with cell growth, when blocked by friction, result in self-propulsion. The elastic engine that drives macrofiber motions generates torques estimated as microdyn cm and femtowatts of power; optical trapping studies yield a first direct measurement of the Young's modulus of the bacterial cell wall, the engine's "working fluid," of ca. 0.05 GPa.