MECHANICS OF BACTERIAL MACROFIBER INITIATION

The twisting and writhing during growth of single-cell filaments of Ba cillus subtilis which lead to macrofiber formation was studied in both left- and right-handed forms of strains FJ7 and RHX. Filament bending , touching, and loop formation (folding), followed by winding up into a double-strand fiber, were documented, Subsequent folds that produced multistrandedness were also examined, The rate of loop rotation durin g winding up was measured for 26 loops from 16 clones, In most cases, the first loop formed turned at a lower rate than those produced by th e following cycles of folding, The sequence of folding topologies diff ered in FJ7 and RHX strains and in left- versus right-handed structure s, Right-handed FJ7 routinely gave rise to four-stranded helices at th e second fold, whereas left-handed FJ7 and both left-handed and right- handed forms of RHX made structures with predominantly two double-stra nded helical regions, Left-handed RHX structures frequently produced s econd folds within the initial loop itself, resulting in T- or Y-shape d fibers, Sixteen cases in which the initial touch of a filament to it self produced a loop that snapped open before it could wind up into a double-strand fiber were found, The snap motions were used to obtain e stimates of the forces generated by helical growth of single filaments and to investigate theoretical models involving the material properti es of cell filaments, In general, the mechanical behavior of growing s ingle-cell filaments and fibers consisting of two-, three-, or four-st rand helices was similar to that described for larger, mature, multifi lament macrofibers. The behavior of multicellular macrofibers can be u nderstood, therefore, in terms of individual cell growth.