Micro-video study of moving bacterial flagellar filaments: III. Cyclic transformation induced by mechanical force

Abstract

Dynamic images of isolated bacterial flagellar filaments undergoing cyclic transformations were recorded by dark-field light microscopy and an ultrasensitive video camera. Flagellar filaments derived from Salmonella SJ25 sometimes stick to a glass surface by short segments near one end. When such a filament, which is a left-handed helix, was subjected to a steady flow of a viscous solution of methylcellulose, its free portion was found to transform cyclically between left-handed (normal) and right-handed (curly or semi-coiled) helical forms. The transformations did not occur simultaneously throughout the whole length of a filament, but occurred at a transition point, which proceeded along the filament. Each transformation process consisted of three phases: initiation, growth and travel. The magnitudes of the mechanical forces, torque and tension, which were generated on a filament by the viscous flow, were obtained by quantitative hydrodynamic analyses. The torque was found responsible for initiating the transformation. The critical magnitude of torque required to induce the normal to semi-coiled transformation was −11 × 10 −19 N m and that for the reverse transformation from the semi-coiled to the normal form was 4 × 10 −19 N m. Therefore, the filaments showed the characteristics of hysteresis during the cyclic transformation. New types of unstable right-handed helical forms (medium and large) were also induced by mechanical force.