Real-time imaging of fluorescent flagellar filaments

Bacteria swim by rotating flagellar filaments that are several micrometers long, but only about 20 nm in diameter. The filaments can exist in differen t polymorphic forms, having distinct values of curvature and twist. Rotatio n rates are on the order of 100 Hz. In the past, the motion of individual f ilaments has been visualized by dark-field or differential-interference-con trast microscopy, methods hampered by intense scattering from the cell body or shallow depth of field, respectively. We have found a simple procedure for fluorescently labeling cells and filaments that allows recording their motion in real time with an inexpensive video camera and an ordinary fluore scence microscope with mercury-are or strobed laser illumination. We report our initial findings with cells of Escherichia coli. Tumbles (events that enable swimming cells to alter course) are remarkably varied. Not every fil ament on a cell needs to change its direction of rotation: different filame nts can change directions at different times, and a tumble can result from the change in direction of only one. Polymorphic transformations tend to oc cur in the sequence normal, semicoiled, curly 1, with changes in the direct ion of movement of the cell body correlated with transformations to the sem icoiled form.