A model of Leptospira motility was recently proposed. One element of the model states that in translating cells the anterior spiral-shaped end gyrates counterclockwise and the posterior hook-shaped end gyrates clockwise. We tested these predictions by analyzing cells tethered to a glass surface. Leptospira illini was incubated with antibody-coated latex beads (Ab-beads). These beads adhered to the cells, and subsequently some cells became attached to either the slide or the cover glass via the

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... b-beads. As previously reported, these cells rapidly moved back and forth across the surface of the beads. In addition, a general trend was observed: cells tethered to the cover glass rotated clockwise around the Ab-bead; cells tethered to the slide rotated counterclockwise around the Ab-bead. A computer-aided microcinematographic analysis of tethered cells indicated that the direction of rotation of cells around the Ab-bead was a function of both the surface of attachment and the shape of the cell ends. The results can best be explained by assuming that the gyrating ends interact with the glass surface to cause rotation around the Ab-beads. The analysis obtained indicates that the hookand spiral-shaped ends rotate in the directions predicted by the model. In addition, the tethered cell assay permitted detection of rapid, coordinated reversals of the cell ends, e.g., cells rapidly switched from a hook-spiral configuration to a spiral-hook configuration. These results suggest the existance of a mechanism which coordinates the shape of the cell ends of L. illini. Leptospira is a spirochete with a unique structure and mode of motility. These right-handed (7, 15, 17, 34) , helically shaped cells have a diameter of approximately 0.12 ptm and