F-pili dynamics by live-cell imaging

Bacteria have evolved numerous mechanisms for cell-cell communication, many of which have important consequences for human health. Among these is conjugation, the direct transfer of DNA from one cell to another. For gram-negative bacteria, conjugation requires thin, flexible filaments (conjugative pili) that are elaborated by DNA donor cells. The structure, function, and especially the dynamics of conjugative pili are poorly understood. Here, we have applied live-cell imaging to characterize the dynamics of F-pili (conjugative pili encoded by the F plasmid of Escherichia coli). We establish that F-pili normally undergo cycles of extension and retraction in the absence of any obvious triggering event, such as contact with a recipient cell. When made, such contacts are able to survive the shear forces felt by bacteria in liquid media. Our data emphasize the role of F-pilus flexibility both in efficiently sampling a large volume surrounding donor cells in liquid culture and in establishing and maintaining cell-cell contact. Additionally and unexpectedly, we infer that extension and retraction are accompanied by rotation about the long axis of the filament.