Direct observation of extension and retraction of type IV pili

Type IV pill are thin filaments that extend from the poles of a diverse gro up of bacteria, enabling them to move at speeds of a few tenths of a microm eter per second. They are required for twitching motility. e,g,, in Pseudom onas aeruginosa and Neisseria gonorrhoeae, and for social gliding motility in Myxococcos xanthus, Here we report direct observation of extension and r etraction of type IV pill in P. aeruginosa, Cells without flagellar filamen ts were labeled with an amino-specific Cy3 fluorescent dye and were visuali zed on a quartz slide by total internal reflection microscopy. When pill we re attached to a cell and their distal ends were free, they extended or ret racted at rates of about 0.5 mum s(-1) (29 degreesC). They also flexed by B rownian motion, exhibiting a persistence length of about 5 mum. Frequently, the distal tip of a filament adsorbed to the substratum and the filament w as pulled taut. From the absence of lateral deflections of such filaments, we estimate tensions of at least 10 pN. Occasionally, cell bodies came free and were pulled forward by pilus retraction. Thus, type IV pill are linear actuators that extend, attach at their distal tips, exert substantial forc e, and retract.