Twitching Pseudomonas aeruginosa
Type IV pili are thin (6 nm dia.) filaments that are essential for twitching motility in Pseudomonas aeruginosa and social gliding in Myxococcus xanthus. How do Type IV pili generate cell movement? The first clues came from electron microscopy studies of bacteriophage infection of a variety of Gram-negative bacteria. After the initial interaction between the phage and the pilus, it appeared that retraction pulled the phage to the cell surface where productive infection could occur.
Using optical tweezers, Mertz et al. (2000) demonstrated that Type IV pili could retract with considerable force (> 80 pN). Mutants that were pili-less or had non-retractile pili were non-motile; thus pilus retraction powers twitching motility. The pili were not observed directly -- pilus function was assayed by monitoring the position of a latex bead attached to pili with antibodies.
We developed a technique to label pili using an amino-reactive Cy3 and observed directly pilus extension, pilus retraction, and retraction-mediated cell movement (Skerker & Berg, 2000).
Labeled cells were videotaped in fluorescence using evanescent wave excitation at a quartz-water interface. Pili tended to stick to the quartz at their distal tips. If the cell body also stuck, pili under retraction were pulled taut. If the cell body was free, retraction pulled it forward. The QuickTime movies shown below are 10 times actual speed.
Mertz, A.J., So, M. and Sheetz, M.P. Pilus retraction powers bacterial twitching motility. Nature 407, 98-102 (2000).
Skerker, J.M. and Berg, H.C. Direct observation of extension and retraction of type IV pili. Proc Natl. Acad. Sci. USA, 98, 6901-6904 (2001).