Trapping bacteria to study movement

Although it sounds like a contradiction, researchers have developed an assay to study how single bacterial cells swim by trapping them with optical tweezers. The assay, developed by Ido Golding and colleagues at the University of Illinois Urbana–Champaign, involves a microfluidic device in which an individual E. coli cell is held steady in a liquid medium with two optical traps placed at either end of the rod-shaped bacterium. Although the cell is trapped in the x direction, it can still rotate its body and move its flagellar bundle in the y and z directions. Movements are detected by position-sensitive detectors that receive light from both trapping beams, and cells are imaged with fluorescence or light microscopy.

Ido Golding

(a) The fluorescence images of a cell exhibiting running motion (1.2 s, 2.2 s, 2.7 s, and 3.2 s) and tumbling motion (1.7 s). (b) The corresponding optical-trap signals for running and tumbling.
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With existing methods, cells are allowed to swim freely in a flow cell or are tethered to a surface, and changes in swimming direction or rotation are observed. Both methods suffer with regard to the quantitation of whole-cell motility, however. The new assay allows quantitation of long-term single-cell movement in a controlled environment with rapid data-acquisition rates.

Optical traps are known to damage cells in high-oxygen environments, so the researchers enhanced cell viability by adding an oxygen scavenger to the medium. Cells in the traps grew and divided at normal rates. With this optimization strategy, cells could be monitored for up to 1 hr; this is in contrast to another trap-based method for motility measurement in which bacteria swimming in an oxygenated environment could be monitored only for intervals of <10 s.

“Run” (oscillatory) and “tumble” (non-oscillatory or erratic) motions of wild-type cells were within the range of values reported in the literature. Mutants that could not run or that could not tumble exhibited the expected motions. With the assay, Golding and colleagues also measured higher-order swimming dynamics, such as changes in velocity and swimming direction. Now that the system has been fully characterized, it could be applied to the study of chemotaxis of individual cells, say the researchers. (Nat. Methods 2009, DOI 10.1038/nmeth.1380)