Synchronized Swimming in Bacteria

Synchronized swimming in bacteria creates dramatic, previously unknown fluid patterns, researchers have discovered. With the Summer Olympics a few weeks away, physicists are showcasing some remarkable water action in aerobic bacteria, those that require oxygen to survive.

Bacteria swim through fluids by quickly rotating corkscrew-shaped "flagella," hair-like appendages that can be up to five times greater than the length of their main body (generally a few microns in size). It's not a routine feat for a bacterium to stay above water: a typical organism is about 10 percent denser than H₂O, so gravity tends to sink the creatures. Nonetheless, aerobic bacteria often swim up to the oxygen-rich surface in order to find and consume the million O₂ molecules per second that they need to survive. Conventional wisdom has been that such swimming does little to stir up the fluid itself.

Now, studying concentrated populations of the common aerobic bacterium Bacillus subtilis in small, half-inch-diameter fluid drops, a group of physicists at the University of Arizona (Raymond Goldstein, gold@physics.arizona.edu and John Kessler, kessler@physics.arizona.edu) has found that the combination of upward swimming and downward sinking in the suspension can produce striking flows that strongly mix the fluid (see pictures at Physics News Graphics) and concentrate the bacteria.

The crowd of swimming bacteria creates arrays of circulating vortices whose size is orders of magnitude larger than an individual bacterium. Jets and surges of fluid that straddle the vortices can move 100 microns per second and be as large as 100 microns. These speeds and lengths greatly exceed the swimming speeds and sizes of the organisms themselves, which move only tens of microns per second.

The new results provide, possibly for the first time, information on the way in which concentrated swimming bacteria order themselves. Such accumulations can have many important consequences. For example, they may greatly aid in the formation of biofilms, and can even be micromixers in tiny quantities of fluid.

In addition, the way the fluid currents concentrate bacteria into small spaces may be crucial for triggering the phenomenon of "quorum sensing," whereby congregated bacteria sense sufficiently high amounts of each other's secreted chemicals to turn on specific capabilities, such as the emission of light in bioluminescent bacteria. Quorum sensing is found in many important bacteria, including those that create gum disease. (Dombrowski et al., Physical Review Letters, upcoming article; also see University of Arizona press release.)