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,
firstname.lastname@example.org and John Kessler, email@example.com)
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.)