Groups of bacteria streamed through the fluid, creating an ever-changing pattern of swirls and blips visible to naked eye. In a bacterial ballet, the tiny organisms seemed to be moving through the fluid of the dish in coordinated fashion, almost like flocking in birds or schooling in fish.
"We all looked at this and said, 'Oh my goodness, why is this happening?' We were all surprised. We are still surprised," said Raymond Goldstein, professor of physics and applied mathematics at the University of Arizona in Tucson. On Monday, Feb. 16, at 5:30 p.m., he will give a presentation at the Biophysical Society annual meeting in Baltimore about the phenomenon, which he and his colleagues call "self-concentration."
Other members of the UA research team are John Kessler, emeritus professor of physics, and UA graduate students Christopher Dombrowski, Luis Cisneros, and Sunita Chatkaew. Some of the research funding was provided by the National Science Foundation.
Although there had been theoretical suggestions that such "flocking" behavior might be not be limited to birds or fish, this could be the first time it's been observed in bacteria.
The bacteria the UA team observed, Bacillus subtilis, swim by rotating a series of corkscrew-like appendages, called flagella, that are about five times the body length of one of the rod-shaped bacteria.
In a culture, when a bacterium uses up the dissolved oxygen nearby, it swims toward the oxygen-rich surface. So do all its fellows. But at the same time gravity acts to pull the bacteria back down. The swimming-up and sinking-down sets up a convective current, much as does cold air sinking toward the floor of a room.
The currents created by one swimming bacterium affect the others. Once a critical concentration of bacteria is reached, the motions of thousands of flagella set up additional large currents in the fluid, creating the organized jets and vortices observed by the UA team.
"This is like a large group of people who, by the very act of swimming in a pool, get carried around by the mutually reinforcing turbulence their swimming created," said Goldstein.
Bacteria sometimes act in concert once a certain number of them are massed in one place. They may detect each other's presence by chemicals each secretes into the medium, a phenomenon called 'quorum sensing.' Bioluminescent bacteria, for example, use quorum sensing to know when to turn on their lights. Quorum sensing is also used by the bacteria that cause gum disease.
Goldstein said the swirls and jets the researchers observed effectively stir the fluid, making it well-mixed. That could help the bacteria detect one another.
The team's next step is developing mathematical models that can predict and describe the phenomena. Ultimately, he said, the work may have applications in biotechnology under circumstances where researchers need to have minute quantities of solutions well-mixed.