Under threat of being scrubbed away with disinfectant,
individual bacteria can improve their odds of survival by joining together to
form colonies, called biofilms. What Arnold Mathijssen, postdoctoral fellow in
bioengineering at Stanford University, wanted to understand was how stationary biofilms
find food once they've devoured nearby nutrients.
When bacteria move, they disturb the liquids that surround them
in the microscopic world. The researchers explored the strength of that
disturbance in a single bacterium that moves in a way that is similar to many
pathogenic species, including those that cause gastritis and cholera. They
found that as this bacterium swims forward, it creates a tiny but stable
current in the surrounding liquid with fluid moving toward its center and away from
the head and tail.
Then, they calculated the flows produced by a colony of randomly
arranged bacteria and were surprised to see that it created a strong,
consistent tide capable of pulling in nutrients. This occurred regardless of
the orientation of each bacterium so long as the colony was thicker in some
areas than others, which causes fluid to move from high points to low points.
Simulations of more orderly bacteria resulted in even stronger circulation.
Within organized biofilms, the researchers found two common
patterns of movement: vortexes and asters. In a vortex pattern, the bacteria
move in concentric circles and produce a flow that brings nutrients down to the
biofilm's center and then pushes the fluid out the sides. In an aster pattern,
the bacteria move toward a central point, creating a flow that moves from the
edge of the biofilm until it rises back up, over the center.
"The powerful thing about this is you can add these
patterns up," Mathijssen said. "Rather than having to know the
position and orientation of every single bacterium, you only need to know the
basic patterns that make up the colony and then it's very easy to derive the
overall transport flow."
The researchers were able to combine vortex and aster patterns
within a single biofilm to determine how the bacteria would push, pull and
whirl the fluids around them. As a final test, the researchers took calculations
representing the complex, realistic motion of bacteria swarming -- as they
might on the surface of a table -- and predicted the strength of that swarm's
transport flow. The result were large vortices that spanned distances beyond
the boundaries of the biofilm, suitable for keeping the colony fed.
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Pharmaceutical Microbiology