Warning: E. Coli Bacteria Can Swim Upstream

Here's a bit of disquieting research news - the bacteria E. coli tend to swim upstream in flowing liquids. The revelation, which appears in a paper to be published Monday, February 5 in the journal Physical Review Letters, could explain how the bacteria manage to make their way far up the urinary tract to cause pyelonephritis, a particularly nasty kidney infection.

Yale University researchers Jane Hill, Jonathan McMurry and Hur Koser collaborated with Ozge Kalkanci of Bogazici University in Istanbul on the work, which they believe is the first observation of the natural tendency of bacteria to swim upstream.

The researchers discovered the phenomenon by filming E. coli being swept along in tiny channels filled with flowing liquid. The bacteria tended to swim to their left (when viewed from above) as they were washed downstream. Eventually, their leftward swimming caused them to move toward the side of the channel, where they promptly turned around to swim back upstream. You can watch the behavior yourself in a video supplement to the paper that the researchers recorded. (The crosshairs in the video highlight the path of one of the bacteria.)

It seems that the leftward swimming and upstream migration result from the mechanical design of the bacteria. E. coli are propelled by whip-like flagella that push the bacteria along by rotating counterclockwise. Their cell bodies rotate clockwise in response to the torque of their twisting flagalla. The combination of motions cause E. coli to swim to the left when they encounter a surface in slowly moving or still fluid - an effect that has been observed in earlier studies.

The surprising result in the recent experiment, however, is that the motions also cause the bacteria to face upstream when they are submersed in a liquid flowing rapidly along a surface, in a manner that the researchers say is "much like a weather vane orienting into the wind."

The researchers suggest that the behavior could explain the incidence of infections in patients fitted with catheters, and could be the cause of the biofilms that form inside some plumbing systems. They even speculate that leaving a running hose in contact with the ground could lead to bacterial migrations out of the dirt and into the toilet tanks and water heater inside your house.

You might wonder what's the Physics of Sex connection to E. coli paddling upstream. Well, if you consider where the largest population of E. coli is in the human body, then you and your partner have yet another excellent reason to use condoms if you happen to practice anal sex. You wouldn't want to give those nasty fellas a chance to swim up anyone's urethra, would you?