The formula that makes bacteria float upstream

Between Physics and Biology

Many types of bacteria, such as the E. coli bacteria, which can often become a health hazard in water, move around with the help of small flagella tails. "This is quite different from the motion of a fish," says Andreas Zöttl from the Institute for Theoretical Physics at Vienna University of Technology. "Fish feel the direction of the current and can decide to swim in a specific direction. Bacteria are much simpler. Their behavior can be explained by very basic physical laws."

Bacteria often accumulate on surfaces overflown by liquids -- this can be the poorly cleaned shower cubicle, a sewage pipe or even a catheter. "The bacteria's behavior is particularly interesting on such surfaces," says Andreas Zöttl, "because it turns out that it is precisely there, directly on the surfaces, that the bacteria often migrate against the current. They are therefore not washed away with the wastewater, but they move upstream." Together with colleagues from Stanford University, Oxford University and the ESPCI in Paris, Andreas Zöttl set out to find a physical explanation for this effect.

Theory and experiment

Andreas Zöttl used mathematical methods: He calculated how a bacterium can be aligned and rotated in a flowing liquid, how the flow interacts with the movement of the flagella and which movement possibilities result from this. "This leads to the remarkable result that there are different, clearly distinguishable types of movement, depending on the strength of the flow," explains Andreas Zöttl.

In slow currents, the bacteria simply rotate in a circle, at a certain point they begin to move against the direction of flow. In even stronger currents, they oscillate back and forth on the surface, or they separate into two different groups that move in different directions. With a single mathematical formula, a whole range of bacterial movement patterns can be explained.

At the same time, new technological methods have been developed in Paris to measure the movements of individual bacteria with specially controlled microscopes -- and these measurements revealed exactly the same clearly distinguishable types of movement that the theoretical calculations had shown before. "This tells us that our theory is correct," says Andreas Zöttl. "What is particularly nice about this is that the results are very robust: They do not depend sensitively on any details, so our formula can be applied to many different types of bacteria." Even DNA strands floating around in the cell plasma can be described correctly with the new theory.

The team hopes that the newly gained understanding of bacterial motion will enable them to find methods that prevent bacteria from moving. "In future, it might be possible to equip catheters with a specific geometric surface structure that prevents bacteria from migrating against the current," hopes Andreas Zöttl.