New scientific explanation for bacteria's ability to swim upstream

The process through which bacteria manage to swim against currents was not yet clear. A research team involving the Vienna Technical University found a physical explanation for this.

Researchers see the possibility of preventing bacterial locomotion

The process through which bacteria manage to swim against currents was not yet clear. A research team involving the Vienna Technical University found a physical explanation for this.

Bacteria can swim against the current - and this is often a serious problem, for example when they spread in water pipes or medical catheters. How they manage to do this has not been clear until now. An international research team, including Andreas Zöttl from the Vienna Technical University (TU Vienna), Austria, was able to answer this question.

With the help of experiments and mathematical calculations, a formula could be found that describes all essential aspects of the amazing bacterial movement. This could make it possible to prevent or at least slow down the spread of bacteria by appropriately designing tube surfaces and expanding the findings into further applications. The results have now been published in the journal Nature Communications.

The overlap between physics and biology

Many types of bacteria, such as the E. coli (which can often become a health hazard when waterborne), move around with the help of small tails: the flagella. "But you can't imagine it like the locomotion of a fish," says Andreas Zöttl from the Institute for Theoretical Physics at the TU Vienna. "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 that are overflowed by liquids - this can be a poorly cleaned shower cubicle, a sewage pipe or even a catheter hose. "The bacteria's behavior is particularly interesting on such surfaces," says 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 swim upstream.” Together with colleagues from Stanford University, Oxford University and the ESPCI in Paris, Zöttl set out to find a physical explanation for this effect.

Theory and experiment

Andreas Zöttl worked with theoretical-mathematical methods: He calculated how a bacterium in a flowing liquid can align and rotate, how the flow interacts with the movement of the flagella and which movement possibilities result from this, in purely mathematical terms. "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 light currents, the bacteria simply rotate in a circle, and 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.

The formula can be applied to many different types of bacteria

In parallel, technical methods were developed at the ESPCI to measure the movements of individual bacteria using specially controlled microscopes, and these measurements revealed exactly the same clearly distinguishable types of movement that the theoretical calculations had shown. "This shows us that our theory is correct," says 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 movement possibilities will enable them to find methods that prevent them from moving. "In the near future, it might be possible to equip catheters with a specific geometric surface structure that prevents bacteria from migrating against the current," hopes Zöttl.

A. Mathijssen et al., Oscillatory surface rheotaxis of swimming E. coli bacteria, Nature Communications, Volume 10, 3434 (2019).

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