How bacteria find their way out of mazes

When bacteria spread through soil, tissue or other environments full of obstacles, they need to be flexible. If they rigidly adhered to a narrow pattern of movement, they would quickly become stranded in dead ends. In fact, bacteria move through open spaces until they get trapped, then they reorient themselves to slip through the next available hole. The model developed by a research team from Princeton University in collaboration with physicists from HHU and TU Darmstadt, among others, explains why this 'hop-and-trap' strategy works for bacteria and how it could be optimised for self-propelled polymer particles.

The model may benefit the development of future microrobots that need to move through complex three-dimensional environments. One application example would be tiny drug transporters that navigate through tumour tissue to release chemotherapeutic drugs at the appropriate location.

Princeton researcher Christina Kurzthaler, first author of the study now published in Nature Communications, says: “We wanted to understand what influence the 'hop-and-trap' mechanism has on the range of motion of bacteria in different environments.” The model organism was E. coli bacteria. Their movements in a porous medium had been measured in advance by the group led by Princeton professor Sujit Datta.

Co-first author Suvendu Mandal, a former staff member at the Institute for Theoretical Physics 2 at HHU and now a researcher at the Technical University of Darmstadt, worked with Kurzthaler to simulate how a bacterium can move randomly through a complex, three-dimensional environment. In the model, the bacteria were represented by plastic caterpillars in an aquarium full of ping-pong balls. Statistical analyses revealed patterns in the simulated paths of plastic caterpillars that closely resemble E. coli movements according to the 'hop-and-trap' mechanism; the model thus fits the natural model.

Most effective way of bacterial dispersal determined

The researchers then developed a simplified model to determine the most efficient way for bacteria to spread. This results in a general rule: a bacterium moves most effectively when it covers a distance that roughly corresponds to the length of the largest pores or holes in the environment before it reorients itself.

“If the path length is very small, the bacteria do not travel very far, they just move backwards and forwards randomly. If the path length is very long, the cells get easily entangled because they never reorient themselves,” Kurzthaler explains.

”The new model also provides a criterion for the development of polymers that are drug transporters capable of transporting drugs through the body or finding and degrading pollutants in the soil,“ says Mandal: ”If you wanted to design such a microrobot it would be important that it can reorient itself to explore the complex environment in which it is to operate.“

On this basis, it is also possible to model the collective behaviour of bacteria, such as how they form biofilms in porous materials. This also has important implications for everyday hospital life, where the aim is to identify sites that are particularly attractive for bacterial colonisation. Such forms can then be avoided in advance, in the design of equipment.

Prof. Dr. Hartmut Löwen, head of the Düsseldorf Institute and co-author of the study, is pleased that theoretical physics can repeatedly make important statements for seemingly very distant fields of science: ”The physics of soft matter in particular has many interfaces with practical life, as this study impressively demonstrates. The model can make statements for pharmacy, microbiology and even hospital hygiene."