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Bacterial motion in polymer solutions and biogels

Periodic Reporting for period 1 - GelSwimming (Bacterial motion in polymer solutions and biogels)

Reporting period: 2016-02-26 to 2018-02-25

The project GelSwimming addressed the question of how bacteria move through biological fluids, and how their motion is governed by physical forces. Interestingly, many cavities in the human body such as the lungs, the stomach or the reproductive tract are covered and protected by a thin layer of mucus gel. Under certain conditions pathogenic bacteria are able to penetrate the mucus layer which leads to serious infections. For example, in cystic fibrosis motile bacteria swim through the mucus layer and form biofilms which can be resistant to antibiotic medication. Interestingly, corals in the ocean are also covered by mucus. Again, pathogenic bacteria are able to penetrate the coral mucus layer and cause serious diseases resulting in a world-wide decline of corals. The underlying mechanisms how the bacteria are able to move through mucus are not yet fully understood. In our project we clarified the physical conditions under which bacterial locomotion in polymer
solutions and gels is possible. In contrast to existing theories on swimming in viscoelastic fluids we explicitly model the polymers and the bacteria via mesoscale computer simulations. Our novel approach enabled us to identify the microscopic conditions for bacterial locomotion in gels and the conditions for immobility (gel-trapping). Our findings may also inspire biologists and medical researchers in designing novel medication and finding strategies to avoid bacterial invasion into mucus, of relevance to both the prevention of human disease and the degradation of corals.
GelSwimming used numerical simulations and mathematical tools to understand the motion of model bacteria and other micro- and nano-particles in complex fluids and confined environments. First a powerful computer simulation code was developed in order to model the dynamics of various microscopic objects (Bacteria, active and passive colloids, ellipsoids and rods) moving in a viscous fluid consisting of water and many large polymers. Using this code, we were running extensive simulations of a swimming bacterium in different polymer solutions. We then analyzed how bacteria move in crosslinked polymer networks, in the presence of other swimmers, and under external fluid flow. We developed an analytical theory for bacteria moving in wall-bounded flow, and compared our results with experiments performed in collaboration with researchers at ESPCI Paris. In addition we studied the dynamics of rodlike particles in flows, und the influence of boundaries, noise, magnetic fields and gravity.
Our work on swimming in polymer solutions is the first who demonstrated the microscopic mechanisms why bacteria can swim faster in polymer solutions than in water despite of their higher viscosity. This is of great importance for the soft and active matter community, but it possibly is of interest for the biomedical community as well who is interested in the speed of bacterial contamination through biological fluids. Interestingly, we showed that including crosslinkers to the polymers and creating a polymer network, can stop bacterial invasion to physiologically relevant biogels such as mucus. This shows that pure physical mechnisms are of great relevance when stopping bacterial infections in such environments. We also showed that magnetic nano- and micro-particles can be used to focus and sort particles in lab-on-a chip devices which is expected to be of great importance for nanotechnologicalapplications in the near future.
Sketch of a bacterium moving in a complex fluid