The dance of bacteria

Bacteria are of immense importance all over the Earth: They maintain soil structure, control the bio-chemistry and the photosynthesis processes of the oceans and clean contaminated land - to cite just a few examples. And yet astonishingly little is known about certain properties of these organisms which are among the oldest and most richly diverse life forms on Earth. Their flowing behaviour is one of these areas. To make more efficient headway, bacteria gladly group together and move on collectively. In highly organized, collective swarming motility, they can cover considerable distances. This behaviour has large advantages over the life of a single individual: It is easier for a whole colony of bacteria to survive in difficult environments, to find food or to conquer new territory. When such a colony of bacteria moves, it resembles - when observed from outside - a fluid which is why the whole thing is also called an "active fluid". Yet if the flowing behaviour is examined more exactly, astounding differences are revealed: In fluids where genuine laminar flows are experienced, there are flows without any disturbances, whereas in the bacterial "fluid", there are chaotic flows and vortices. A totally different fluid dynamics ultimately prevails. This is because the motion is caused completely differently: In a conventional fluid external influences cause turbulence, in contrast, in a bacterial fluid the propulsion comes from deep within the bacterial fluid itself, namely from the many millions of flagella. These are thread-like structures on the surface of bacteria which enable them to move.

There is a good reason for mathematicians at PTB to deal with fluids: Their numerical simulations might immediately be important for several branches of industry. Thus, the description of conventional fluids by the Navier-Stokes equation is of enormous importance for industrial applications, such as in the use of simulations of fluid and structural interactions or as becomes apparent in connection with flow measurements. The understanding and the simulation of the novel type of active fluids represent an important first step towards a multitude of future applications. For example, microbes could be added to a fluid, and in this way their flowing characteristics could be manipulated in a targeted manner, they could be effectively stirred or medication could be transported within the body. Although the collective behaviour of microbes is the subject of this research, much too little is still known about the properties of active fluids. In particular, the models developed are very complicated and require many parameters, making the quantitative comparison with experiments impossible.

PTB researchers and scientists at the University of Cambridge have jointly suggested a simple extension of the Navier-Stokes equation for active fluids, which even becomes instable without external influences. In the publication, three-dimensional simulations (PTB) of the model with experiments of viscous Bacillus subtilis suspensions (Cambridge, Princeton) were quantitatively compared. For the first time, it was possible to compare a model with experimental data and model parameters. In this way physical quantities which are difficult to access such as elasticity or anisotropic viscosity of the active fluid could be measured indirectly.

The results of the international research group are certain to be followed up by interesting new studies, to get to know the origin of collective behaviour in still more depth, and possibly initiate future practical applications.

Contact

Dr. Sebastian Heidenreich,
PTB Working Group 8.41 Mathematical Modelling and Simulation,
Phone: +49(0)30 3481-7726,
E-mail: sebastian.heidenreich(at)ptb.de

Original scientific publication

J. Dunkel (Cambridge), S. Heidenreich (PTB), K. Drescher (Princeton), H. H. Wensink (Paris), M. Bär (PTB), R. E. Goldstein (Cambridge): Fluid Dynamics of Bacterial Turbulence. Physical Review Letters 110, 228102 (2013)