Swirling and swarming in bacterial colonies: Interfacial driven flows and rheological complexity
AbstractCo-authors: firstname.lastname@example.org (chemical engineering, KU Leuven), Jan.email@example.com (CPMG KU Leuven), firstname.lastname@example.org (Chemistry, KU Leuven), email@example.com (Chemistry, KU Leuven)
Bacterial colonies have interesting dynamics and pattern formation, when moving atop a solid surface. In the present work we discuss how autoproduced bio-surfactants play a dominant role in pattern formation during either drying or swarming.
First, bacterial swarming is one of the most efficient methods by which bacteria colonize nutrient-rich environments and host tissues. Several mechanisms have been proposed to explain the phenomenon and the associated intricate macroscopic pattern formation. Here, by using a series of complementary genetic and physicochemical experiments and a simple mathematical analysis, we show how the bacterial swarming can be caused by a surface tension driven flow. The opportunistic pathogen, Pseudomonas aeruginosa, is studied, as it is relevant for such bacteria to control and arrest swarming. Moreover, P. aeruginosa bacteria secrete strong surface active component.
Second, auto-production or exogenous addition of a soluble non-ionic surfactant during the drying stages of the colonies induces complex flow patterns in a region near the edge of an evaporating droplet, even at very high surfactant concentrations. This is due to the generation of a Marangoni flow, itself being created by a heterogeneous distribution of surfactant molecules along the interface by an outward capillary flow, creating oscillatory or vortex dominated flows.
In all these systems Marangoni stresses can generate sufficiently strong forces to drive both surface and bulk flows, either in swarming colonies or during drying of bacterial systems.