in the presence of an air bubble*

Shewanella oneidensis MR-1 is a common bacterium, that can be found in natural water reservoirs and can live both in aerobic and anaerobic conditions. It has substantial technological relevance, as it can be exploited to produce biofuel. In nature one can observe the formation of areas with higher bacterial density (chemotactic band), next to metallic mineral rocks. This process takes place over a period of few days. Our experiments show that, in the proximity of an air bubble, the chemotactic band originates in a much shorter time -few hours- as the oxygen acts as a chemoattractant for the bacteria (aerotaxis). We develop a continuum model to track the evolution of the bacterial density, the oxygen concentration in the liquid and the bubble dynamics, finding good agreement with the experiments. We perform a numerical parametric study to assess the dependence of the characteristic properties of the chemotactic band on experimentally tuneable parameters, such as the radius of the bubble and the initial bacterial concentration. We find the optimal conditions for a fast formation of the aerotactic band.