  Aerotaxis is the ability of motile cells to navigate toward oxygen. A key
question is the dependence of the aerotactic velocity with the local oxygen
concentration c. Here we combine simultaneous bacteria tracking and local
oxygen concentration measurements using Ruthenium encapsulated in micelles to
characterize the aerotactic response of Burkholderia contaminans, a motile
bacterium ubiquitous in the environment. In our experiments, an oxygen gradient
is produced by the bacterial respiration in a sealed glass capillary permeable
to oxygen at one end, producing a bacterial band traveling toward the oxygen
source. We compute the aerotactic response \chi(c) both at the population
scale, from the drift velocity in the bacterial band, and at the bacterial
scale, from the angular modulation of the run times. Both methods are
consistent with a power-law \chi \propto c^{-2}, in good agreement with
existing models based on the biochemistry of bacterial membrane receptors.
