Microbiological Garden - Bacterial bands

Bacterial bands

Close-up showing bands of sulfate-reducing bacteria around an oxygen
bubble. The bacteria are in three flat chambers filled with
oxygen-free medium. A single oxygen bubble was brought separately into each capillary. Bacteria density in the suspension decreases from
the left to the right picture. The red color of indicates the presence of
oxygen whereas the surrounding bacteria band appears whitish.
The bacteria bands appear more defined and dense around the
air bubble with increasing cell density. Scale bar on the left border represents 10 millimeter.

Aerotaxis
Motile bacteria are able to respond directly to chemical signals of their environment. Nutrients often serve as attractant and cause a positive movement to the origin whereas the presence of poisonous materials may result in the opposite way. However, the relation between bacteria and free molecular oxygen is more complicated. On one hand oxygen can serve in a positive manner as terminal electron acceptor for respiration to generate energy; on the other hand it can be toxic for many bacteria in higher concentrations. One example of this controversial situation, in particular in sulfate-reducing bacteria, is presented here.