Bacterial bands
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Bioconvection. Closely packed swarms of bacteria sink down into the lower solution with a higher density.
Click at the picture to watch a video (AVI-File, 308 kB) in fast motion (200-fold).
The specific weight of the bacteria is a little higher as those of the surrounding medium. Usually, single bacteria do not sink down because of the great friction of the surrounding water sheath. Furthermore single bacteria are pushed around by movements of molecules (Brown movement) and all kinds of flows keeping them away from sedimentation. The only way to sink down is in dense cell-suspension. Under these conditions bacteria can form aggregates heavy and frictionless enough to sediment and generate a flow down that is strong enough to tear other bacteria to depth.
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