Flexibacter is a filamentous bacterium which possesses the following characteristics: 
- not branched;
- mobile (gliding motion);
- straight/bowed filaments, mainly in the water between the flocs;
- filament length < 200 µm;
- cell diameter 0.5 - 0.8 µm;
- no attached growth;
- no sheath;
- septa not visible;
- Gram negative;
- Neisser negative. 

The thin, moving filaments are so characteristic that confusion with other filamentous species is practically impossible.

Occurrence in activated sludge

Flexibacter never occurs en masse in activated sludge. It is probably an indicator organism for oxygen poor conditions in the treatment plant.
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