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The swimming track of the sulphur/sulphide-oxidizing bacterium Thiovulum majus is a left-handed helix. The cells modulate swimming speed by changing the tangential speed andlor the pitch and radius of the helix. Whether attached (to a mucous thread) or swimming, the spherical cells rotate around their anterior-posterior axis in a counter-clockwise direction when observed from the posterior pole. Swimming speeds may exceed 600 pm s-l, which is 5-6 times faster than recorded for any other bacterium. Thiovulum cells congregate at oxygen tensions of about 4% atmospheric saturation (085 kPa). Cells which accidentally leave the optimum zone make a U-turn within 150-200 pm, thus returning to where they came from. This represents a type of phobic response in which the eventual swimming direction is correlated with the initial direction; it is not a true chemotactic response in the sense that the cells can orient themselves in 0,-gradients. The 180"lbend of the swimming path is probably accomplished by changes in the rotational velocity component which take place when the cells swim into an adverse environment. The U-turn response allows the bacteria to maintain the characteristic 100-200 pm thick veils which separate the sulphidic and the oxygenated zone in or above sediments. Evidence for a chemosensory response to sulphide could not be found.

Motility and chemosensory behaviour of the sulphur bacterium Thiovulum majus