The bacterial flagellum uses rotation for generating forward motion, just like a helicopter main rotor does. Helicopters must utilize a secondary rotor to avoid self-rotation due to preservation of angular momentum. Alternatively they can have multiple rotors rotating at opposite directions. But all flagella rotate in the same direction. Does this mean that the bacterial cell rotates in the opposite direction?

If so, what is the typical frequency of such rotation? Shouldn't the rotation disturb sensing and nevigation in the environment by the cell?

  • 2
    $\begingroup$ Welcome to Biology.SE! What research have you done before asking it here? $\endgroup$ Apr 15, 2017 at 8:18
  • 1
    $\begingroup$ Google, and a bit of common sense. But no definitive conclusion... $\endgroup$
    – Uri M
    Apr 16, 2017 at 8:44

2 Answers 2


My opinion as a physicist would be that yes, they do.

Assuming a sperm-like structure with a single flagellum at the centre of a long, thin cell of uniform density. They would rotate around the axis of travel.

However, for other cells, this could be mitigated in several ways:

  1. If the cell was submersed in or floating on a liquid, it is likely that the effects of Archimede's Principal would be sufficient to keep it in a fixed orientation (this is how ships and submarines stay upright despite rotors that all go in the same direction - the least dense parts of a body always attempt to restore themselves to the highest position).

  2. Fluid in the cytoplasm could act as ballast, providing a counterrotation force thanks to the gravitational effect on it.

  3. If the cell happened to be particularily masseous in relation to the force the flaggela were providing, this would make any rotational force negligible.

  4. If the flagella were located away from the axis of travel (through the centre of gravity - ie to the sides) the torque would have less of an effect.

As for the navigational aspect of your question, if these cells do auto-rotate, they will have done so throughout their evoloutionary histories and therefore in all likelihood will have evolved navigation methods capable of dealing with this.

I will however put a disclaimer on this, I am not a biologist and am weighing in from the point of view of a physicist.

  • $\begingroup$ 1+2 are indeed nice ways to stabilize the cell. Has any such mechanism been found? $\endgroup$
    – Uri M
    Apr 16, 2017 at 8:47
  • $\begingroup$ 3 sounds wrong. if the cell is too massive in relation to the force, then the flagella will not be useful for motility. I would say that if the flagella applies a forward force F on the cell, it must generate torque of approximately F*r, where r is the radius of the flagellum helix, probably ~1 micron. Thus, with similar approximations I would say that using flagellum to move in velocity of 100 body lengths per second must compel auto-rotation with frequency of about 100 Hz. This is reasonable as it is a typical rotation frequency of flagella. $\endgroup$
    – Uri M
    Apr 16, 2017 at 9:01
  • $\begingroup$ @UriM, I agree, the flagella wouldn't be much use in that situation, but the cell would still be fairly stable $\endgroup$ Apr 16, 2017 at 17:43
  • $\begingroup$ As for whether or not any such mechanism has been found, 1 + 2 must be true for any cell of non-uniform density with a non-rigidly located cytoplasm $\endgroup$ Apr 16, 2017 at 17:44
  • $\begingroup$ point 4 applies to E. coli which has peritrichous flagella i.e. they arise at points over the entire cell surface and coalesce to form a flagellar bundle. There is a flexible component, the hook, between the motor and the filament which enables this. $\endgroup$
    – Alan Boyd
    May 16, 2017 at 6:01

Yes, the bacterial cell body rotates, but more slowly than you might think.

Bacteria are examples of 'life at low Reynold's number'. Here are some extracts from Howard C. Berg Random Walks in Biology (1993) Princeton University Press pp 75-80.

The Reynolds number is a dimensionless parameter in the equations of motion of a fluid that indicates the relative size of terms that describe intertial forces (forces required to accelerate masses) and viscous forces (forces due to viscous shear).

For a fish R is approximately 105; for a bacterium it is approximately 10-5. This difference has a profound effect on the physics of swimming.

The fish propels itself by accelerating water, the bacterium by using viscous shear.

To illustrate the lack of inertial effects at low Reynolds number, Berg estimates that if a bacterium stops swimming it comes to rest within about 1 µs and coasts for approximately 0.4 nm.

Flagellated bacteria swim by rotating one or more thin helical filaments... Torque generated by rotation of the filaments is balanced by viscous drag due to the counter-rotation of the cell and thrust generated by rotation of the filaments is balanced by viscous drag due to translation of the body of the cell.

In a recent paper from Berg's group direct measurements of the rotation of the flagellar filament and the cell body (for E. coli) are presented: the filament rotates at approximately 100 Hz; the cell body at 20 Hz.

The supplementary question is: Shouldn't the rotation disturb sensing and nevigation in the environment by the cell? Bacterial chemotaxis, at least in E. coli, relies upon time-averaged sensing of the concentrations of attractants in the surroundings via ligand-receptor interactions. Rotation of the cell body will not have any effect on these interactions.


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