4.1 Prokaryotic Flagella
67
Fig. 4.3 Simulated time-averaged flow field near a bacterium of length 8 μm. Left: Axisymmetric
flow field in a plane of motion. Right: Swirling flow in planes normal to the direction of motion at
the positions marked by white lines in the left panel. The magnitude of the velocity is color-coded,
increasing from blue to red (Hu et al, 2015)
(1883) proved this by switching illumination on and off in a homogeneous solution,
and bacteria reacted by backing up and stopping to steer their path. If a change in time
is beneficial in some respect, bacteria increase the duration of runs and, respectively,
decrease the frequency of tumbles. This results in a net motion along the gradient of
an attractant or against the gradient of a repellent.
During a run, usually lasting about one second, a flagellum rotates counterclockwise, propelling the cell forward. The motion of the helix is irreversible in time, so
that the “scallop theorem” is not applicable, even if the filament does not change
its shape. The simulated flow field in a plane of motion shown in the left panel of
Fig. 4.3 is close to the theoretical flow field of a pusher in Fig. 3.1.
When a tumble is signaled, the rotation direction is reversed, which causes the filament to change its conformation from “normal” (helical) to “semicoiled” or “curly”,
as sketched in Fig. 4.2. Following the tumble interval of about 0.1 second, as counterclockwise rotation is resumed, the helical conformation is restored but the new run
is directed randomly at some angle to the preceding one. Buckling instabilities play
an important role in reconfigurations in the course of a tumbling event (Vogel and
Stark, 2012). Detailed records of trajectories (Son et al, 2013) exhibit alternations
between reversals and large reorientations with a broad angular distribution centered
about 90 ◦ , shown, respectively, by green circles and red squares in Fig. 4.4. As is
evident from the right panel of this figure, where bacteria are attracted to a dead
copepod, run-and-tumble motion is not at all effective in minimizing the path to the
target.
Most bacteria have several flagella, some of them lophotrichous, with multiple
flagella located at the same spot on their surface, and many others (including the
“model bacterium” E. coli), peritrichous, with flagella projecting in all directions
67
Fig. 4.3 Simulated time-averaged flow field near a bacterium of length 8 μm. Left: Axisymmetric
flow field in a plane of motion. Right: Swirling flow in planes normal to the direction of motion at
the positions marked by white lines in the left panel. The magnitude of the velocity is color-coded,
increasing from blue to red (Hu et al, 2015)
(1883) proved this by switching illumination on and off in a homogeneous solution,
and bacteria reacted by backing up and stopping to steer their path. If a change in time
is beneficial in some respect, bacteria increase the duration of runs and, respectively,
decrease the frequency of tumbles. This results in a net motion along the gradient of
an attractant or against the gradient of a repellent.
During a run, usually lasting about one second, a flagellum rotates counterclockwise, propelling the cell forward. The motion of the helix is irreversible in time, so
that the “scallop theorem” is not applicable, even if the filament does not change
its shape. The simulated flow field in a plane of motion shown in the left panel of
Fig. 4.3 is close to the theoretical flow field of a pusher in Fig. 3.1.
When a tumble is signaled, the rotation direction is reversed, which causes the filament to change its conformation from “normal” (helical) to “semicoiled” or “curly”,
as sketched in Fig. 4.2. Following the tumble interval of about 0.1 second, as counterclockwise rotation is resumed, the helical conformation is restored but the new run
is directed randomly at some angle to the preceding one. Buckling instabilities play
an important role in reconfigurations in the course of a tumbling event (Vogel and
Stark, 2012). Detailed records of trajectories (Son et al, 2013) exhibit alternations
between reversals and large reorientations with a broad angular distribution centered
about 90 ◦ , shown, respectively, by green circles and red squares in Fig. 4.4. As is
evident from the right panel of this figure, where bacteria are attracted to a dead
copepod, run-and-tumble motion is not at all effective in minimizing the path to the
target.
Most bacteria have several flagella, some of them lophotrichous, with multiple
flagella located at the same spot on their surface, and many others (including the
“model bacterium” E. coli), peritrichous, with flagella projecting in all directions