76
4 Motion of Microorganisms
Fig. 4.15 Top row: Collisions of individual bacteria. Two counter-propagating myxobacteria become anti-parallel. Middle row: Collision of two small clusters moving in opposite directions and
passing each other. Bottom row: Merging of two clusters (Harvey et al, 2013)
Further clustering may lead to the formation of bands propagating along a chemical gradient, similar to polar (Sect. 1.2) or nematic (Sect. 2.2) active particles. The
model put forward by Keller and Segel (1971) couples a diffusion–drift equation for
the bacterial density to a reaction–diffusion equation for the nutrient to arrive at a
traveling band solution. The model has acquired a “classical” status, and found wide
applications in studies of bacterial chemotaxis.
Yet, bacteria are more sophisticated than that. Just as people, while belonging
to the same species H. sapiens, have different physical (and other) abilities, so
too do bacteria. They use run-and-tumble motion to detect chemical gradients (see
Sect. 4.1), but the ratio between the durations of the run and tumble periods, what
is called the tumble bias (TB), varies among individuals. Procrastinating fellows
with a high TB waste more time tumbling and diffuse more slowly in a uniform
environment (Fig. 4.16a). This makes them less sensitive to chemical gradients, and
Fig. 4.16 (a) Trajectories of bacteria with different tumble bias (color coded) in a uniform environment. Scale bar 0.2 mm. (b) Simulated density profiles of migrating bacteria with different tumble
bias. The black curve shows the nutrient concentration distribution (Fu et al, 2018)
4 Motion of Microorganisms
Fig. 4.15 Top row: Collisions of individual bacteria. Two counter-propagating myxobacteria become anti-parallel. Middle row: Collision of two small clusters moving in opposite directions and
passing each other. Bottom row: Merging of two clusters (Harvey et al, 2013)
Further clustering may lead to the formation of bands propagating along a chemical gradient, similar to polar (Sect. 1.2) or nematic (Sect. 2.2) active particles. The
model put forward by Keller and Segel (1971) couples a diffusion–drift equation for
the bacterial density to a reaction–diffusion equation for the nutrient to arrive at a
traveling band solution. The model has acquired a “classical” status, and found wide
applications in studies of bacterial chemotaxis.
Yet, bacteria are more sophisticated than that. Just as people, while belonging
to the same species H. sapiens, have different physical (and other) abilities, so
too do bacteria. They use run-and-tumble motion to detect chemical gradients (see
Sect. 4.1), but the ratio between the durations of the run and tumble periods, what
is called the tumble bias (TB), varies among individuals. Procrastinating fellows
with a high TB waste more time tumbling and diffuse more slowly in a uniform
environment (Fig. 4.16a). This makes them less sensitive to chemical gradients, and
Fig. 4.16 (a) Trajectories of bacteria with different tumble bias (color coded) in a uniform environment. Scale bar 0.2 mm. (b) Simulated density profiles of migrating bacteria with different tumble
bias. The black curve shows the nutrient concentration distribution (Fu et al, 2018)