Run-and-tumble motion

[1] The tumbling is erratic or "random" in the sense of a stochastic process—that is, the new direction is sampled from a probability density function, which may depend on the organism's local environment (e.g., chemical gradients).

[1][5] Run-and-tumble motion is found in many peritrichous bacteria, including E. coli, Salmonella typhimurium, and Bacillus subtilis.

[8] Genetically diverse groups of microorganisms rely upon directed motility (taxis), such as chemotaxis or phototaxis, to optimally navigate through complex environments or colonise host tissues.

[9][8] In the model organisms Escherichia coli and Salmonella, bacteria swim in a random pattern produced by alternating counterclockwise (CCW) and clockwise (CW) flagellar rotation.

Chemoreceptors detect attractants or repellents and stimulate responses through a signalling cascade that controls the direction of the flagellar motor.

[1][5] In a uniform medium, run-and-tumble trajectories appear as a sequence of nearly straight segments interspersed by erratic reorientation events, during which the bacterium remains stationary.

[11][12] For example, a tumbling distribution that depends on a chemical gradient can guide bacteria toward a food source or away from a repellant, a behavior referred to as chemotaxis.

Nonetheless, Synechocystis species can move in cell suspensions and on moist surfaces and by using retractile type IV pili, displaying an intermittent two phase motion; a high-motility run and a low-motility tumble (see diagram).

These results were well described by a mathematical model based on a linear response theory proposed by Vourc’h et al.[15][8] Synechocystis cells can also undergo biased motility under directional illumination.

Vourc’h et al. (2020) showed that this biased motility stems from the averaged displacements during run periods, which is no longer random (as it was in the uniform illumination).

[16] Increasing the light intensity of more than ~475 μmol m−2 s−1 reverses the direction of Synechocystis cells to move away from the high levels of radiation source.

Detecting by transmembrane chemoreceptors [23][24][25] the microorganism performs a three-dimensional random walk is observed in a homogenous environment, and the direction of each run is identified after a tumble.