Chemotaxis in external fields: Simulations for active magnetic biological matter

Abstract

The movement of microswimmers is often described by active Brownian particle models. Here we introduce a variant of these models with several internal states of the swimmer to describe stochastic strategies for directional swimming such as run and tumble or run and reverse that are used by microorganisms for chemotaxis. The model includes a mechanism to generate a directional bias for chemotaxis and interactions with external fields (e.g., gravity, magnetic field, fluid flow) that impose forces or torques on the swimmer. We show how this modified model can be applied to various scenarios: First, the run and tumble motion of E. coli is used to establish a paradigm for chemotaxis and investigate how it is affected by external forces. Then, we study magneto-aerotaxis in magnetotactic bacteria, which is biased not only by an oxygen gradient towards a preferred concentration, but also by magnetic fields, which exert a torque on an intracellular chain of magnets. We study the competition of magnetic alignment with active reorientation and show that the magnetic orientation can improve chemotaxis and thereby provide an advantage to the bacteria, even at rather large inclination angles of the magnetic field relative to the oxygen gradient, a case reminiscent of what is expected for the bacteria at or close to the equator. The highest gain in chemotactic velocity is obtained for run and tumble with a magnetic field parallel to the gradient, but in general a mechanism for reverse motion is necessary to swim against the magnetic field and a run and reverse strategy is more advantageous in the presence of a magnetic torque. This finding is consistent with observations that the dominant mode of directional changes in magnetotactic bacteria is reversal rather than tumbles. Moreover, it provides guidance for the design of future magnetic biohybrid swimmers.

Single-cell motion of magnetotactic bacteria in microfluidic confinement: interplay between surface interaction and magnetic torque

“…The simulation is based on an Active Brownian Particle method (ABP) in 3 dimensions 33 . Bacteria are represented by spheres, whose single trajectories are simulated by integrating the following equations: where t is the time, r is the position of the bacterium, e is the direction vector, T is the temperature, k B is the Boltzmann constant, γ t and γ r the translational and rotational friction coefficients, respectively, v is the speed of self propulsion, m × B is the the external magnetic torque with m being the magnetic moment of the bacterium and B the external magnetic field, F WCA and T reor .…”

“…The simulation is based on an Active Brownian Particle method (ABP) in 3 dimensions 33 . Bacteria are represented by spheres, whose single trajectories are simulated by integrating the following equations:…”

“…The tactic behavior of microorganisms is known to help their navigation and proliferation in complex environments. Accordingly, a line of research has also developed, which seeks to understand the directionality of microswimmers and how to control them due to their tactic behaviors or due to interactions with external fields 8,32,33 . This regard, magnetic fields are particularly interesting, as they .…”

“…The tactic behavior of microorganisms is known to help their navigation and proliferation in complex environments. Accordingly, a line of research has also developed, which seeks to understand the directionality of microswimmers and how to control them due to their tactic behaviors or due to interactions with external fields 8,32,33 This regard, magnetic fields are particularly interesting, as they allow the remote steering of microswimmers exhibiting a magnetic moment, a feature suitable for the remote control of the swimmers performing biomedical tasks in the human body 7 . Such microswimmers include magnetotactic bacteria 34–36 , i.e.…”

Magnetic Actuation Methods in Bio/Soft Robotics

“…Instead of the typical run‐and‐tumble known for E. coli , a run and reverse is typically observed for MTB. [ 191 ] This is, without any U‐turn of the cell body, the bacteria inverse their propulsion direction while simultaneously maintaining the direction of their magnetic moment. An overview of flagella apparatus from a set of typical bacteria species is provided in Table 1.…”