Bacterial spinning top

Abstract

We have investigated the dynamics of a monotrichous bacteria cell near a wall boundary, taking elastic hook flexibility into consideration. Combining theoretical linear stability analysis and direct numerical computations via the boundary element method, we have found that the elastohydrodynamic coupling between the hook elasticity and cell rotational motion enables a stable vertical spinning behaviour like a low-Reynolds-number spinning top. The forwardly rotated flagellum, which generates the force exertion pushing towards the cell body, typically destabilizes the vertical upright position and leads to a boundary-following motion. In contrast, the backward rotation of the flagellum, generating a force pulling the cell body, contributes to stable upright behaviour in a large range of hook rigidity. Further numerical investigations have demonstrated that the non-spherical geometry of the cell body and boundary adhesive interactions affect the bacterial dynamics, leading to complex behaviours such as horizontal spinning and unstable vertical spinning motions, both of which are experimentally observed in Pseudomonas aeruginosa bacteria. These results highlight the rich diversity of bacterial surface motility emerging from mechanical boundary interactions coupled with the cell swimming and hook flexibility.

Transition to bound states for bacteria swimming near surfaces

“…Note. Recently, the authors were made aware of work by K. Ishimoto similar to that presented here [37].…”

Motile Bacteria at Oil–Water Interfaces: Pseudomonas aeruginosa

“…Over these time spans, there is evidence of switching between CW and CCW modes. The stability of vertical spinning of monotrichous bacteria near a solid surface has been analyzed hydrodynamically; elastohydrodynamic coupling between the hook elasticity and cell rotational motion played a critical role in this process . Our observations at the fluid interface might spur further research to relate the stability of this configuration to analysis and simulation of bacteria orientation near fluid boundaries, and the flexibility of the hook that connects the cell body to the motors that drive the flagellum.…”

Helical trajectories of swimming cells with a flexible flagellar hook

“…Although it has been observed in generic settings that a helical flagellum does not substantially deform under rotation [14], a more recent investigation by Jabbarzadeh & Fu indicates that both hook as well as flagellum deformability is needed to account for the large hook angles seen in such flicks [15,16], consistent with experimental observations [13]. Even without the added complexity of a cell body, the chirality of a helical filament results in coupling of translation and rotation which can lead to surprisingly rich dynamics under gravity [17], under magnetic actuation [18,19], near surfaces [20,21], in a background flow [22,23] or even double-helical trajectories for double-helical "superhelices" like insect spermatozoa [24,25]. For very soft filaments, other instabilities and dynamics abound [26][27][28] The end result of such flagellar activity is the body trajectory, itself an object of intense scrutiny.…”

“…The model of the flagellum will incorporate its geometry and the nontrivial relationship between the motor torque and its dynamics via the flexible hook, to second order in the flagellum amplitude. For this purpose we use the simplest resistive force theory approximation [47,48] as recently used in similar contexts, including the instability of bodies propelled by N flagella or swimming with a flexible flagellum near a wall [20,21,49], and neglecting hydrodynamic interactions with the cell body. Comparisons between this resistive force theory and full hydrodynamic theory have been explored in detail [48,50]; a comparison for this precise context in Ref.…”