Modeling of lophotrichous bacteria reveals key factors for swimming reorientation

Abstract

Lophotrichous bacteria swim through fluid by rotating their flagellar bundle extended collectively from one pole of the cell body. Cells experience modes of motility such as push, pull, and wrapping, accompanied by pauses of motor rotation in between. We present a mathematical model of a lophotrichous bacterium and investigate the hydrodynamic interaction of cells to understand their swimming mechanism. We classify the swimming modes which vary depending on the bending modulus of the hook and the magnitude of applied torques on the motor. Given the hook’s bending modulus, we find that there exist corresponding critical thresholds of the magnitude of applied torques that separate wrapping from pull in CW motor rotation, and overwhirling from push in CCW motor rotation, respectively. We also investigate reoriented directions of cells in three-dimensional perspectives as the cell experiences different series of swimming modes. Our simulations show that the transition from a wrapping mode to a push mode and pauses in between are key factors to determine a new path and that the reoriented direction depends upon the start time and duration of the pauses. It is also shown that the wrapping mode may help a cell to escape from the region where the cell is trapped near a wall.

Role of the two flagellar stators in swimming motility ofPseudomonas putida

“…The increased wrapped mode formation in agar that we observed for wild-type as well as for both stator mutant strains agrees with earlier observations that the wrapped mode typically occurs under external confinement and most likely serves as a strategy of flagellated cells to escape from mechanical traps (18, 19) or to enhance environmental spreading (31). It also fits the prediction that P. putida preferentially uses this slow swimming mode to navigate its natural environment (16, 22). How bacteria sense mechanical confinement and trapping remains an open question that will be addressed in subsequent studies.…”

“…It also fits the prediction that P. putida preferentially uses this slow swimming mode to navigate its natural environment (16,22). How bacteria sense mechanical confinement and trapping remains an open question that will be addressed in subsequent studies.…”

“…It was previously conjectured that wrapped mode formation results from an instability of the pulling bundle that is triggered by an increase in motor torque (18, 15). Numerical simulations of lophotrichously flagellated bacteria have confirmed this torque dependent filament wrapping (22). We thus assume that, under low viscosity conditions, Δ motAB mutant cells are not able to form the wrapped mode because not enough torque is generated by the MotCD stators that are less active in fluids, cf.…”

“…It is assumed that the wrapped mode is formed due to changes in motor torque (15, 18). Recently, numerical simulations supported this conjecture, showing increased wrapped mode formation under higher torque (22). In this study, we knocked out the two stators MotAB and MotCD and analysed swimming motility under different environmental conditions to address these questions.…”

Role of the Two Flagellar Stators in Swimming Motility of Pseudomonas putida

“…It is assumed that the wrapped mode is formed due to changes in motor torque ( 19 , 22 ). Recently, numerical simulations supported this conjecture, showing increased wrapped mode formation under higher torque ( 26 ). In this study, we knocked out the two stators, MotAB and MotCD, and analyzed swimming motility under different environmental conditions to address these questions.…”