Microswimmer Propulsion by Two Steadily Rotating Helical Flagella.

Henry Shum

doi:10.3390/mi10010065

Abstract

Many theoretical studies of bacterial locomotion adopt a simple model for the organism consisting of a spheroidal cell body and a single corkscrew-shaped flagellum that rotates to propel the body forward. Motivated by experimental observations of a group of magnetotactic bacterial strains, we extended the model by considering two flagella attached to the cell body and rotating about their respective axes. Using numerical simulations, we analyzed the motion of such a microswimmer in bulk fluid and close to a solid surface. We show that positioning the two flagella far apart on the cell body reduces the rate of rotation of the body and increases the swimming speed. Near surfaces, we found that swimmers with two flagella can swim in relatively straight trajectories or circular orbits in either direction. It is also possible for the swimmer to escape from surfaces, unlike a model swimmer of similar shape but with only a single flagellum. Thus, we conclude that there are important implications of swimming with two flagella or flagellar bundles rather than one. These considerations are relevant not only for understanding differences in bacterial morphology but also for designing microrobotic swimmers.

Highlights

The locomotion of microscopic organisms, such as bacteria, has been a topic of fascination and mathematical analysis for well over sixty years [1]
Studied magnetotactic bacteria include the strains MO-1 and MC-1 (Magnetococcus marinus), which are similar in morphology and differ from the species most widely studied in other contexts, such as Escherichia coli, Bacillus subtilis, or Vibrio alginolyticus
Experiments with MC-1 near flat surfaces do not indicate a tendency for such circular motion [19], suggesting that either the bacteria swim away from surfaces or that they exhibit small curvatures even close to a surface. Motivated by this unexpected behaviour at surfaces, we examined the simulated motion of bacteria with two flagella and compared the results to the well-studied model of propulsion by a single flagellum

Summary

Introduction

The locomotion of microscopic organisms, such as bacteria, has been a topic of fascination and mathematical analysis for well over sixty years [1]. Of particular interest are magnetotactic bacteria, which can readily be steered by applying magnetic fields [7,8]. Organisms that are not naturally magnetotactic can be controlled with magnetic fields after incorporation of magnetic particles; this has recently been demonstrated with the alga Chlamydomonas reinhardtii [9]. Studied magnetotactic bacteria include the strains MO-1 and MC-1 (Magnetococcus marinus), which are similar in morphology and differ from the species most widely studied in other contexts, such as Escherichia coli, Bacillus subtilis, or Vibrio alginolyticus. Each bundle consists of seven flagellar filaments and numerous fibrils enveloped in a sheath [12]. This complex structure propels the bacterium at speeds of up to 300 μm/s, an order of magnitude faster than many other flagellated bacteria

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