Abstract
In this study, in a bio-hybrid microswimmer system driven by multiple Serratia marcescens bacteria, we quantify the chemotactic drift of a large number of microswimmers towards L-serine and elucidate the associated collective chemotaxis behavior by statistical analysis of over a thousand swimming trajectories of the microswimmers. The results show that the microswimmers have a strong heading preference for moving up the L-serine gradient, while their speed does not change considerably when moving up and down the gradient; therefore, the heading bias constitutes the major factor that produces the chemotactic drift. The heading direction of a microswimmer is found to be significantly more persistent when it moves up the L-serine gradient than when it travels down the gradient; this effect causes the apparent heading preference of the microswimmers and is the crucial reason that enables the seemingly cooperative chemotaxis of multiple bacteria on a microswimmer. In addition, we find that their chemotactic drift velocity increases superquadratically with their mean swimming speed, suggesting that chemotaxis of bio-hybrid microsystems can be enhanced by designing and building faster microswimmers. Such bio-hybrid microswimmers with chemotactic steering capability may find future applications in targeted drug delivery, bioengineering, and lab-on-a-chip devices.
Highlights
Relying on a precise microfluidic chemical gradient generator, we first traced out the chemotactic response of the free swimming bacteria S. marcescens to L-serine, and an optimal concentration gradient that leads to the strongest chemotactic response was empirically determined
To quantify the apparent chemotactic drift of the microswimmer swarm, we examined the center of mass (COM) position of the microswimmers captured in each imaging frame and plotted its y component (COM-y) over time (Fig. 3(b))
Chemotaxis is a rather common and understood behavior of individual flagellated bacteria, such as S. marcescens and E. coli; it is crucial for bacteria survival because chemotaxis navigates them towards nutrient sources and away from hazardous environments
Summary
Relying on a precise microfluidic chemical gradient generator, we first traced out the chemotactic response of the free swimming bacteria S. marcescens to L-serine (chemoattractant), and an optimal concentration gradient that leads to the strongest chemotactic response was empirically determined. Chemotactic drifting process of multi-bacteria-driven microswimmer swarms were imaged and quantified. By tracking the individual microswimmers and statistically analyzing the trajectories, we identified the critical factors and the behind physical mechanisms which enabled the chemotaxis in the multi-bacteria-driven microswimmers
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