Abstract
Directed motility enables swimming microbes to navigate their environment for resources via chemo-, photo-, and magneto-taxis. However, directed motility competes with fluid flow in porous microbial habitats, affecting biofilm formation and disease transmission. Despite this broad importance, a microscopic understanding of how directed motility impacts the transport of microswimmers in flows through constricted pores remains unknown. Through microfluidic experiments, we show that individual magnetotactic bacteria directed upstream through pores display three distinct regimes, whereby cells swim upstream, become trapped within a pore, or are advected downstream. These transport regimes are reminiscent of the electrical conductivity of a diode and are accurately predicted by a comprehensive Langevin model. The diode-like behavior persists at the pore scale in geometries of higher dimension, where disorder impacts conductivity at the sample scale by extending the trapping regime over a broader range of flow speeds. This work has implications for our understanding of the survival strategies of magnetotactic bacteria in sediments and for developing their use in drug delivery applications in vascular networks.
Highlights
Directed motility enables swimming microbes to navigate their environment for resources via chemo, photo, and magneto-taxis
Beyond gaining insight into the evolutionary advantages of natural magnetotactism, Magnetotactic bacteria (MTB) are a promising candidate in biomedical applications for targeted drug delivery due to their potential to be directed through heterogeneous vascular network flows[25,26]
The flow field in the micro-channel is measured by particle tracking velocimetry (Fig. 1c and Supplementary Fig. 2; see “Methods”), and the flow strength is characterized by the maximum, Umax, and minimum, Umin, centerline flow speeds, measured in the throat and pore, respectively
Summary
Directed motility enables swimming microbes to navigate their environment for resources via chemo-, photo-, and magneto-taxis. Hydraulic networks, ranging from saturated soils and sediments to human tissues and filtration media, play host to diverse swimming microbes[1,2,3] To navigate these complex geometries, cells use environmental cues to direct their motility towards favorable conditions via an array of sensing (e.g., chemotaxis)[4,5,6] and physical reorientation mechanisms (e.g., magnetotaxis)[7]. We use microfluidic experiments (Fig. 1) in 1D porous media to unveil a novel, pore-scale transport mechanism for dilute suspensions of magnetotactic bacteria: cells, directed to swim upstream against a continuous, externally controlled flow, become trapped in striking vortical orbits when traversing a constriction. These nonlinear transport properties pose new challenges for modeling directed active matter[15,33], far beyond the paradigms of classical passive scalar transport[34]
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