Mechanical shear controls bacterial penetration in mucus.

Mucus plays crucial roles in higher organisms, from aiding fertilization to protecting the female reproductive tract. Here, we investigate how anisotropic organization of mucus affects bacterial motility. We demonstrate by cryo electron micrographs and elongated tracer particles imaging, that mucus anisotropy and heterogeneity depend on how mechanical stress is applied. In shallow mucus films, we observe bacteria reversing their swimming direction without U-turns. During the forward motion, bacteria burrowed tunnels that last for several seconds and enable them to swim back faster, following the same track. We elucidate the physical mechanism of direction reversal by fluorescent visualization of the flagella: when the bacterial body is suddenly stopped by the mucus structure, the compression on the flagellar bundle causes buckling, disassembly and reorganization on the other side of the bacterium. Our results shed light into motility of bacteria in complex visco-elastic fluids and can provide clues in the propagation of bacteria-born diseases in mucus.

Fight the flow: the role of shear in artificial rheotaxis for individual and collective motion.

To navigate in complex fluid environments, swimming organisms like fish or bacteria often reorient their bodies antiparallel or against the flow, more commonly known as rheotaxis. This reorientation motion enables the organisms to migrate against the fluid flow, as observed in salmon swimming upstream to spawn. Rheotaxis can also be realized in artificial microswimmers - self-propelled particles that mimic swimming microorganisms. Here we study experimentally and by computer simulations the rheotaxis of self-propelled gold-platinum nanorods in microfluidic channels. We observed two distinct modes of artificial rheotaxis: a high shear domain near the bottom wall of the microfluidic channel and a low shear regime in the corners. Reduced fluid drag in the corners promotes the formation of many particle aggregates that rheotax collectively. Our study provides insight into the biomimetic functionality of artificial self-propelled nanorods for dynamic self-assembly and the delivery of payloads to targeted locations.