Congratulations to Yangguang TIAN, for his success of Master degree, and PhD position in EPFL

Congratulations to Yangguang TIAN, for success of defense for Master degree of Physics, on bacterial interaction with cells. Afterwords, he basically was accepted being PhD candidate in EPFL.

From 2022 to 2025, Yangguang investigated the bacterial interaction with cells in fluids, working as a master student in our group. Flagella-driven motility is a key mechanism for bacteria to achieve autonomous migration, and their movement behavior is closely related to the formation and spread of lesions within biological tissues, particularly in the blood system, which serves as a crucial medium for bacterial migration and widespread infection. From a fluid dynamics perspective, systematic physical models have been established to explain bacterial behavior in various inorganic complex environments, including run and tumble in fluid environments, circular motion under surface constraints, surface capture induced by self-alignment, and upstream and drift motion in shear flow. However, despite excluding the influence of biochemical factors, the high deformability of cells, complex surface components, extreme spatial confinement, and diverse flow environments within the blood system present new challenges for understanding bacterial movement and migration mechanisms in this context. Therefore, this study constructs a biological interface model composed of red blood cells (RBCs) and human vascular endothelial cells (HUVECs) to systematically explore the regulatory mechanisms of bacterial capture, escape, and accumulation behaviors at cell interfaces.

First, to investigate the effects of naturally occurring soft interfaces and confined crowded features in biological tissues on bacterial behavior, we constructed a confined space with soft and constrained characteristics based on the unique biconcave shape of red blood cells. This model simulates bacterial capture and escape behavior in cellular environments. Through dynamic imaging of flagellated bacteria, we identified three typical bacterial escape modes, which exhibited significant differences in bacterial flagellar status, escape kinematics, and capture duration. By regulating RBC stiffness and adhesion, the study systematically clarified the impact of soft and confined spatial characteristics on bacterial escape behavior.

Next, to determine whether theoretical models based on the assumption of ideal interfaces (rigid, smooth, and without biological interactions) are applicable to biological cell interfaces, we used microfluidic technology to construct an in vitro endothelial cell layer model system. We compared bacterial movement and accumulation behavior on glass and cell surfaces under both static and flow conditions. The study found that the cell surface.

significantly influenced the bacterial circular motion radius by regulating the effective distance between bacteria and the surface. Furthermore, bacteria on cell surfaces exhibited two distinct adhesion states: tight and loose adhesion. Bacteria in different adhesion states responded very differently to shear flow. In terms of bacterial surface accumulation, despite the positive contribution of adhesion behavior, physical effects, particularly shear-induced drift motion and shear trapping, still played a dominant role.

Finally, to explore bacterial movement behavior in extremely confined blood flow environments (analogous to capillaries), we constructed a single line red blood cell flow simulating capillaries using microfluidic technology and studied the dynamic bacterial entrainment effect induced by red blood cell vortices. The results showed that flow parameters (such as red blood cell spacing and flow rate) affected the bacterial orbital period in vortices, while bacterial morphology and movement characteristics determined their trajectory and spatial distribution. Spherical bacteria oscillated with vortices, non-motile long-rod bacteria were captured and rotated at the front of vortex, and motile long-rod bacteria drift with upstream orientation and accumulated at the vortex rear. After long-distance transport, a significant phase separation between bacteria and RBCs occurred, with different bacterial morphologies and motility characteristics exhibiting characteristic spatial distribution patterns.

In conclusion, this thesis systematically investigates bacterial capture, escape, and accumulation behaviors in soft confined spaces, cell surfaces, and capillary dynamic vortex environments under the constraints of red blood cells and vascular endothelial cells. These findings provide new experimental evidence for understanding bacterial movement and colonization in blood systems and biological tissues, and also contribute to the understanding of lesion formation and diffusion dynamics during blood infections.