Bacteria have the remarkable ability to swim upstream, a process called rheotaxis. This motion against flows can cause not only respiratory, gastrointestinal, and urinary tract infections, but also the contamination of medical devices and hospital equipment. However, it remains unknown how bacteria navigate upstream through these microstructured environments with narrow channels and wide cavities. Here, combining microbiology experiments with nanofabrication and mathematical modeling, we reveal how Escherichia coli bacteria invade in four stages: The (I) break-out from colonized cavities against the current, (II) propagation upstream in narrow connectors, (III) infiltration of new cavities, and (IV) colonization with biofilms under flow. Surprisingly, we find that wider channels with faster counterflows are actually more prone to invasion, but these incursions can be inhibited effectively with sharp corner designs. Next, we explore the serial invasion of multiple cavities in a row. We discover that instead of colonizing these nodes one by one slowly, the bacteria rapidly swim all the way upstream and form biofilm streamers there to take possession of the entire channel three times faster. These results shed new light on pathogen motility in host-relevant shear regimes, and they offer solutions that can be implemented directly in biomedical devices.

BIO: Arnold Mathijssen completed his undergraduate at University College London (2012), his PhD in biophysics with Julia Yeomans FRS at the University Oxford (2016), and a postdoc in bioengineering with Manu Prakash at Stanford University (2020). He is now a faculty member at the University of Pennsylvania and director of the Penn Working Group on Environmental and Biological Fluid Dynamics. Arnold was awarded the Sir Sam Edwards PhD Thesis Prize by the UK Institute of Physics, the ‘30 under 30’ Award by Scientific American, the HFSP Cross-Disciplinary Fellowship, the Charles Kittel Award by the American Physical Society, the Paul Sniegowski Award for Mentorship of Undergraduate Research, and the Cottrell Scholars Award. Arnold is also a popular science communicator known for culinary fluid mechanics and the science of pour-over coffee, with coverage in The New York Times, The Guardian, CNN, FOX, USA Today, and Food & Wine Magazine.

Morphogenesis is a developmental process through which plants and animals acquire their shape and form. This talk will focus on tissue mechanics underlying massive transformations in the shape of epithelial tissues during gastrulation of animal embryos. This transformation of tissue is often described in terms of a (visco-elastic) flow of cells driven by active, myosin motor generated, internal forces. Yet, despite their apparent “fluidity” epithelial tissue can support shear stress, so a deeper examination is in order. This talk will argue that morphogenetic transformations of tissue can be understood in terms of an adiabatic remodeling of the force balance governed by myosin motor activity and controlled by feedback. The ability of cells to directly control active cytoskeletal tension -much like muscles! - deprives tissue mechanics of a constitutive relation, pulling the rug under conventional elasticity as a starting point for a theory. This talk will formulate an alternative continuum mechanics of Active Solids, starting with prescribed internal active tension, that will exhibit “emergent elasticity” on large scales and provide a principled description of active plastic flow as controlled “morphing” transformation of shape that takes place on the force-balanced manifold. This new theory of Active Solids will also provide an example of how the study of biological phenomena pushes the envelope of Physics beyond what we have learned from textbooks.

Replaced by Center for Living Systems student-organized special lecture.