Multivalent binding through force activation of fimbriated bacteria revealed by digital holographic phase contrast microscopy
Dr. Peter van Oostrum (University of Natural Resources and Life Science, Vienna)

Many bacteria interact with surfaces via long (µm) and thin (nm) tethers called fimbriae or pili. These are implicated in pathogenic biofilm formation via both specific and nonspecific bonds. We investigated the interaction of bacteria under shear with surfaces displaying different densities of mannose or hydrophobic domains of similar size as the fimbriae tip. The fimbriae tips form specific bonds with mannose, while only non-specific bonds can form to hydrophobic patches. Digital holographic phase contrast microscopy was implemented to map 3D bacterium trajectories with ~50 nm precision and millisecond time resolution. Simultaneously, shear forces on the bacteria were measured by resolving the laminar flow profile. Our results show transient bonds that are formed and, irrespective of binding specificity, a rolling motion regulated by the binding valency; the higher the number of bound fimbriae, the slower the movement. Upon increased flow, the bacteria slowed down and eventually stopped. This behaviour correlates with a force-induced displacement towards the interface, affording shorter fimbria to also bind to surface domains, thereby increasing the binding valency. Our data suggests that fimbriae allow bacteria to explore different surface niches, responding both to the affinity and number of available contact points, as well as to flow. In contrast to other force-controlled binding mechanisms, e.g. catch-bonds, force actuation of the binding valency is generic; it is not limited to a specific interaction, but merely dependent on general properties such as the number, length distribution and mechanical properties of the tethers. We argue that this effect may be ubiquitous in biology and play an important, but overlooked, role in cellular interactions under physiological conditions.