Figure 2.
Self-generated hydrodynamic flows cause BV to interact strongly with surfaces. The motility of active BV was monitored at various planes between a microscope slide and a coverslip (A) separated by 40–90 μm (in such a way that the bacterium does not interact with surfaces when freely swimming through the middle plane). BV trajectories were recorded on the surface of the coverslip (B), the middle plane (C), and slide (D) (a sum of 290, 296, and 305 trajectories drawn from four samples, respectively). The signed curvature (helicity) histograms on the coverslip (E), middle plane (F), and slide (G) show a transition from counterclockwise to clockwise rotations. Positive curvature values indicate clockwise rotation, while negative curvature values indicate counterclockwise rotation. The helicity data are summarized in (H) by approximating trajectories as circles with the radius Reff of 2/(κave + κmedian), where κave and κmedian are the mean and median for the corresponding curvature histogram (see the Supporting Material for details of the calculation). The helicity of the circles on the surfaces depend on the bacterium’s distance from the surface, the bacterium’s shape, its propulsion mechanism, and the size and shape of the bacterium (29) (see the Supporting Material for detailed discussion). In addition, there are differences in the surface roughnesses that are reflected as slight differences in how bacteria interact with the coverslip and the microscope slide surface. These differences arise, for instance, because the coverslip has more debris while dead bacteria tend to stick to the slide. In (I), we show histograms of speed as well as duration and lengths of trajectories on each plane shown for those trajectories given in (B)–(D). Frequency in all plots represents trajectory count. (J) Hydrodynamic simulations from Spagnolie and Lauga (29)—adapted to match our boundary conditions—demonstrate that hydrodynamic interactions are sufficient to account for switching helicity (and trajectory radius size changes) as bacteria move between two surfaces with a z-range set arbitrarily between 1 and 3. BV dwell longer at the coverslip and microscope slide (0 being the coverslip plane), indicating that mobile BV is hydrodynamically forced onto surfaces (K). Each data point here represents the average dwell time of 20 trajectories recorded at that specific plane. Tracking criteria are explained in Materials and Methods. The helicity of all trajectories in figures in the main body and Supporting Material are opposite to the observations as seen in the movies (in other words, as seen from the coverslip side of our inverted setup). To see this figure in color, go online.