Understanding how bacteria move close to surfaces is crucial for a broad range of microbial processes including biofilm formation, bacterial dispersion, and pathogenic infections. We used digital holographic microscopy to capture a large number (> 10 3 ) of three-dimensional Escherichia coli trajectories near and far from a surface. We found that within 20 μm from a surface tumbles are suppressed by 50% and reorientations are largely confined to surface-parallel directions, preventing escape of bacteria from the near-surface region. A hydrodynamic model indicates that the tumble suppression is likely due to a surfaceinduced reduction in the hydrodynamic force responsible for the flagellar unbundling that causes tumbling. These findings imply that tumbling does not provide an effective means to escape trapping near surfaces. The motility of bacteria near surfaces is relevant in a broad range of applications, from biofilm formation on medical instruments and wounds [1], to biofouling of engineered surfaces [2], and bioremediation of pollutants in the environment [3,4]. The presence of surfaces is known to alter bacterial motility by inducing circular swimming trajectories [5,6] and trapping cells in the near-surface region [7][8][9][10][11][12]. These near-surface behaviors have been attributed to longrange hydrodynamic interactions between swimming bacteria and the nearby surface [5,[7][8][9][10][11][12][13]: the surface modifies velocity and pressure fields around a swimming cell, and consequently forces and torques on the cell. Surfaces can also interfere with motility through steric interactions. For phytoplankton and spermatozoa, direct interaction of flagella with the surface is an important driver of surface scattering [14]. For smaller bacterial cells, measurements of the flow field around individual swimmers [15] indicate that hydrodynamic interactions are weak, suggesting that physical contact is critical in determining cell-surface interactions.Efforts to understand the trapping of bacteria by surfaces have largely neglected the effect of tumbles, the reorientations exhibited by wild-type peritrichous bacteria in their swimming trajectories. Studies have focused instead on smooth-swimming mutants for a range of species, including Escherichia coli [6,7,9,15,16], Caulobacter crescentus [17], and Bacillus subtilis [18]. When tumbling has been considered, in the context of surface interactions in E. coli, it was suggested to act as a mechanism that favors the cells' escape from the near-surface region [7,19]. However, subsequent observations have shown that wild-type (i.e., tumbling) E. coli attach to surfaces as effectively as a smooth-swimming mutant [20]. This inconsistency highlights the current limitations in our understanding of the surface interactions of bacteria. Here, we describe the effect of a surface on wild-type E. coli, and specifically on its ability to tumble, by capturing three-dimensional swimming trajectories of thousands of individual cells in a microfluidic device. We discovered that...
Recent advances in optical microscopy instrumentation and processing techniques have led to imaging that both breaks the diffraction barrier and enables sub-pixel resolution. This enhanced resolution has expanded the capabilities of particle tracking to nanoscale processes in soft matter including biomolecular, colloidal, and polymeric materials. This tutorial provides a basic understanding of particle tracking instrumentation, the fundamentals of tracking analysis, and potential sources of error and bias inherent in analyzing particle tracking. Finally, we provide a brief outlook for the future of particle tracking through the lens of machine learning.
The complex rotational and translational Brownian motion of anisotropic particles depends on their shape and the viscoelasticity of their surroundings. Because of their strong optical scattering and chemical versatility, gold nanorods would seem to provide the ultimate probes of rheology at the nanoscale, but the suitably accurate orientational tracking required to compute rheology has not been demonstrated. Here we image single gold nanorods with a laser-illuminated dark-field microscope and use optical polarization to determine their three-dimensional orientation to better than one degree. We convert the rotational diffusion of single nanorods in viscoelastic polyethylene glycol solutions to rheology and obtain excellent agreement with bulk measurements. Extensions of earlier models of anisotropic translational diffusion to three dimensions and viscoelastic fluids give excellent agreement with the observed motion of single nanorods. We find that nanorod tracking provides a uniquely capable approach to microrheology and provides a powerful tool for probing nanoscale dynamics and structure in a range of soft materials.
Bacteria are important examples of active or self-propelled colloids. Because of their directed motion, they accumulate near interfaces. There, they can become trapped and swim adjacent to the interface via hydrodynamic interactions, or they can adsorb directly and swim in an adhered state with complex trajectories that differ from those in bulk in both form and spatiotemporal implications. We have adopted the monotrichous bacterium Pseudomonas aeruginosa PA01 as a model species and have studied its motion at oil–aqueous interfaces. We have identified conditions in which bacteria swim persistently without restructuring the interface, allowing detailed and prolonged study of their motion. In addition to characterizing the ensemble behavior of the bacteria, we have observed a gallery of distinct trajectories of individual swimmers on and near fluid interfaces. We attribute these diverse swimming behaviors to differing trapped states for the bacteria in the fluid interface. These trajectory types include Brownian diffusive paths for passive adsorbed bacteria, curvilinear trajectories including curly paths with radii of curvature larger than the cell body length, and rapid pirouette motions with radii of curvature comparable to the cell body length. Finally, we see interfacial visitors that come and go from the interfacial plane. We characterize these individual swimmer motions. This work may impact nutrient cycles for bacteria on or near interfaces in nature. This work will also have implications in microrobotics, as active colloids in general and bacteria in particular are used to carry cargo in this burgeoning field. Finally, these results have implications in engineering of active surfaces that exploit interfacially trapped self-propelled colloids.
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