Full 3D translational and rotational optical control of multiple rod-shaped bacteria

Towards non-blind optical tweezing by finding 3D refractive index changes through off-focus interferometric tracking

In modern 3D microscopy, holding and orienting arbitrary biological objects with optical forces instead of using coverslips and gel cylinders is still a vision. Although optical trapping forces are strong enough and related photodamage is acceptable, the precise (re-) orientation of large specimen with multiple optical traps is difficult, since they grab blindly at the object and often slip off. Here, we present an approach to localize and track regions with increased refractive index using several holographic optical traps with a single camera in an off-focus position. We estimate the 3D grabbing positions around several trapping foci in parallel through analysis of the beam deformations, which are continuously measured by defocused camera images of cellular structures inside cell clusters. Although non-blind optical trapping is still a vision, this is an important step towards fully computer-controlled orientation and feature-optimized laser scanning of sub-mm sized biological specimen for future 3D light microscopy.

Optimal trapping stability of Escherichia coli in oscillating optical tweezers

Single-beam oscillating optical tweezers can be used to trap rod-shaped bacterial cells and align them with their long axis lying within the focal plane. While such configuration is useful for imaging applications, the corresponding imaging resolution is limited by the fluctuations of the trapped cell. We study the fluctuations of four of the coordinates of the trapped cell, two for its center of mass position and two for its angular orientation, showing the way they depend on the trap length and the trapping beam power. We find that optimal trapping stability is obtained when the trap length is about the same as the cell length and that cell fluctuations in the focal plane decrease like the inverse of the trapping power.

Synchronization in pairs of rotating active biomotors

Although synchronization is a well-known physical phenomenon, experimental studies of its emergence in living bacterial cells are still scarce. The difficulty in generating a controlled scenario to detect synchronization has limited the experimental outcomes so far. We present a realization based on holographic optical tweezers in which adhered pairs of self-propelled bacteria rotate in a plane. The separation distance between the bacteria determines the strength of the hydrodynamic coupling. Despite the noisy environment and autonomous dynamics of the living bacteria, we find evidence of phase locking and frequency entrainment in their rotation. The observation of higher order frequency synchronization is also discussed.

Conical Refraction Bottle Beams for Entrapment of Absorbing Droplets

Conical refraction (CR) optical bottle beams for photophoretic trapping of airborne absorbing droplets are introduced and experimentally demonstrated. CR describes the circular split-up of unpolarised light propagating along an optical axis in a biaxial crystal. The diverging and converging cones lend themselves to the construction of optical bottle beams with flexible entry points. The interaction of single inkjet droplets with an open or partly open bottle beam is shown implementing high-speed video microscopy in a dual-view configuration. Perpendicular image planes are visualized on a single camera chip to characterize the integral three-dimensional movement dynamics of droplets. We demonstrate how a partly opened optical bottle transversely confines liquid objects. Furthermore we observe and analyse transverse oscillations of absorbing droplets as they hit the inner walls and simultaneously measure both transverse and axial velocity components.

Optically driven full-angle sample rotation for tomographic imaging in digital holographic microscopy

This study presents a novel tomographic imaging technique for living biomedical samples using an optically driven full-angle rotation scheme based on digital holographic microscopy, in which the three-dimensional refractive index distribution inside the sample can be measured and analyzed. To accomplish the full-angle sample rotation, two optical traps are driven by highly focused spots on the top and bottom of the sample. The rim image of the sample outside the focal depth at the different rotation angles and propagation distances can be corrected and compensated, respectively, via numerical focusing; therefore, tomographic imaging of the sample can be conducted. The proposed approach shows that an entire symmetric spectrum can be acquired for tomographic reconstruction without the missing apple core problem as in traditional sample-rotation schemes. The three-dimensional refractive index of living yeast in a fluid medium is measured and verified.

Role of Cyclic Di-GMP and Exopolysaccharide in Type IV Pilus Dynamics

For Pseudomonas aeruginosa, levels of cyclic di-GMP (c-di-GMP) govern the transition from the planktonic state to biofilm formation. Type IV pili (T4P) are crucial determinants of biofilm structure and dynamics, but it is unknown how levels of c-di-GMP affect pilus dynamics. Here, we scrutinized how c-di-GMP affects molecular motor properties and adhesive behavior of T4P. By means of retraction, T4P generated forces of ∼30 pN. Deletion mutants in the proteins with known roles in biofilm formation, swarming motility, and exopolysaccharide (EPS) production (specifically, the diguanylate cyclases sadC and roeA or the c-di-GMP phosphodiesterase bifA) showed only modest effects on velocity or force of T4P retraction. At high levels of c-di-GMP, the production of exopolysaccharides, particularly of Pel, is upregulated. We found that Pel production strongly enhances T4P-mediated surface adhesion of P. aeruginosa, suggesting that T4P-matrix interactions may be involved in biofilm formation by P. aeruginosa Finally, our data support the previously proposed model of slingshot-like "twitching" motility of P. aeruginosaIMPORTANCE Type IV pili (T4P) play various important roles in the transition of bacteria from the planktonic state to the biofilm state, including surface attachment and surface sensing. Here, we investigate adhesion, dynamics, and force generation of T4P after bacteria engage a surface. Our studies showed that two critical components of biofilm formation by Pseudomonas aeruginosa, T4P and exopolysaccharides, contribute to enhanced T4P-mediated force generation by attached bacteria. These data indicate a crucial role for the coordinated impact of multiple biofilm-promoting factors during the early stages of attachment to a surface. Our data are also consistent with a previous model explaining why pilus-mediated motility in P. aeruginosa results in characteristic "twitching" behavior.

Spatiotemporally Resolved Tracking of Bacterial Responses to ROS-Mediated Damage at the Single-Cell Level with Quantitative Functional Microscopy

Herein we report on the implementation of photofunctional microparticles in combination with optical tweezers for the investigation of bacterial responses to oxidative stress by means of quantitative functional microscopy. A combination of a strongly hydrophobic axially substituted Si(IV) phthalocyanine adsorbed onto silica microparticles was developed, and the structural and photophysical characterization was carried out. The microparticles are able to produce reactive oxygen species under the fluorescence microscope upon irradiation with red light, and the behavior of individual bacteria can be consequently investigated in situ and in real time at the single cell level. For this purpose, a methodology was introduced to monitor phototriggered changes with spatiotemporal resolution. The defined distance between the photoactive particles and individual bacteria can be fixed under the microscope before the photosensitization process is started, and the photoinduced damage can be monitored by tracing the time-dependent fluorescence turn-on of a suitable marker. The results showed a distance-dependent photoinduced death time, defined as the onset of the incorporation of propidium iodide. Our methodology constitutes a new tool for the in vitro design and evaluation of photosensitizers for the treatment of cancer and infectious diseases with the aid of functional optical microscopy, as it enables a quantitative response evaluation of living systems toward oxidative stress. More generally, it provides a way to understand the response of an ensemble of living entities to reactive oxygen species by analyzing the behavior of a set of individual organisms.

Optical assembly of bio-hybrid micro-robots

The combination of micro synthetic structures with bacterial flagella motors represents an actual trend for the construction of self-propelled micro-robots. The development of methods for fabrication of these bacteria-based robots is a first crucial step towards the realization of functional miniature and autonomous moving robots. We present a novel scheme based on optical trapping to fabricate living micro-robots. By using holographic optical tweezers that allow three-dimensional manipulation in real time, we are able to arrange the building blocks that constitute the micro-robot in a defined way. We demonstrate exemplarily that our method enables the controlled assembly of living micro-robots consisting of a rod-shaped prokaryotic bacterium and a single elongated zeolite L crystal, which are used as model of the biological and abiotic components, respectively. We present different proof-of-principle approaches for the site-selective attachment of the bacteria on the particle surface. The propulsion of the optically assembled micro-robot demonstrates the potential of the proposed method as a powerful strategy for the fabrication of bio-hybrid micro-robots.

Optical disassembly of cellular clusters by tunable 'tug-of-war' tweezers

Bacterial biofilms underlie many persistent infections, posing major hurdles in antibiotic treatment. Here we design and demonstrate 'tug-of-war' optical tweezers that can facilitate the assessment of cell-cell adhesion-a key contributing factor to biofilm development, thanks to the combined actions of optical scattering and gradient forces. With a customized optical landscape distinct from that of conventional tweezers, not only can such 'tug-of-war' tweezers stably trap and stretch a rod-shaped bacterium in the observing plane, but, more importantly, they can also impose a tunable lateral force that pulls apart cellular clusters without any tethering or mechanical movement. As a proof of principle, we examined a Sinorhizobium meliloti strain that forms robust biofilms and found that the strength of intercellular adhesion depends on the growth medium. This technique may herald new photonic tools for optical manipulation and biofilm study, as well as other biological applications.

Synthesis and photo-postmodification of zeolite L based polymer brushes

Zeolite L macroinitiators are used for controlled radical copolymerization of a photo-active monomer and subsequent spin trapping of nitroxides results in diversely functionalized particles.

Controlled 3D rotation of biological cells using optical multiple-force clamps

Controlled three-dimensional (3D) rotation of arbitrarily shaped objects in the observation space of optical microscopes is essential for realizing tomographic microscope imaging and offers great flexibility as a noncontact micromanipulation tool for biomedical applications. Herein, we present 3D rotational control of inhomogeneous biological samples using 3D optical multiple-force clamps based on time-shared scanning with a fast focus-tunable lens. For inhomogeneous samples with shape and optical anisotropy, we choose diatoms and their fragments, and demonstrate interactive and controlled 3D rotation about arbitrary axes in 3D Cartesian coordinates. We also outline the hardware setup and 3D rotation method for our demonstrations.

Trapping and delivery of Escherichia coli in a microfluidic channel using an optical nanofiber

The stable trapping and direct delivery of bacteria are very important in studying their nanoscopic biochemical processes. Here, we report the stable optical trapping and direct delivery of Escherichia coli in a microfluidic channel. By placing an optical nanofiber (NF, diameter: 600 nm) in the channel and directing a laser beam with a wavelength of 980 nm and an optical power over 25 mW into the NF, E. coli were stably trapped by the NF. The trapping stability was tested in flowing suspensions and the delivery efficiency of the trapped E. coli was demonstrated. Experimental results were interpreted by numerical simulations and theoretical calculations.

Towards 3D modelling and imaging of infection scenarios at the single cell level using holographic optical tweezers and digital holographic microscopy

The analysis of dynamic interactions of microorganisms with a host cell is of utmost importance for understanding infection processes. We present a biophotonic holographic workstation that allows optical manipulation of bacteria by holographic optical tweezers and simultaneously monitoring of dynamic processes with quantitative multi-focus phase imaging based on self-interference digital holographic microscopy. Our results show that several bacterial cells, even with non-spherical shape, can be aligned precisely on the surface of living host cells and localized reproducibly in three dimensions. In this way a new label-free multipurpose device for modelling and quantitative analysis of infection scenarios at the single cell level is provided.

Extended Kramers-Moyal analysis applied to optical trapping

The Kramers-Moyal analysis is a well-established approach to analyze stochastic time series from complex systems. If the sampling interval of a measured time series is too low, systematic errors occur in the analysis results. These errors are labeled as finite time effects in the literature. In the present article, we present some new insights about these effects and discuss the limitations of a previously published method to estimate Kramers-Moyal coefficients at the presence of finite time effects. To increase the reliability of this method and to avoid misinterpretations, we extend it by the computation of error estimates for estimated parameters using a Monte Carlo error propagation technique. Finally, the extended method is applied to a data set of an optical trapping experiment yielding estimations of the forces acting on a Brownian particle trapped by optical tweezers. We find an increased Markov-Einstein time scale of the order of the relaxation time of the process, which can be traced back to memory effects caused by the interaction of the particle and the fluid. Above the Markov-Einstein time scale, the process can be very well described by the classical overdamped Markov model for Brownian motion.

Optical trapping of microrods: variation with size and refractive index

Optical traps can be characterized in terms of two simple parameters: the stiffness, given by the gradient of the force at mechanical equilibrium, and the strength, as expressed by the maximum restoring force available for displacement in a given direction. We present numerical calculations of these quantities for dielectric microrods of varying radius and refractive index held horizontally in pairs of holographically generated Gaussian beams. The resulting variations are seen to be influenced by optical resonances, as well as by the relative sizes of the beam waist and rod diameter. In addition, it is shown that trapping in these systems is sensitive to the polarization state of the incident field; i.e., for certain rods, trapping will occur for beams polarized perpendicular to the long axis of the rod, but not for beams polarized parallel to the long axis. Finally, friction coefficients are evaluated and used to estimate the maximum rates at which the rods may be dragged through the ambient medium.

Assessing bacterial adhesion on an individual adhesin and single pili level using optical tweezers

Optical tweezers (OT) are a technique that, by focused laser light, can both manipulate micrometer sized objects and measure minute forces (in the pN range) in biological systems. The technique is therefore suitable for assessment of bacterial adhesion on an individual adhesin-receptor and single attachment organelle (pili) level. This chapter summarizes the use of OT for assessment of adhesion mechanisms of both non-piliated and piliated bacteria. The latter include the important helix-like pili expressed by uropathogenic Escherichia coli (UPEC), which have shown to have unique and intricate biomechanical properties. It is conjectured that the large flexibility of this type of pili allows for a redistribution of an external shear force among several pili, thereby extending the adhesion lifetime of bacteria. Systems with helix-like adhesion organelles may therefore act as dynamic biomechanical machineries, enhancing the ability of bacteria to withstand high shear forces originating from rinsing flows such as in the urinary tract. This implies that pili constitute an important virulence factor and a possible target for future anti-microbial drugs.

Dynamic multiple-beam counter-propagating optical traps using optical phase-conjugation

Counter-propagating optical traps are widely used where long working distances, axially symmetric trapping potentials, or standing light waves are required. We demonstrate that optical phase-conjugation can automatically provide a counter-propagating replica of a wide range of incident light fields in an optical trapping configuration. The resulting counter-propagating traps are self-adjusting and adapt dynamically to changes of the input light field. It is shown that not only single counter-propagating traps can be implemented by phase-conjugation, but also structured light fields can be used. This step towards more complex traps enables advanced state-of-the-art applications where multiple traps or other elaborated trapping scenarios are required. The resulting traps cannot only be used statically, but they can be rearranged in real-time and allow for interactive dynamic manipulation.