Bacteria-driven Microswimmers Research Articles

Red/Far-Red Light Switchable Cargo Attachment and Release in Bacteria-Driven Microswimmers.

In bacteria-driven microswimmers, i.e., bacteriabots, artificial cargos are attached to flagellated chemotactic bacteria for active delivery with potential applications in biomedical technology. Controlling when and where bacteria bind and release their cargo is a critical step for bacteriabot fabrication and efficient cargo delivery/deposition at the target site. Toward this goal, photoregulating the cargo integration and release in bacteriabots using red and far-red light, which are noninvasive stimuli with good tissue penetration and provide high spatiotemporal control, is proposed. In the bacteriabot design, the surfaces of E. coli and microsized model cargo particles with the proteins PhyB and PIF6, which bind to each other under red light and dissociate from each other under far-red light are functionalized. Consequently, the engineered bacteria adhere and transport the model cargo under red light and release it on-demand upon far-red light illumination due to the photoswitchable PhyB-PIF6 protein interaction. Overall, the proof-of-concept for red/far-red light switchable bacteriabots, which opens new possibilities in the photoregulation in biohybrid systems for bioengineering, targeted drug delivery, and lab-on-a-chip devices, is demonstrated.

A stochastic model for bacteria-driven micro-swimmers.

Experiments have recently shown the feasibility of utilising bacteria as micro-scale robotic devices, with special attention paid to the development of bacteria-driven micro-swimmers taking advantage of built-in actuation and sensing mechanisms of cells. Here we propose a stochastic fluid dynamic model to describe analytically and computationally the dynamics of microscopic particles driven by the motion of surface-attached bacteria undergoing run-and-tumble motion. We compute analytical expressions for the rotational diffusion coefficient, the swimming speed and the effective diffusion coefficient. At short times, the mean squared displacement (MSD) is proportional to the square of the swimming speed, which is independent of the particle size (for fixed density of attached bacteria) and scales linearly with the number of attached bacteria; in contrast, at long times the MSD scales quadratically with the size of the swimmer and is independent of the number of bacteria. We then extend our result to the situation where the surface-attached bacteria undergo chemotaxis within the linear response regime. We demonstrate that bacteria-driven particles are capable of performing artificial chemotaxis, with a chemotactic drift velocity linear in the chemical concentration gradient and independent of the size of the particle. Our results are validated against numerical simulations in the Brownian dynamics limit and will be relevant to the optimal design of micro-swimmers for biomedical applications.

Motility and chemotaxis of bacteria-driven microswimmers fabricated using antigen 43-mediated biotin display

Bacteria-driven biohybrid microswimmers (bacteriabots) combine synthetic cargo with motile living bacteria that enable propulsion and steering. Although fabrication and potential use of such bacteriabots have attracted much attention, existing methods of fabrication require an extensive sample preparation that can drastically decrease the viability and motility of bacteria. Moreover, chemotactic behavior of bacteriabots in a liquid medium with chemical gradients has remained largely unclear. To overcome these shortcomings, we designed Escherichia coli to autonomously display biotin on its cell surface via the engineered autotransporter antigen 43 and thus to bind streptavidin-coated cargo. We show that the cargo attachment to these bacteria is greatly enhanced by motility and occurs predominantly at the cell poles, which is greatly beneficial for the fabrication of motile bacteriabots. We further performed a systemic study to understand and optimize the ability of these bacteriabots to follow chemical gradients. We demonstrate that the chemotaxis of bacteriabots is primarily limited by the cargo-dependent reduction of swimming speed and show that the fabrication of bacteriabots using elongated E. coli cells can be used to overcome this limitation.

Multifunctional Bacteria-Driven Microswimmers for Targeted Active Drug Delivery.

High-performance, multifunctional bacteria-driven microswimmers are introduced using an optimized design and fabrication method for targeted drug delivery applications. These microswimmers are made of mostly single Escherichia coli bacterium attached to the surface of drug-loaded polyelectrolyte multilayer (PEM) microparticles with embedded magnetic nanoparticles. The PEM drug carriers are 1 μm in diameter and are intentionally fabricated with a more viscoelastic material than the particles previously studied in the literature. The resulting stochastic microswimmers are able to swim at mean speeds of up to 22.5 μm/s. They can be guided and targeted to specific cells, because they exhibit biased and directional motion under a chemoattractant gradient and a magnetic field, respectively. Moreover, we demonstrate the microswimmers delivering doxorubicin anticancer drug molecules, encapsulated in the polyelectrolyte multilayers, to 4T1 breast cancer cells under magnetic guidance in vitro. The results reveal the feasibility of using these active multifunctional bacteria-driven microswimmers to perform targeted drug delivery with significantly enhanced drug transfer, when compared with the passive PEM microparticles.

Microemulsion-Based Soft Bacteria-Driven Microswimmers for Active Cargo Delivery.

Biohybrid cell-driven microsystems offer unparalleled possibilities for realization of soft microrobots at the micron scale. Here, we introduce a bacteria-driven microswimmer that combines the active locomotion and sensing capabilities of bacteria with the desirable encapsulation and viscoelastic properties of a soft double-micelle microemulsion for active transport and delivery of cargo (e.g., imaging agents, genes, and drugs) to living cells. Quasi-monodisperse double emulsions were synthesized with an aqueous core that encapsulated the fluorescence imaging agents, as a proof-of-concept cargo in this study, and an outer oil shell that was functionalized with streptavidin for specific and stable attachment of biotin-conjugated Escherichia coli. Motile bacteria effectively propelled the soft microswimmers across a Transwell membrane, actively delivering imaging agents (i.e., dyes) encapsulated inside of the micelles to a monolayer of cultured MCF7 breast cancer and J744A.1 macrophage cells, which enabled real-time, live-cell imaging of cell organelles, namely mitochondria, endoplasmic reticulum, and Golgi body. This in vitro model demonstrates the proof-of-concept feasibility of the proposed soft microswimmers and offers promise for potential biomedical applications in active and/or targeted transport and delivery of imaging agents, drugs, stem cells, siRNA, and therapeutic genes to live tissue in in vitro disease models (e.g., organ-on-a-chip devices) and stagnant or low-flow-velocity fluidic regions of the human body.

Propulsion and Chemotaxis in Bacteria-Driven Microswimmers.

Despite the large body of experimental work recently on biohybrid microsystems, few studies have focused on theoretical modeling of such systems, which is essential to understand their underlying functioning mechanisms and hence design them optimally for a given application task. Therefore, this study focuses on developing a mathematical model to describe the 3D motion and chemotaxis of a type of widely studied biohybrid microswimmer, where spherical microbeads are driven by multiple attached bacteria. The model is developed based on the biophysical observations of the experimental system and is validated by comparing the model simulation with experimental 3D swimming trajectories and other motility characteristics, including mean squared displacement, speed, diffusivity, and turn angle. The chemotaxis modeling results of the microswimmers also agree well with the experiments, where a collective chemotactic behavior among multiple bacteria is observed. The simulation result implies that such collective chemotaxis behavior is due to a synchronized signaling pathway across the bacteria attached to the same microswimmer. Furthermore, the dependencies of the motility and chemotaxis of the microswimmers on certain system parameters, such as the chemoattractant concentration gradient, swimmer body size, and number of attached bacteria, toward an optimized design of such biohybrid system are studied. The optimized microswimmers would be used in targeted cargo, e.g., drug, imaging agent, gene, and RNA, transport and delivery inside the stagnant or low‐velocity fluids of the human body as one of their potential biomedical applications.

Chemotaxis of bio-hybrid multiple bacteria-driven microswimmers

In this study, in a bio-hybrid microswimmer system driven by multiple Serratia marcescens bacteria, we quantify the chemotactic drift of a large number of microswimmers towards L-serine and elucidate the associated collective chemotaxis behavior by statistical analysis of over a thousand swimming trajectories of the microswimmers. The results show that the microswimmers have a strong heading preference for moving up the L-serine gradient, while their speed does not change considerably when moving up and down the gradient; therefore, the heading bias constitutes the major factor that produces the chemotactic drift. The heading direction of a microswimmer is found to be significantly more persistent when it moves up the L-serine gradient than when it travels down the gradient; this effect causes the apparent heading preference of the microswimmers and is the crucial reason that enables the seemingly cooperative chemotaxis of multiple bacteria on a microswimmer. In addition, we find that their chemotactic drift velocity increases superquadratically with their mean swimming speed, suggesting that chemotaxis of bio-hybrid microsystems can be enhanced by designing and building faster microswimmers. Such bio-hybrid microswimmers with chemotactic steering capability may find future applications in targeted drug delivery, bioengineering, and lab-on-a-chip devices.

Patterned and Specific Attachment of Bacteria on Biohybrid Bacteria-Driven Microswimmers.

A surface patterning technique and a specific and strong biotin-streptavidin bonding of bacteria on patterned surfaces are proposed to fabricate Janus particles that are propelled by the attached bacteria. Bacteria-driven Janus microswimmers with diameters larger than 3 μm show enhanced mean propulsion speed. Such microswimmers could be used for future applications such as targeted drug delivery and environmental remediation.

Bio-hybrid cell-based actuators for microsystems.

As we move towards the miniaturization of devices to perform tasks at the nano and microscale, it has become increasingly important to develop new methods for actuation, sensing, and control. Over the past decade, bio-hybrid methods have been investigated as a promising new approach to overcome the challenges of scaling down robotic and other functional devices. These methods integrate biological cells with artificial components and therefore, can take advantage of the intrinsic actuation and sensing functionalities of biological cells. Here, the recent advancements in bio-hybrid actuation are reviewed, and the challenges associated with the design, fabrication, and control of bio-hybrid microsystems are discussed. As a case study, focus is put on the development of bacteria-driven microswimmers, which has been investigated as a targeted drug delivery carrier. Finally, a future outlook for the development of these systems is provided. The continued integration of biological and artificial components is envisioned to enable the performance of tasks at a smaller and smaller scale in the future, leading to the parallel and distributed operation of functional systems at the microscale.