Bacterial sunscreen: layer-by-layer deposition of UV-absorbing polymers on whole-cell biosensors

Nematode surface functionalization with hydrogel sheaths tailored in situ

Engineering the surfaces of biological organisms allows the introduction of novel functions and enhances their native functions. However, studies on surface engineering remained limited to unicellular organisms. Herein, nematode surfaces are engineered through in situ hydrogelation mediated by horseradish peroxidase (HRP) anchored to nematode cuticles. With this method, hydrogel sheaths of approximately 10-μm thickness are fabricated from a variety of polysaccharides, proteins, and synthetic polymers. Caenorhabditis elegans and Anisakis simplex coated with a hydrogel sheath showed a negligible decrease in viability, chemotaxis and locomotion. Hydrogel sheaths containing UV-absorbable groups and catalase functioned as shields to protect nematodes from UV and hydrogen peroxide, respectively. The results also showed that hydrogel sheaths containing glucose oxidase have the potential to be used as living drug delivery systems for cancer therapy. The nematode functionalization method developed in this study has the potential to impact a wide range of fields from agriculture to medicine.

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Technique to modify the surface of nematode with hydrogel is presented.

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Hydrogel is fabricated in situ using enzymatic cross-linking reaction.

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The technique is compatible with different materials and different species of nematodes.

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Hydrogel sheath protects the nematode from environmental stress.

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Nematode coated with drug-loaded hydrogel has a potential as living drug delivery system.

Cell-based biocomposite engineering directed by polymers

Biological cells such as bacterial, fungal, and mammalian cells always exploit sophisticated chemistries and exquisite micro- and nano-structures to execute life activities, providing numerous templates for engineering bioactive and biomorphic materials, devices, and systems. To transform biological cells into functional biocomposites, polymer-directed cell surface engineering and intracellular functionalization have been developed over the past two decades. Polymeric materials can be easily adopted by various cells through polymer grafting or in situ hydrogelation and can successfully bridge cells with other functional materials as interfacial layers, thus achieving the manufacture of advanced biocomposites through bioaugmentation of living cells and transformation of cells into templated materials. This review article summarizes the recent progress in the design and construction of cell-based biocomposites by polymer-directed strategies. Furthermore, the applications of cell-based biocomposites in broad fields such as cell research, biomedicine, and bioenergy are discussed. Last, we provide personal perspectives on challenges and future trends in this interdisciplinary area.

X-Ray Photoelectron Spectroscopy on Microbial Cell Surfaces: A Forgotten Method for the Characterization of Microorganisms Encapsulated With Surface-Engineered Shells

Encapsulation of single microbial cells by surface-engineered shells has great potential for the protection of yeasts and bacteria against harsh environmental conditions, such as elevated temperatures, UV light, extreme pH values, and antimicrobials. Encapsulation with functionalized shells can also alter the surface characteristics of cells in a way that can make them more suitable to perform their function in complex environments, including bio-reactors, bio-fuel production, biosensors, and the human body. Surface-engineered shells bear as an advantage above genetically-engineered microorganisms that the protection and functionalization added are temporary and disappear upon microbial growth, ultimately breaking a shell. Therewith, the danger of creating a "super-bug," resistant to all known antimicrobial measures does not exist for surface-engineered shells. Encapsulating shells around single microorganisms are predominantly characterized by electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, particulate micro-electrophoresis, nitrogen adsorption-desorption isotherms, and X-ray diffraction. It is amazing that X-ray Photoelectron Spectroscopy (XPS) is forgotten as a method to characterize encapsulated yeasts and bacteria. XPS was introduced several decades ago to characterize the elemental composition of microbial cell surfaces. Microbial sample preparation requires freeze-drying which leaves microorganisms intact. Freeze-dried microorganisms form a powder that can be easily pressed in small cups, suitable for insertion in the high vacuum of an XPS machine and obtaining high resolution spectra. Typically, XPS measures carbon, nitrogen, oxygen and phosphorus as the most common elements in microbial cell surfaces. Models exist to transform these compositions into well-known, biochemical cell surface components, including proteins, polysaccharides, chitin, glucan, teichoic acid, peptidoglycan, and hydrocarbon like components. Moreover, elemental surface compositions of many different microbial strains and species in freeze-dried conditions, related with zeta potentials of microbial cells, measured in a hydrated state. Relationships between elemental surface compositions measured using XPS in vacuum with characteristics measured in a hydrated state have been taken as a validation of microbial cell surface XPS. Despite the merits of microbial cell surface XPS, XPS has seldom been applied to characterize the many different types of surface-engineered shells around yeasts and bacteria currently described in the literature. In this review, we aim to advocate the use of XPS as a forgotten method for microbial cell surface characterization, for use on surface-engineered shells encapsulating microorganisms.

High-Yield Production of Biohybrid Microalgae for On-Demand Cargo Delivery

Biohybrid microswimmers exploit the swimming and navigation of a motile microorganism to target and deliver cargo molecules in a wide range of biomedical applications. Medical biohybrid microswimmers suffer from low manufacturing yields, which would significantly limit their potential applications. In the present study, a biohybrid design strategy is reported, where a thin and soft uniform coating layer is noncovalently assembled around a motile microorganism. Chlamydomonas reinhardtii (a single-cell green alga) is used in the design as a biological model microorganism along with polymer-nanoparticle matrix as the synthetic component, reaching a manufacturing efficiency of ≈90%. Natural biopolymer chitosan is used as a binder to efficiently coat the cell wall of the microalgae with nanoparticles. The soft surface coating does not impair the viability and phototactic ability of the microalgae, and allows further engineering to accommodate biomedical cargo molecules. Furthermore, by conjugating the nanoparticles embedded in the thin coating with chemotherapeutic doxorubicin by a photocleavable linker, on-demand delivery of drugs to tumor cells is reported as a proof-of-concept biomedical demonstration. The high-throughput strategy can pave the way for the next-generation generation microrobotic swarms for future medical active cargo delivery tasks.

The Theophylline Aptamer: 25 Years as an Important Tool in Cellular Engineering Research

The theophylline aptamer was isolated from an oligonucleotide library in 1994. Since that time, the aptamer has found wide utility, particularly in synthetic biology, cellular engineering, and diagnostic applications. The primary application of the theophylline aptamer is in the construction and characterization of synthetic riboswitches for regulation of gene expression. These riboswitches have been used to control cellular motility, regulate carbon metabolism, construct logic gates, screen for mutant enzymes, and control apoptosis. Other applications of the theophylline aptamer in cellular engineering include regulation of RNA interference and genome editing through CRISPR systems. Here we describe the uses of the theophylline aptamer for cellular engineering over the past 25 years. In so doing, we also highlight important synthetic biology applications to control gene expression in a ligand-dependent manner.

Controlling the Growth of Staphylococcus epidermidis by Layer-By-Layer Encapsulation

Commensal skin bacteria such as Staphylococcus epidermidis are currently being considered as possible components in skin-care and skin-health products. However, considering the potentially adverse effects of commensal skin bacteria if left free to proliferate, it is crucial to develop methodologies that are capable of maintaining bacteria viability while controlling their proliferation. Here, we encapsulate S. epidermidis in shells of increasing thickness using layer-by-layer assembly, with either a pair of synthetic polyelectrolytes or a pair of oppositely charged polysaccharides. We study the viability of the cells and their delay of growth depending on the composition of the shell, its thickness, the charge of the last deposited layer, and the degree of aggregation of the bacteria which is varied using different coating procedures-among which is a new scalable process that easily leads to large amounts of nonaggregated bacteria. We demonstrate that the growth of bacteria is not controlled by the mechanical properties of the shell but by the bacteriostatic effect of the polyelectrolyte complex, which depends on the shell thickness and charge of its outmost layer, and involves the diffusion of unpaired amine sites through the shell. The lag times of growth are sufficient to prevent proliferation for daily topical applications.

Single cells in nanoshells for the functionalization of living cells

Inspired by the characteristics of cells in live organisms, new types of hybrids have been designed comprising live cells and abiotic materials having a variety of structures and functionalities. The major goal of these studies is to uncover hybridization approaches that promote cell stabilization and enable the introduction of new functions into living cells. Single-cells in nanoshells have great potential in a large number of applications including bioelectronics, cell protection, cell therapy, and biocatalysis. In this review, we discuss the results of investigations that have focused on the synthesis, structuration, functionalization, and applications of these single-cells in nanoshells. We describe synthesis methods to control the structural and functional features of single-cells in nanoshells, and further develop their applications in sustainable energy, environmental remediation, green biocatalysis, and smart cell therapy. Perceived limitations of single-cells in nanoshells have been also identified.

Biphasic Supramolecular Self-Assembly of Ferric Ions and Tannic Acid across Interfaces for Nanofilm Formation

Cell nanoencapsulation provides a chemical tool for the isolation and protection of living cells from harmful, and often lethal, external environments. Although several strategies are available to form nanometric films, most methods heavily rely on time-consuming multistep processes and are not biocompatible. Here, the interfacial supramolecular self-assembly and film formation of ferric ions (Fe^III ) and tannic acid (TA) in biphasic systems is reported, where Fe^III and TA come into contact each other and self-assemble across the interface of two immiscible phases. The interfacial nanofilm formation developed is simple, fast, and cytocompatible. Its versatility is demonstrated with various biphasic systems: hollow microcapsules, encasing microbial or mammalian cells, that are generated at the water-oil interface in a microfluidic device; a cytoprotective Fe^III -TA shell that forms on the surface of the alginate microbead, which then entraps probiotic Lactobacillus acidophilus; and a pericellular Fe^III -TA shell that forms on individual Saccharomyces cerevisiae. This biphasic interfacial reaction system provides a simple but versatile structural motif in materials science, as well as advancing chemical manipulability of living cells.

One-step deposition of a melanin-like polymer on individual Escherichia coli cells exhibiting a special UV resistance effect

Melanin-like polydopamine encapsulated E. coli cells could retained cells viability, inhibited cell division and protected cells from UV radiation. These provide both fundamental research and applications of cell encapsulation for UV resistance.

Tuning cell surface charge in E. coli with conjugated oligoelectrolytes

Conjugated oligoelectrolytes intercalate into and associate with membranes, thereby changing the surface charge of microbes, as determined by zeta potential measurements.

Cationic conjugated oligoelectrolytes (COEs) varying in length and structural features are compared with respect to their association with E. coli and their effect on cell surface charge as determined by zeta potential measurements. Regardless of structural features, at high staining concentrations COEs with longer molecular dimensions associate less, but neutralize the negative surface charge of E. coli to a greater degree than shorter COEs.

Pendant ionic groups of conjugated oligoelectrolytes govern their ability to intercalate into microbial membranes

The ionic groups of lipid membrane intercalating conjugated oligoelectrolytes affect their interaction with E. coli and application in microbial fuel cells.

Spectral and structural comparison between bright and dim green fluorescent proteins in Amphioxus

The cephalochordate Amphioxus naturally co-expresses fluorescent proteins (FPs) with different brightness, which thus offers the rare opportunity to identify FP molecular feature/s that are associated with greater/lower intensity of fluorescence. Here, we describe the spectral and structural characteristics of green FP (bfloGFPa1) with perfect (100%) quantum efficiency yielding to unprecedentedly-high brightness, and compare them to those of co-expressed bfloGFPc1 showing extremely-dim brightness due to low (0.1%) quantum efficiency. This direct comparison of structure-function relationship indicated that in the bright bfloGFPa1, a Tyrosine (Tyr159) promotes a ring flipping of a Tryptophan (Trp157) that in turn allows a cis-trans transformation of a Proline (Pro55). Consequently, the FP chromophore is pushed up, which comes with a slight tilt and increased stability. FPs are continuously engineered for improved biochemical and/or photonic properties, and this study provides new insight to the challenge of establishing a clear mechanistic understanding between chromophore structural environment and brightness level.

Cell surface engineering with edible protein nanoshells

Natural protein (silk fibroin) nanoshells are assembled on the surface of Saccharomyces cerevisiae yeast cells without compromising their viability. The nanoshells facilitate initial protection of the cells and allow them to function in encapsulated state for some time period, afterwards being completely biodegraded and consumed by the cells. In contrast to a traditional methanol treatment, the gentle ionic treatment suggested here stabilizes the shell silk fibroin structure but does not compromise the viability of the cells, as indicated by the fast response of the encapsulated cells, with an immediate activation by the inducer molecules. Extremely high viability rates (up to 97%) and preserved activity of encapsulated cells are facilitated by cytocompatibility of the natural proteins and the formation of highly porous shells in contrast to traditional polyelectrolyte-based materials. Moreover, in a high contrast to traditional synthetic shells, the silk proteins are biodegradable and can be consumed by cells at a later stage of growth, thus releasing the cells from their temporary protective capsules. These on-demand encapsulated cells can be considered a valuable platform for biocompatible and biodegradable cell encapsulation, controlled cell protection in a synthetic environment, transfer to a device environment, and cell implantation followed by biodegradation and consumption of protective protein shells.