Cellular Biosensors with Engineered Genetic Circuits

A single-microbe living bioelectronic sensor for intracellular amperometric analysis

Subcellularly amperometric analysis in situ is crucial for understanding intracellular redox biochemistry and subcellular heterogeneity. Unfortunately, the ultra-small size and complex microenvironment inside the cell pose a great challenge to achieve this goal. To address the challenge, a minimized living microbial sensor has been fabricated in this work for amperometric analysis. Here, by fabricating the dimidiate microelectrode as the working electrode, while fitting a living electroactive bacterium (EAB) as the transducer, outward extracellular electron transfer (EET) of the sensory EAB is correlated with the concentration of lactic acid, which is electrochemically recorded and thus displays an electrical signal output for detection. In specific, the S. oneidensis modified dimidiate microelectrode (S.O.@GNE-NPE) acts as an integrated electroanalytical device to generate the electrical signal in situ. The established microcircuit provides unprecedented precision and sensitivity, contributing to subcellular amperometric measurement. The microbial sensor shows a linear response in the concentration range of 0-60 mM, with a limit of detection (LOD) at 0.3 mM. The microsensor also demonstrates good selectivity against interferences. Additionally, intracellular analysis of lactic acid provides direct evidence of enhanced lactic metabolism in cancer cells as a result of "Warburg Effect". This work shows an example of nano-, bio- and electric technologies that have been integrated on the EAB-modified dimidiate microelectrode, and achieves intracellular biosensing application through such integration. It may give a new strategy on the combination of micro/nanotechnologies with sensory EAB for the necessary development of bioelectronic devices.

Yeast Surface-Displayed Quenchbody as a Novel Whole-Cell Biosensor for One-Step Detection of Influenza A (H1N1) Virus

Timely surveillance of airborne pathogens is essential to preventing the spread of infectious diseases and safeguard human health. Methods for sensitive, efficient, and cost-effective detection of airborne viruses are needed. With advances in synthetic biology, whole-cell biosensors have emerged as promising platforms for environmental monitoring and medical diagnostics. However, the current design paradigm of whole-cell biosensors is mostly based on intracellular detection of analytes that can transport across the cell membrane, which presents a critical challenge for viral pathogens and large biomolecules. To address this challenge, we developed a new type of whole-cell biosensor by expressing and displaying VHH-based quenchbody (Q-body) on the surface of the yeast Saccharomyces cerevisiae for simple one-step detection of influenza A (H1N1) virus. Seventeen VHH antibody fragments targeting the hemagglutinin protein H1N1-HA were displayed on the yeast cells and screened for the H1N1-HA binding affinity. The functionally displayed VHHs were selected to create surface-displayed Q-body biosensors. The surface-displayed Q-body exhibiting the highest quenching and dequenching efficiency was identified. The biosensor quantitatively detected H1N1-HA in a range from 0.5 to 16 μg/mL, with a half-maximal concentration of 2.60 μg/mL. The biosensor exhibited high specificity for H1N1-HA over other hemagglutinin proteins from various influenza A virus subtypes. Moreover, the biosensor succeeded in detecting the H1N1 virus at concentrations from 2.4 × 104 to 1.5 × 107 PFU/mL. The results from this study demonstrated a new whole-cell biosensor design that circumvents the need for transport of analytes into biosensor cells, enabling efficient detection of the target virus particles.

Customizable gene sensing and response without altering endogenous coding sequences

Synthetic biology aims to modify cellular behaviors by implementing genetic circuits that respond to changes in cell state. Integrating genetic biosensors into endogenous gene coding sequences using clustered regularly interspaced short palindromic repeats and Cas9 enables interrogation of gene expression dynamics in the appropriate chromosomal context. However, embedding a biosensor into a gene coding sequence may unpredictably alter endogenous gene regulation. To address this challenge, we developed an approach to integrate genetic biosensors into endogenous genes without modifying their coding sequence by inserting into their terminator region single-guide RNAs that activate downstream circuits. Sensor dosage responses can be fine-tuned and predicted through a mathematical model. We engineered a cell stress sensor and actuator in CHO-K1 cells that conditionally activates antiapoptotic protein BCL-2 through a downstream circuit, thereby increasing cell survival under stress conditions. Our gene sensor and actuator platform has potential use for a wide range of applications that include biomanufacturing, cell fate control and cell-based therapeutics.

Engineered Bacteria as Living Biosensors in Dermal Tattoos

Dermal tattoo biosensors are promising platforms for real-time monitoring of biomarkers, with skin used as a diagnostic interface. Traditional tattoo sensors have utilized small molecules as biosensing elements. However, the rise of synthetic biology has enabled the potential employment of engineered bacteria as living analytical tools. Exploiting engineered bacterial sensors will allow for potentially more sensitive detection across a broad biomarker range, with advanced processing and sense/response functionalities using genetic circuits. Here, the interfacing of bacterial biosensors as living analytics in tattoos is shown. Engineered bacteria are encapsulated into micron-scale hydrogel beads prepared through scalable microfluidics. These biosensors can sense both biochemical cues (model biomarkers) and biophysical cues (temperature changes, using RNA thermometers), with fluorescent readouts. By tattooing beads into skin models and confirming sensor activity post-tattooing, our study establishes a foundation for integrating bacteria as living biosensing entities in tattoos.

Expanding salivary biomarker detection by creating a synthetic neuraminic acid sensor via chimeragenesis

Accurate and timely diagnosis of oral squamous cell carcinoma (OSCC) is crucial in preventing its progression to advanced stages with a poor prognosis. As such, the construction of sensors capable of detecting previously established disease biomarkers for the early and non-invasive diagnosis of this and many other conditions has enormous therapeutic potential. In this work, we apply synthetic biology techniques for the development of a whole-cell biosensor (WCB) that leverages the physiology of engineered bacteria in vivo to promote the expression of an observable effector upon detection of a soluble molecule. To this end, we have constructed a bacterial strain expressing a novel chimeric transcription factor (Sphnx) for the detection of N-acetylneuraminic acid (Neu5Ac), a salivary biomolecule correlated with the onset of OSCC. This WCB serves as the proof-of-concept of a platform that can eventually be applied to clinical screening panels for a multitude of oral and systemic medical conditions whose biomarkers are present in saliva.

Cascaded amplifying circuit enables sensitive detection of fungal pathogens

The rapid and accurate detection of fungal pathogens is of utmost importance in the fields of healthcare, food safety, and environmental monitoring. In this study, we implemented a cascaded amplifying circuit in Saccharomyces cerevisiae to improve the G protein-coupled receptor (GPCR) mediated fungal detection. The GPCR signaling pathway was coupled with the galactose-regulated (GAL) system and a positive feedback loop was implemented to enhance the performance of yeast biosensor. We systematically compared four generations of biosensors for detecting the mating pheromone of Candida albicans, and the best biosensor exhibited the limit of detection (LOD) as low as 0.25 pM and the limit of quantification (LOQ) of 1 pM after 2 h incubation. Subsequently, we developed a betaxanthin-based colorimetric module for the easy visualization of signal outputs, and the resulting biosensors can give reliable naked-eye readouts. In summary, we demonstrated that cascaded amplifying circuits could substantially improve the engineered yeast biosensors with a better sensitivity and signal output magnitude, which will pave the way for their real-world applications in public health.

Validation and calibration of a novel GEM biosensor for specific detection of Cd2+, Zn2+, and Pb2

BACKGROUND

In this study, we designed a novel genetic circuit sensitive to Cd2+, Zn2+ and Pb2+ by mimicking the CadA/CadR operon system mediated heavy metal homeostasis mechanism of Pseudomonas aeruginosa. The regular DNA motifs on natural operon were reconfigured and coupled with the enhanced Green Fluorescent Protein (eGFP) reporter to develop a novel basic NOT type logic gate CadA/CadR-eGFP to respond metal ions mentioned above. A Genetically Engineered Microbial (GEM)-based biosensor (E.coli-BL21:pJET1.2-CadA/CadR-eGFP) was developed by cloning the chemically synthesised CadA/CadR-eGFP gene circuit into pJET1.2-plasmid and transforming into Escherichia coli (E. coli)-BL21 bacterial cells.

RESULTS

The GEM-based biosensor cells indicated the reporter gene expression in the presence of Cd2+, Zn2+ and Pb2+ either singly or in combination. Further, the same biosensor cells calibrated for fluorescent intensity against heavy metal concentration generated linear graphs for Cd2+, Zn2+ and Pb2+ with the R2 values of 0.9809, 0.9761 and 0.9758, respectively as compared to non-specific metals, Fe3+ (0.0373), AsO43- (0.3825) and Ni2+ (0.8498) making our biosensor suitable for the detection of low concentration of the former metal ions in the range of 1-6 ppb. Furthermore, the GEM based biosensor cells were growing naturally within the concentration range of heavy metals, at 37 °C and optimum pH = 7.0 in the medium, resembling the characteristics of wildtype E.coli.

CONCLUSION

Finally, the novel GEM based biosensor cells developed in this study can be applied for detection of targeted heavy metals in low concentration ranges (1-6 ppb) at normal bacterial physiological conditions.

An Intelligent Synthetic Bacterium for Chronological Toxicant Detection, Biodegradation, and Its Subsequent Suicide

Modules, toolboxes, and synthetic biology systems may be designed to address environmental bioremediation. However, weak and decentralized functional modules require complex control. To address this issue, an integrated system for toxicant detection and biodegradation, and subsequent suicide in chronological order without exogenous inducers is constructed. Salicylic acid, a typical pollutant in industrial wastewater, is selected as an example to demonstrate this design. Biosensors are optimized by regulating the expression of receptors and reporters to get 2-fold sensitivity and 6-fold maximum output. Several stationary phase promoters are compared, and promoter Pfic is chosen to express the degradation enzyme. Two concepts for suicide circuits are developed, with the toxin/antitoxin circuit showing potent lethality. The three modules are coupled in a stepwise manner. Detection and biodegradation, and suicide are sequentially completed with partial attenuation compared to pre-integration, except for biodegradation, being improved by the replacements of ribosome binding site. Finally, a long-term stability test reveals that the engineered strain maintained its function for ten generations. The study provides a novel concept for integrating and controlling functional modules that can accelerate the transition of synthetic biology from conceptual to practical applications.

RuX: A Novel, Flexible, and Sensitive Mifepristone-Induced Transcriptional Regulation System

Inducible gene regulation methods are indispensable in diverse biological applications, yet many of them have severe limitations in their applicability. These include inducer toxicity, a limited variety of organisms the given system can be used in, and side effects of the induction method. In this study, a novel inducible system, the RuX system, was created using a mutant ligand-binding domain of the glucocorticoid receptor (CS1/CD), used together with various genetic elements such as the Gal4 DNA-binding domain or Cre recombinase. The RuX system is shown to be capable of over 1000-fold inducibility, has flexible applications, and is offered for use in cell cultures.

Engineered live bacteria as disease detection and diagnosis tools

Sensitive and minimally invasive medical diagnostics are essential to the early detection of diseases, monitoring their progression and response to treatment. Engineered bacteria as live sensors are being developed as a new class of biosensors for sensitive, robust, noninvasive, and in situ detection of disease onset at low cost. Akin to microrobotic systems, a combination of simple genetic rules, basic logic gates, and complex synthetic bioengineering principles are used to program bacterial vectors as living machines for detecting biomarkers of diseases, some of which cannot be detected with other sensing technologies. Bacterial whole-cell biosensors (BWCBs) can have wide-ranging functions from detection only, to detection and recording, to closed-loop detection-regulated treatment. In this review article, we first summarize the unique benefits of bacteria as living sensors. We then describe the different bacteria-based diagnosis approaches and provide examples of diagnosing various diseases and disorders. We also discuss the use of bacteria as imaging vectors for disease detection and image-guided surgery. We conclude by highlighting current challenges and opportunities for further exploration toward clinical translation of these bacteria-based systems.

Exploring the Application and Prospects of Synthetic Biology in Engineered Living Materials

At the intersection of synthetic biology and materials science, engineered living materials (ELMs) exhibit unprecedented potential. Possessing unique "living" attributes, ELMs represent a significant paradigm shift in material design, showcasing self-organization, self-repair, adaptability, and evolvability, surpassing conventional synthetic materials. This review focuses on reviewing the applications of ELMs derived from bacteria, fungi, and plants in environmental remediation, eco-friendly architecture, and sustainable energy. The review provides a comprehensive overview of the latest research progress and emerging design strategies for ELMs in various application fields from the perspectives of synthetic biology and materials science. In addition, the review provides valuable references for the design of novel ELMs, extending the potential applications of future ELMs. The investigation into the synergistic application possibilities amongst different species of ELMs offers beneficial reference information for researchers and practitioners in this field. Finally, future trends and development challenges of synthetic biology for ELMs in the coming years are discussed in detail.

Engineering living materials by synthetic biology

Natural biological materials are programmed by genetic information and able to self-organize, respond to environmental stimulus, and couple with inorganic matter. Inspired by the natural system and to mimic their complex and delicate fabrication process and functions, the field of engineered living materials emerges at the interface of synthetic biology and materials science. Here, we review the recent efforts and discuss the challenges and future opportunities.

Multiplexed cell-based diagnostic devices for detection of renal biomarkers

The number of synthetic biology-based solutions employed in the medical industry is growing every year. The whole cell biosensors being one of them, have been proven valuable tools for developing low-cost, portable, personalized medicine alternatives to conventional techniques. Based on this concept, we targeted one of the major health problems in the world, Chronic Kidney Disease (CKD). To do so, we developed two novel biosensors for the detection of two important renal biomarkers: urea and uric acid. Using advanced gene expression control strategies, we improved the operational range and the response profiles of each biosensor to meet clinical specifications. We further engineered these systems to enable multiplexed detection as well as an AND-logic gate operating system. Finally, we tested the applicability of these systems and optimized their working dynamics inside complex medium human blood serum. This study could help the efforts to transition from labor-intensive and expensive laboratory techniques to widely available, portable, low-cost diagnostic options.

Shoot-root signal circuit: Phytoremediation of heavy metal contaminated soil

High concentrations of heavy metals in the environment will cause serious harm to ecosystems and human health. It is urgent to develop effective methods to control soil heavy metal pollution. Phytoremediation has advantages and potential for soil heavy metal pollution control. However, the current hyperaccumulators have the disadvantages of poor environmental adaptability, single enrichment species and small biomass. Based on the concept of modularity, synthetic biology makes it possible to design a wide range of organisms. In this paper, a comprehensive strategy of "microbial biosensor detection - phytoremediation - heavy metal recovery" for soil heavy metal pollution control was proposed, and the required steps were modified by using synthetic biology methods. This paper summarizes the new experimental methods that promote the discovery of synthetic biological elements and the construction of circuits, and combs the methods of producing transgenic plants to facilitate the transformation of constructed synthetic biological vectors. Finally, the problems that should be paid more attention to in the remediation of soil heavy metal pollution based on synthetic biology were discussed.

Soft hydrogel-shell confinement systems as bacteria-based bioactuators and biosensors

Genetically engineered (GE) bacteria were utilized for developing functional systems upon confinement within a restricted space. Use of natural soft hydrogel such as alginate, gelatin, and agarose, have been investigated as promising approaches to design functional architectures. Nevertheless, a challenge is to develop functional microenvironments that support biofilm-like confinement in a relevant three-dimensional (3D) format for long-term studies. We demonstrate a natural soft hydrogel bioactuator based on alginate core-shell structures (0.25-2 mm core and 50-300 μm shell thickness) that enables 3D microbial colonization upon confinement with high cell density. Specially, our study evaluates the efficiency of bacteria-functional system by recapitulating various GE bacteria which can produce common reporter proteins, to demonstrate their actuator functions as well as dynamic pair-wise interactions. The structural design of the hydrogel can endure continued growth of various bacteria colonies within the confined space for over 10 days. The total amount of cellular biomass upon hydrogel-shell confinement was increased 5-fold compared to conventional techniques without hydrogel-shell. Furthermore, the enzymatic activity increased 3.8-fold and bioluminescence signal by 8-fold compared to the responses from conventional hydrogel systems. The conceptualized platform and our workflow represent a reliable strategy with core-shell structures to develop artificial hydrogel habitats as bacteria-based functional systems for bioactuation.

Genetically encoded biosensors for microbial synthetic biology: From conceptual frameworks to practical applications

Genetically encoded biosensors are the vital components of synthetic biology and metabolic engineering, as they are regarded as powerful devices for the dynamic control of genotype metabolism and evolution/screening of desirable phenotypes. This review summarized the recent advances in the construction and applications of different genetically encoded biosensors, including fluorescent protein-based biosensors, nucleic acid-based biosensors, allosteric transcription factor-based biosensors and two-component system-based biosensors. First, the construction frameworks of these biosensors were outlined. Then, the recent progress of biosensor applications in creating versatile microbial cell factories for the bioproduction of high-value chemicals was summarized. Finally, the challenges and prospects for constructing robust and sophisticated biosensors were discussed. This review provided theoretical guidance for constructing genetically encoded biosensors to create desirable microbial cell factories for sustainable bioproduction.

CRISPRi-based programmable logic inverter cascade for antibiotic-free selection and maintenance of multiple plasmids

Antibiotics have been widely used for plasmid-mediated cell engineering. However, continued use of antibiotics increases the metabolic burden, horizontal gene transfer risks, and biomanufacturing costs. There are limited approaches to maintaining multiple plasmids without antibiotics. Herein, we developed an inverter cascade using CRISPRi by building a plasmid containing a single guide RNA (sgRNA) landing pad (pSLiP); this inhibited host cell growth by repressing an essential cellular gene. Anti-sgRNAs on separate plasmids restored cell growth by blocking the expression of growth-inhibitory sgRNAs in pSLiP. We maintained three plasmids in Escherichia coli with a single antibiotic selective marker. To completely avoid antibiotic use and maintain the CRISPRi-based logic inverter cascade, we created a novel d-glutamate auxotrophic E. coli. This enabled the stable maintenance of the plasmid without antibiotics, enhanced the production of the terpenoid, (-)-α-bisabolol, and generation of an antibiotic-resistance gene-free plasmid. CRISPRi is therefore widely applicable in genetic circuits and may allow for antibiotic-free biomanufacturing.

Modular, Synthetic Boolean Logic Gates Enabled in Saccharomyces cerevisiae through T7 Polymerases/CRISPR dCas9 Designs

Synthetic control of gene expression, whether simply promoter selection or higher-order Boolean-style logic, is an important tool for metabolic engineering and synthetic biology. This work develops a suite of orthogonal T7 RNA polymerase systems capable of exerting AND/OR switchlike control over transcription in the yeastSaccharomyces cerevisiae. When linked with CRISPR dCas9-based regulation systems, more complex circuitry is possible including AND/OR/NAND/NOR style control in response to combinations of extracellular copper and galactose. Additionally, we demonstrate that these T7 system designs are modular and can accommodate alternative stimuli sensing as demonstrated through blue light induction. These designs should greatly reduce the time and labor necessary for developing Boolean gene circuits in yeast with novel applications including metabolic pathway control in the future.

The protein scaffold calibrates metal specificity and activation in MerR sensors

MerR metalloregulators are the central components of many biosensor platforms designed to report metal contamination. However, most MerR proteins are non-specific. This makes it difficult to apply these biosensors in the analysis of real environmental samples. On-demand implementation of molecular engineering to modify the MerR metal preferences is innovative, although it does not always yield the expected results. As the metal binding loop region (MBL) of these sensors has been proposed to be the major modulator of their specificity, we surgically switched this region for that of well-characterized specific and non-specific homologues. We found that identical modifications in different MerR proteins result in synthetic sensors displaying particular metal-detection patterns that cannot be predicted from the nature of the assembled modules. For instance, the MBL from a native Hg(II) sensor provided non-specificity or specificity toward Hg(II) or Cd(II) depending on the MerR scaffold into which it was integrated. These and other evidences reveal that residues outside the MBL are required to modulate ion recognition and transduce the input signal to the target promoter. Revealing their identity and their interactions with other residues is a critical step toward the design of more efficient biosensor devices for environmental metal monitoring.

Improving the Robustness of Engineered Bacteria to Nutrient Stress Using Programmed Proteolysis

The use of short peptide tags in synthetic genetic circuits allows for the tuning of gene expression dynamics and release of amino acid resources through targeted protein degradation. Here, we use elements of the Escherichia coli and Mesoplasma florum transfer-mRNA (tmRNA) ribosome rescue systems to compare endogenous and foreign proteolysis systems in E. coli. We characterize the performance and burden of each and show that, while both greatly shorten the half-life of a tagged protein, the endogenous system is approximately 10 times more efficient. On the basis of these results we then demonstrate using mathematical modeling and experiments how proteolysis can improve cellular robustness through targeted degradation of a reporter protein in auxotrophic strains, providing a limited secondary source of essential amino acids that help partially restore growth when nutrients become scarce. These findings provide avenues for controlling the functional lifetime of engineered cells once deployed and increasing their tolerance to fluctuations in nutrient availability.

A Recombinase-Based Genetic Circuit for Heavy Metal Monitoring

Rapid progress in the genetic circuit design enabled whole-cell biosensors (WCBs) to become prominent in detecting an extensive range of analytes with promise in many fields, from medical diagnostics to environmental toxicity assessment. However, several drawbacks, such as high background signal or low precision, limit WCBs to transfer from proof-of-concept studies to real-world applications, particularly for heavy metal toxicity monitoring. For an alternative WCB module design, we utilized Bxb1 recombinase that provides tight control as a switch to increase dose-response behavior concerning leakiness. The modularity of Bxb1 recombinase recognition elements allowed us to combine an engineered semi-specific heat shock response (HSR) promoter, sensitive to stress conditions including toxic ions such as cadmium, with cadmium resistance regulatory elements; a cadmium-responsive transcription factor and its cognitive promoter. We optimized the conditions for the recombinase-based cadmium biosensor to obtain increased fold change and shorter response time. This system can be expanded for various heavy metals to make an all-in-one type of WCB, even using semi-specific parts of a sensing system.

Programming living sensors for environment, health and biomanufacturing

Synthetic biology offers new tools and capabilities of engineering cells with desired functions for example as new biosensing platforms leveraging engineered microbes. In the last two decades, bacterial cells have been programmed to sense and respond to various input cues for versatile purposes including environmental monitoring, disease diagnosis and adaptive biomanufacturing. Despite demonstrated proof-of-concept success in the laboratory, the real-world applications of microbial sensors have been restricted due to certain technical and societal limitations. Yet, most limitations can be addressed by new technological developments in synthetic biology such as circuit design, biocontainment and machine learning. Here, we summarize the latest advances in synthetic biology and discuss how they could accelerate the development, enhance the performance and address the present limitations of microbial sensors to facilitate their use in the field. We view that programmable living sensors are promising sensing platforms to achieve sustainable, affordable and easy-to-use on-site detection in diverse settings.

Constructing a Facile Biocomputing Platform Based on Smart Supramolecular Hydrogel Film Electrodes with Immobilized Enzymes and Gold Nanoclusters

Herein, fluorescent gold nanoclusters (AuNCs) and horseradish peroxidase (HRP) were simultaneously embedded into self-assembled dipeptide supramolecular films of N-fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF) on the surface of ITO electrodes (Fmoc-FF/AuNCs/HRP) by using a simple single-step process. In the films, both the fluorescence property of AuNCs and the bioelectrocatalytic property of HRP were well maintained and could be reversibly regulated by pH-sensitive structural changes in the Fmoc-FF hydrogel films. Cu(II)/EDTA in the solution could lead to the aggregation/disaggregation of AuNCs and further quenching/dequenching the fluorescence signal from the films. Meanwhile, the blue complexes formed by Cu(II) and EDTA could produce a UV-vis signal in the solution. In addition, the coordinated Cu(II) in the films enhanced the electrocatalytic capacity toward the reduction of H₂O₂ and could switch the current signal. A biomolecular logic circuit was built based on the smart film electrode system by using pH, the concentrations of EDTA, Cu(II) and H₂O₂ as inputs, while the fluorescence intensity (FL), current (I) and UV-vis extinction (E) of the solution as outputs. Various logic devices were fabricated using the uniform platform, consisting of an encoder/decoder, demultiplexer, dual-transfer gate, keypad lock, digital comparator, half adder, and controlled NOT (CNOT) gate. Specifically, an electronic three-value logic gate, gullibility (ANY) gate, was first mimicked in this biocomputing system. This work not only demonstrated the construction of a new type of multivalued logic gate by using a dipeptide micromolecular matrix but also provided a new approach for designing sophisticated biologic functions, establishing smart multianalyte biosensing or fabricating biology information processing through the use of a simple film system.

Multilayer Genetic Circuits for Dynamic Regulation of Metabolic Pathways

The dynamic regulation of metabolic pathways is based on changes in external signals and endogenous changes in gene expression levels and has extensive applications in the field of synthetic biology and metabolic engineering. However, achieving dynamic control is not trivial, and dynamic control is difficult to obtain using simple, single-level, control strategies because they are often affected by native regulatory networks. Therefore, synthetic biologists usually apply the concept of logic gates to build more complex and multilayer genetic circuits that can process various signals and direct the metabolic flux toward the synthesis of the molecules of interest. In this review, we first summarize the applications of dynamic regulatory systems and genetic circuits and then discuss how to design multilayer genetic circuits to achieve the optimal control of metabolic fluxes in living cells.

In-situ monitoring of xenobiotics using genetically engineered whole-cell-based microbial biosensors: recent advances and outlook

Industrialisation, directly or indirectly, exposes humans to various xenobiotics. The increased magnitude of chemical pesticides and toxic heavy metals in the environment, as well as their intrusion into the food chain, seriously threatens human health. Therefore, the surveillance of xenobiotics is crucial for social safety and security. Online investigation by traditional methods is not sufficient for the detection and identification of such compounds because of the high costs and their complexity. Advancement in the field of genetic engineering provides a potential opportunity to use genetically modified microorganisms. In this regard, whole-cell-based microbial biosensors (WCBMB) represent an essential tool that couples genetically engineered organisms with an operator/promoter derived from a heavy metal-resistant operon combined with a regulatory protein in the gene circuit. The plasmid controls the expression of the reporter gene, such as gfp, luc, lux and lacZ, to an inducible gene promoter and has been widely applied to assay toxicity and bioavailability. This review summarises the recent trends in the development and application of microbial biosensors and the use of mobile genes for biomedical and environmental safety concerns.

Whole-Cell Microbial Bioreporter for Soil Contaminants Detection

Anthropogenic activities have released various contaminants into soil that pose a serious threat to the ecosystem and human well-being. Compared to conventional analytical methodologies, microbial cell-based bioreporters are offering a flexible, rapid, and cost-effective strategy to assess the environmental risks. This review aims to summarize the recent progress in the application of bioreporters in soil contamination detection and provide insight into the challenges and current strategies. The biosensing principles and genetic circuit engineering are introduced. Developments of bioreporters to detect and quantify heavy metal and organic contaminants in soil are reviewed. Moreover, future opportunities of whole-cell bioreporters for soil contamination monitoring are discussed.

CRISPRi-Mediated NIMPLY Logic Gate for Fine-Tuning the Whole-Cell Sensing toward Simple Urine Glucose Detection

Whole-cell biosensors have been regarded as a prominent alternative to chemical and physical biosensors due to their renewability, environmental friendliness, and biocompatibility. However, there is still a lack of noninvasive measurements of urine glucose, which plays a vital role in monitoring the risk of diabetes in the healthcare system, via whole-cell biosensors. In this study, we characterized a glucose-inducible promoter and further enhanced the sensing performance using three genetic effectors, which encompassed ribozyme regulator (RiboJ), clustered regularly interspaced short palindromic repeat interference (CRISPRi), and plasmid-based T7RNA polymerase (PDT7), to develop the noninvasive glucose biosensor by fluorescent signal. As a result, RiboJ increased dynamic range to 2989 au, but declined signal-to-noise (S/N) to 1.59, while CRISPRi-mediated NIMPLY gate intensified both dynamic range to 5720 au and S/N to 4.58. The use of single PDT7 orthogonal with T7 promoter in cells (i.e., P strain) achieved a 44 180 au of dynamic range with S/N at 3.08. By coupling the PDT7 and NIMPLY-mediated CRISPRi, we constructed an optimum PIGAS strain with the highest S/N value of 4.95. Finally, we adopted the synthetic bacteria into a microdevice to afford an integrative and portable system for daily urine glucose inspection, which would be an alternative approach for medical diagnosis in the future.

Stimuli-responsive engineered living materials

Stimuli-responsive materials are able to undergo controllable changes in materials properties in response to external cues. Increasing efforts have been directed towards building materials that mimic the responsive nature of biological systems. Nevertheless, limitations remain surrounding the way these synthetic materials interact and respond to their environment. In particular, it is difficult to synthesize synthetic materials that respond with specificity to poorly differentiated (bio)chemical and weak physical stimuli. The emerging area of engineered living materials (ELMs) includes composites that combine living cells and synthetic materials. ELMs have yielded promising advances in the creation of stimuli-responsive materials that respond with diverse outputs in response to a broad array of biochemical and physical stimuli. This review describes advances made in the genetic engineering of the living component and the processing-property relationships of stimuli-responsive ELMs. Finally, the implementation of stimuli-responsive ELMs as environmental sensors, biomedical sensors, drug delivery vehicles, and soft robots is discussed.

Artificial intelligence and synthetic biology approaches for human gut microbiome

The gut microbiome comprises a variety of microorganisms whose genes encode proteins to carry out crucial metabolic functions that are responsible for the majority of health-related issues in human beings. The advent of the technological revolution in artificial intelligence (AI) assisted synthetic biology (SB) approaches will play a vital role in the modulating the therapeutic and nutritive potential of probiotics. This can turn human gut as a reservoir of beneficial bacterial colonies having an immense role in immunity, digestion, brain function, and other health benefits. Hence, in the present review, we have discussed the role of several gene editing tools and approaches in synthetic biology that have equipped us with novel tools like Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR-Cas) systems to precisely engineer probiotics for diagnostic, therapeutic and nutritive value. A brief discussion over the AI techniques to understand the metagenomic data from the healthy and diseased gut microbiome is also presented. Further, the role of AI in potentially impacting the pace of developments in SB and its current challenges is also discussed. The review also describes the health benefits conferred by engineered microbes through the production of biochemicals, nutraceuticals, drugs or biotherapeutics molecules etc. Finally, the review concludes with the challenges and regulatory concerns in adopting synthetic biology engineered microbes for clinical applications. Thus, the review presents a synergistic approach of AI and SB toward human gut microbiome for better health which will provide interesting clues to researchers working in the area of rapidly evolving food and nutrition science.

Machine learning-aided quantification of antibody-based cancer immunotherapy by natural killer cells in microfluidic droplets

Natural killer (NK) cells have emerged as an effective alternative option to T cell-based immunotherapies, particularly against liquid (hematologic) tumors. However, the effectiveness of NK cell therapy has been less than optimal for solid tumors, partly due to the heterogeneity in target interaction leading to variable anti-tumor cytotoxicity. This paper describes a microfluidic droplet-based cytotoxicity assay for quantitative comparison of immunotherapeutic NK-92 cell interaction with various types of target cells. Machine learning algorithms were developed to assess the dynamics of individual effector-target cell pair conjugation and target death in droplets in a semi-automated manner. Our results showed that while short contacts were sufficient to induce potent killing of hematological cancer cells, long-lasting stable conjugation with NK-92 cells was unable to kill HER2+ solid tumor cells (SKOV3, SKBR3) significantly. NK-92 cells that were engineered to express FcγRIII (CD16) mediated antibody-dependent cellular cytotoxicity (ADCC) selectively against HER2+ cells upon addition of Herceptin (trastuzumab). The requirement of CD16, Herceptin and specific pre-incubation temperature served as three inputs to generate a molecular logic function with HER2+ cell death as the output. Mass proteomic analysis of the two effector cell lines suggested differential changes in adhesion, exocytosis, metabolism, transport and activation of upstream regulators and cytotoxicity mediators, which can be utilized to regulate specific functionalities of NK-92 cells in future. These results suggest that this semi-automated single cell assay can reveal the variability and functional potency of NK cells and may be used to optimize immunotherapeutic efficacy for preclinical analyses.

Engineering of a Au-sensor to develop a Hg-specific, sensitive and robust whole-cell biosensor for on-site water monitoring

A highly sensitive and specific Hg-whole-cell biosensor was developed from a non-selective variant of the Au sensor GolS and its regulatory pathway. The performance of this analytical tool was validated under laboratory and field-like conditions. This biosensor can be easily applied in cost-effective and portable semiquantitative devices to report Hg contamination in water.

AND logic gate based fluorescence probe for simultaneous detection of peroxynitrite and hypochlorous acid

Hypochlorous acid (HOCl) and peroxynitrite (ONOO^-) are two of the most important reactive species and associated with various diseases in various physiological and pathological processes. Nonetheless, many of their roles are still vague due to the shortage of methods for simultaneously detecting HOCl and ONOO^-. Herein, three simple yet useful fluorogenic probes, LG-1, LG-2 and LG-3, have been fabricated with facile synthesis route and used to monitor the coexistence of HOCl and ONOO^- as AND-based logic gate fluorescent probe firstly. LG-1 and LG-2, which consists of 1,3-oxathiolane group and boronate group respectively, were designed to verify the capacity of monitoring HOCl and ONOO^- without interference from each other. The result showed that these two groups are perfect reaction sites of detecting HOCl and ONOO^- respectively via specific analyte-induced reactions. Hence, LG-3, which is attached by these two groups to suppress the fluorophore core, can response to HOCl and ONOO^- simultaneously without mutual interference and generate the significant time-dependent fluorescence enhancement. By investigating the absorption and fluorescence properties of LG-3 towards HOCl and ONOO^- individually and collectively, the result confirmed clearly that LG-3 has the capacity of monitoring the coexistence of HOCl and ONOO^-, which could act as a two-input AND-based logic gate fluorescent probe.

Development of High-Performance Whole Cell Biosensors Aided by Statistical Modeling

Whole cell biosensors are genetic systems that link the presence of a chemical, or other stimulus, to a user-defined gene expression output for applications in sensing and control. However, the gene expression level of biosensor regulatory components required for optimal performance is nonintuitive, and classical iterative approaches do not efficiently explore multidimensional experimental space. To overcome these challenges, we used a design of experiments (DoE) methodology to efficiently map gene expression levels and provide biosensors with enhanced performance. This methodology was applied to two biosensors that respond to catabolic breakdown products of lignin biomass, protocatechuic acid and ferulic acid. Utilizing DoE we systematically modified biosensor dose-response behavior by increasing the maximum signal output (up to 30-fold increase), improving dynamic range (>500-fold), expanding the sensing range (∼4-orders of magnitude), increasing sensitivity (by >1500-fold), and modulated the slope of the curve to afford biosensors designs with both digital and analogue dose-response behavior. This DoE method shows promise for the optimization of regulatory systems and metabolic pathways constructed from novel, poorly characterized parts.

Synthetic Biology Speeds Up Drug Target Discovery

As a rising emerging field, synthetic biology intends to realize precise regulations of cellular network by constructing artificial synthetic circuits, and it brings great opportunities to treat diseases and discover novel drug targets. Depending on the combination mode of different logic gates, various synthetic circuits are created to carry out multilevel regulations. In given synthetic circuits, drugs often act as inputs to drive circuits operation. It is becoming available to construct drug-responsive gene circuits for experimentally treating various disease models, including metabolic disease, immunity disease, cancer and bacterial infection. Synthetic biology works well in association with the CRISPR system for drug target functional screening. Remarkably, more and more well-designed circuits are developed to discover novel drug targets and precisely regulate drug therapy for diseases.

Active Analyte Import Improves the Dynamic Range and Sensitivity of a Vitamin B12 Biosensor

Cell-free systems provide a versatile platform for the development of low-cost, easy-to-use sensors for diverse analytes. However, sensor affinity dictates response sensitivity, and improving binding affinity can be challenging. Here, we describe efforts to address this problem while developing a biosensor for vitamin B₁₂, a critical micronutrient. We first use a B₁₂-responsive transcription factor to enable B₁₂-dependent output in a cell-free reaction, but the resulting sensor responds to B₁₂ far above clinically relevant concentrations. Surprisingly, when expressed in cells, the same sensor mediates a much more sensitive response to B₁₂. The sensitivity difference is partly due to regulated import that accumulates cytoplasmic B₁₂. Overexpression of importers further improves sensitivity, demonstrating an inherent advantage of whole-cell sensors. The resulting cells can respond to B₁₂ in serum, can be lyophilized, and are functional in a minimal-equipment environment, showing the potential utility of whole-cell sensors as sensitive, field-deployable diagnostics.

Genetic Circuits To Detect Nanomaterial Triggered Toxicity through Engineered Heat Shock Response Mechanism

Biocompatibility assessment of nanomaterials has been of great interest due to their potential toxicity. However, conventional biocompatibility tests fall short of providing a fast toxicity report. We developed a whole cell based biosensor to track biocompatibility of nanomaterials with the aim of providing fast feedback to engineer them with lower toxicity levels. We engineered promoters of four heat shock response (HSR) proteins utilizing synthetic biology approaches. As an initial design, a reporter coding gene was cloned downstream of the selected promoter regions. Initial results indicated that native heat shock protein (HSP) promoter regions were not very promising to generate signals with low background signals. Introducing riboregulators to native promoters eliminated unwanted background signals almost entirely. Yet, this approach also led to a decrease in expected sensor signal upon stress treatment. Thus, a repression based genetic circuit, inspired by the HSR mechanism of Mycobacterium tuberculosis, was constructed. These genetic circuits could report the toxicity of quantum dot nanoparticles in 1 h. Our designed nanoparticle toxicity sensors can provide quick reports, which can lower the demand for additional experiments with more complex organisms.

Synthetic Genetic Circuits for Self-Actuated Cellular Nanomaterial Fabrication Devices

Genetically controlled synthetic biosystems are being developed to create nanoscale materials. These biosystems are modeled on the natural ability of living cells to synthesize materials: many organisms have dedicated proteins that synthesize a wide range of hard tissues and solid materials, such as nanomagnets and biosilica. We designed an autonomous living material synthesizing system consisting of engineered cells with genetic circuits that synthesize nanomaterials. The circuits encode a nanomaterial precursor-sensing module (sensor) coupled with a materials synthesis module. The sensor detects the presence of cadmium, gold, or iron ions, and this detection triggers the synthesis of the related nanomaterial-nucleating extracellular matrix. We demonstrate that when engineered cells sense the availability of a precursor ion, they express the corresponding extracellular matrix to form the nanomaterials. This proof-of-concept study shows that endowing cells with synthetic genetic circuits enables nanomaterial synthesis and has the potential to be extended to the synthesis of a variety of nanomaterials and biomaterials using a green approach.

Regulating, Measuring, and Modeling the Viscoelasticity of Bacterial Biofilms

Biofilms occur in a broad range of environments under heterogeneous physicochemical conditions, such as in bioremediation plants, on surfaces of biomedical implants, and in the lungs of cystic fibrosis patients. In these scenarios, biofilms are subjected to shear forces, but the mechanical integrity of these aggregates often prevents their disruption or dispersal. Biofilms' physical robustness is the result of the multiple biopolymers secreted by constituent microbial cells which are also responsible for numerous biological functions. A better understanding of the role of these biopolymers and their response to dynamic forces is therefore crucial for understanding the interplay between biofilm structure and function. In this paper, we review experimental techniques in rheology, which help quantify the viscoelasticity of biofilms, and modeling approaches from soft matter physics that can assist our understanding of the rheological properties. We describe how these methods could be combined with synthetic biology approaches to control and investigate the effects of secreted polymers on the physical properties of biofilms. We argue that without an integrated approach of the three disciplines, the links between genetics, composition, and interaction of matrix biopolymers and the viscoelastic properties of biofilms will be much harder to uncover.

Genetic Logic Gates Enable Patterning of Amyloid Nanofibers

Distinct spatial patterning of naturally produced materials is observed in many cellular structures and even among communities of microorganisms. Reoccurrence of spatially organized materials in all branches of life is clear proof that organization is beneficial for survival. Indeed, organisms can trick the evolutionary process by using organized materials in ways that can help the organism to avoid unexpected conditions. To expand the toolbox for synthesizing patterned living materials, Boolean type "AND" and "OR" control of curli fibers expression is demonstrated using recombinases. Logic gates are designed to activate the production of curli fibers. The gates can be used to record the presence of input molecules and give output as CsgA expression. Two different curli fibers (CsgA and CsgA-His-tag) production are then selectively activated to explore distribution of monomers upon coexpression. To keep track of the composition of fibers, CsgA-His-tag proteins are labeled with nickel-nitrilotriacetic acid (Ni-NTA-) conjugated gold nanoparticles. It is observed that an organized living material can be obtained upon inducing the coexpression of different CsgA fibers. It is foreseen that living materials with user-defined curli composition hold great potential for the development of living materials for many biomedical applications.

Quality and Capacity Analysis of Molecular Communications in Bacterial Synthetic Logic Circuits

Synthetic logic circuits have been proposed as potential solutions for theranostics of biotechnological problems. One proposed model is the engineering of bacteria cells to create logic gates, and the communication between the bacteria populations will enable the circuit operation. In this paper, we analyze the quality of bacteria-based synthetic logic circuit through molecular communications that represent communication along a bus between three gates. In the bacteria-based synthetic logic circuit, the system receives environmental signals as molecular inputs and will process this information through a cascade of synthetic logic gates and free diffusion channels. We analyze the performance of this circuit by evaluating its quality and its relationship to the channel capacity of the molecular communications links that interconnect the bacteria populations. Our results show the effect of the molecular environmental delay and molecular amplitude differences over both the channel capacity and circuit quality. Furthermore, based on these metrics, we also obtain an optimum region for the circuit operation resulting in an accuracy of 80% for specific conditions. These results show that the performance of synthetic biology circuits can be evaluated through molecular communications, and lays the groundwork for combined systems that can contribute to future biomedical and biotechnology applications.

Decoding the Time-Dependent Response of Bioluminescent Metal-Detecting Whole-Cell Bacterial Sensors

The signal produced by aqueous dispersions of bioluminescent, metal-responsive whole-cell bacterial sensors is indicative of the concentration of bioavailable metal ions in solution. The conventional calibration-based strategy followed for measuring this concentration is however inadequate to provide any quantitative prediction of the cell response over time as a function of, e.g., their growth features, their defining metal accumulation properties, or the physicochemical medium composition. Such an evaluation is still critically needed for assessing on a mechanistic level the performance of biosensors in terms of metal bioavailability and toxicity monitoring. Herein we report a comprehensive formalism unraveling how the dependence of bioluminescence on time is governed by the dynamics of metal biouptake, by the activation kinetics of lux-based reporter gene, and by the ensuing rate of luciferase production, the kinetics of light emission, and quenching. It is shown that the bioluminescence signal corresponds to the convolution product between two time-dependent functions, one detailing the dynamic interplay of the above micro- and nanoscale processes, and the other pertaining to the change in concentration of photoactive cell sensors over time. Numerical computations illustrate how the shape and magnitude of the bioluminescence peak(s) are intimately connected to the dependence of the photoactive cell concentration on time and to the magnitudes of Deborah numbers that compare the relevant time scales of the biointerfacial and intracellular events controlling light emission. Explicit analytical expressions are further derived for practical situations where bioluminescence is proportional to the concentration of metal ions in solution. The theory is further quantitatively supported by experiments performed on luminescent cadmium-responsive lux-based Escherichia coli biosensors.

Extracellular electron transfer features of Gram-positive bacteria

Electroactive microorganisms possess the unique ability to transfer electrons to or from solid phase electron conductors, e.g., electrodes or minerals, through various physiological mechanisms. The processes are commonly known as extracellular electron transfer and broadly harnessed in microbial electrochemical systems, such as microbial biosensors, microbial electrosynthesis, or microbial fuel cells. Apart from a few model microorganisms, the nature of the microbe-electrode conductive interaction is poorly understood for most of the electroactive species. The interaction determines the efficiency and a potential scaling up of bioelectrochemical systems. Gram-positive bacteria generally have a thick electron non-conductive cell wall and are believed to exhibit weak extracellular electron shuttling activity. This review highlights reported research accomplishments on electroactive Gram-positive bacteria. The use of electron-conducting polymers as mediators is considered as one promising strategy to enhance the electron transfer efficiency up to application scale. In view of the recent progress in understanding the molecular aspects of the extracellular electron transfer mechanisms of Enterococcus faecalis, the electron transfer properties of this bacterium are especially focused on. Fundamental knowledge on the nature of microbial extracellular electron transfer and its possibilities can provide insight in interspecies electron transfer and biogeochemical cycling of elements in nature. Additionally, a comprehensive understanding of cell-electrode interactions may help in overcoming insufficient electron transfer and restricted operational performance of various bioelectrochemical systems and facilitate their practical applications.

A Self-Actuated Cellular Protein Delivery Machine

Engineered bacterial cells have great promise to solve global problems, yet they are hampered by a lack of convenient strategy for controlled protein release. A well-controlled protein translocation through cellular membranes is essential for cell-based protein delivery. Here we have developed a controlled protein release system by programming a bacterial autotransporter system named Ag43. Ag43 protein is engineered by adding a protease digestion site between its translocation and cargo domains. Once it is displayed on the cell surface, we managed to release the cargo proteins in defined conditions by processing environmental signals. The protein release in terms of time and quantity can be controlled through changing the inducer conditions. We thought that the release system can be adopted for complex genetic circuitries due to its simplicity. We implemented the protein release system to develop a cellular device that is able to release proteins in a sequence response to ordered chemical signals. We envision that development of genetically controlled protein release systems will improve the applications of synthetic organisms in cell based therapies, especially for cases with a need for controlled protein release using the cues from the biological environment.

Determination of Gold Ions in Human Urine Using Genetically Engineered Microorganisms on a Paper Device

This paper presents a whole-cell biosensor that operates in conjunction with a smartphone-based fluorescence diagnostic system on a paper device to monitor the concentration of gold ions in human urine. The heavy metal-tolerant bacteria Cupriavidus metallidurans was genetically engineered for use as a chassis in a red fluorescent protein (RFP)-based microbial sensor. The biosensor is highly sensitive to gold ions, with a detection limit of 110 nM. The proposed smartphone-based analysis system provides a user-friendly approach to design tools of personal health monitoring for reporting the presence of gold ions in human urine.