RNA detection with high specificity and sensitivity using nested fluorogenic Mango NASBA

Molecular methods for rapid detection and identification of foodborne pathogenic bacteria

Foodborne pathogenic bacteria are one of the main factors causing food safety issues. The rapid and accurate detection of pathogenic bacteria using molecular techniques is an effective and powerful strategy for preventing and controlling outbreaks of foodborne diseases, thereby ensuring food safety. This article summarizes the rapid and efficient molecular diagnostic techniques for detecting pathogenic bacteria, including polymerase chain reaction and its derivatives, isothermal amplification, DNA hybridization, genomic sequencing, and Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)/CRISPR-associated (CRISPR/Cas)-based detection technique. Through a comparative analysis of the technical principles, advantages, and potential limitations of these diagnostic methods, as well as an outlook on the future development directions for molecular biological detection technology, which will provide a valuable reference for developing more accurate, convenient, and sensitive methods for foodborne pathogens detection, and will help better address the challenges posed by foodborne diseases, thereby ensuring public health and safety.

SATCAS: A CRISPR/Cas13a-based simultaneous amplification and testing platform for one-pot RNA detection and SNPs distinguish in clinical diagnosis

The clinical diagnosis of pathogen infectious diseases increasingly requires sensitive and rapid RNA detection technologies. The RNA-guided clustered regularly interspaced short palindromic repeats (CRISPR)/Cas13a system has shown immense potential in molecular diagnostics due to its trans-cleavage activity. However, most Cas13a-based detection methods require an amplicon transcription step, and the multi-step open-tube operations are prone to contamination, limiting their widespread application. Here, we propose an ultrasensitive (single-copy range, ∼aM) and rapid (within 40 min) isothermal one-pot RNA detection platform, termed SATCAS (Simultaneous Amplification and Testing platform based on Cas13a). This method effectively distinguishes viable bacteria (0%-100%) under constant total bacterial conditions, demonstrating its robustness and universality. SATCAS excels in identifying single nucleotide polymorphisms (SNPs), particularly detecting 0.5% drug-resistant mutations. We validated SATCAS by detecting infections in biological samples from 68 HBV, 23 EBV, and 48 SARS-CoV-2 patients, achieving 100% sensitivity, 92.86% specificity, and 97.06% accuracy in HBV infection testing. We anticipate that SATCAS has broad application potential in the early diagnosis, subtyping, drug resistance detection, and point-of-care monitoring of pathogen infectious diseases.

Recent advances of food safety detection by nucleic acid isothermal amplification integrated with CRISPR/Cas

Food safety problems have become one of the most important public health issues worldwide. Therefore, the development of rapid, effective and robust detection is of great importance. Amongst a range of methods, nucleic acid isothermal amplification (NAIA) plays a great role in food safety detection. However, the widespread application remains limited due to a few shortcomings. CRISPR/Cas system has emerged as a powerful tool in nucleic acid detection, which could be readily integrated with NAIA to improve the detection sensitivity, specificity, adaptability versatility and dependability. However, currently there was a lack of a comprehensive summary regarding the integration of NAIA and CRISPR/Cas in the field of food safety detection. In this review, the recent advances in food safety detection based on CRISPR/Cas-integrated NAIA were comprehensively reviewed. To begin with, the development of NAIA was summarized. Then, the types and working principles of CRISPR/Cas were introduced. The applications of the integration of NAIA and CRISPR/Cas for food safety were mainly introduced and objectively discussed. Lastly, current challenges and future opportunities were proposed. In summary, this technology is expected to become an important approach for food safety detection, leading to a safer and more reliable food industry.

Cross-chiral exponential amplification of an RNA enzyme

An RNA ligase ribozyme that catalyzes the joining of RNA molecules of the opposite chiral handedness was optimized for the ability to synthesize its own enantiomer from two component fragments. The mirror-image D- and L-ligases operate in concert to provide a system for cross-chiral replication, whereby they catalyze each other's synthesis and undergo mutual amplification at constant temperature, with apparent exponential growth and a doubling time of about 1 h. Neither the D- nor the L-RNA components alone can achieve autocatalytic self-replication. Cross-chiral exponential amplification can be continued indefinitely through a serial-transfer process that provides an ongoing supply of the component D- and L-substrates. Unlike the familiar paradigm of semiconservative nucleic acid replication that relies on Watson-Crick pairing between complementary strands, cross-chiral replication relies on tertiary interactions between structured nucleic acids "across the mirror." There are few examples, outside of biology, of autocatalytic self-replication systems that undergo exponential amplification and there are no prior examples, in either biological or chemical systems, of cross-chiral replication enabling exponential amplification.

Miniaturized Devices for Isothermal Amplification and Photometric Quantification of Pseudomonas Aeruginosa

Goal: This study introduced a proof-of-concept prototype for isothermal recombinase polymerase amplification (RPA) with miniaturized photometric detection, enabling rapid P. aeruginosa detection. Methods: The researchers conducted the amplification process within a microchamber with a diameter of 10 mm, utilizing a standalone Thermostat driven thermal management setup. RPA, an amplification technique was employed, which required a lower operating temperature of 37 °C-40 °C to complete the reaction. The amplified amplicon was labeled with a fluorophore reporter, stimulated by an LED light source, and detected in real-time using a photodiode. Results: The developed prototype successfully demonstrated the rapid detection of P. aeruginosa using the RPA assay. The process only required the utilization of 0.04 ng of working concentration of DNA. The entire process, from amplification to detection, could be completed in over 15 minutes. The platform showed enhanced sensitivity and specificity, providing a cost-effective and accurate solution for on-site detection/quantification of pathogens. Conclusions: The integration of isothermal RPA with the miniaturized photometric detection platform proved successful in achieving the goal of rapid and specific pathogen detection. This study proved the benefits of Isothermal Nucleic Acid Amplification Technology (INAAT), emphasizing its potential as an accessible, user-friendly point-of-care technology for resource-constrained institutions. The RPA-based prototype demonstrated capability without requiring costly laboratory equipment or expertise. The developed platform, when combined with Internet of Things (IoT) enabled cloud platform, also allowed remote monitoring of data. Overall, the methodology presented in this study offered a cost-effective, accurate, and convenient solution for on-site testing in resource-limited settings.

Symmetry breaking of fluorophore binding to a G-quadruplex generates an RNA aptamer with picomolar KD

Fluorogenic RNA aptamer tags with high affinity enable RNA purification and imaging. The G-quadruplex (G4) based Mango (M) series of aptamers were selected to bind a thiazole orange based (TO1-Biotin) ligand. Using a chemical biology and reselection approach, we have produced a MII.2 aptamer-ligand complex with a remarkable set of properties: Its unprecedented KD of 45 pM, formaldehyde resistance (8% v/v), temperature stability and ligand photo-recycling properties are all unusual to find simultaneously within a small RNA tag. Crystal structures demonstrate how MII.2, which differs from MII by a single A23U mutation, and modification of the TO1-Biotin ligand to TO1-6A-Biotin achieves these results. MII binds TO1-Biotin heterogeneously via a G4 surface that is surrounded by a stadium of five adenosines. Breaking this pseudo-rotational symmetry results in a highly cooperative and homogeneous ligand binding pocket: A22 of the G4 stadium stacks on the G4 binding surface while the TO1-6A-Biotin ligand completely fills the remaining three quadrants of the G4 ligand binding face. Similar optimization attempts with MIII.1, which already binds TO1-Biotin in a homogeneous manner, did not produce such marked improvements. We use the novel features of the MII.2 complex to demonstrate a powerful optically-based RNA purification system.

PACRAT: pathogen detection with aptamer-observed cascaded recombinase polymerase amplification-in vitro transcription

The SARS-CoV-2 pandemic underscored the need for early, rapid, and widespread pathogen detection tests that are readily accessible. Many existing rapid isothermal detection methods use the recombinase polymerase amplification (RPA), which exhibits polymerase chain reaction (PCR)-like sensitivity, specificity, and even higher speed. However, coupling RPA to other enzymatic reactions has proven difficult. For the first time, we demonstrate that with tuning of buffer conditions and optimization of reagent concentrations, RPA can be cascaded into an in vitro transcription reaction, enabling detection using fluorescent aptamers in a one-pot reaction. We show that this reaction, which we term PACRAT (pathogen detection with aptamer-observed cascaded recombinase polymerase amplification-in vitro transcription) can be used to detect SARS-CoV-2 RNA with single-copy detection limits, Escherichia coli with single-cell detection limits, and 10-min detection times. Further demonstrating the utility of our one-pot, cascaded amplification system, we show PACRAT can be used for multiplexed detection of the pathogens SARS-CoV-2 and E. coli, along with multiplexed detection of two variants of SARS-CoV-2.

BRET-based biosensors for SARS-CoV-2 oligonucleotide detection

The need for the early detection of emerging pathogenic viruses and their newer variants has driven the urgent demand for developing point-of-care diagnostic tools. Although nucleic acid-based methods such as reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and loop-mediated isothermal amplification (LAMP) have been developed, a more facile and robust platform is still required. To address this need, as a proof-of-principle study, we engineered a prototype-the versatile, sensitive, rapid, and cost-effective bioluminescence resonance energy transfer (BRET)-based biosensor for oligonucleotide detection (BioOD). Specifically, we designed BioODs against the SARS-CoV-2 parental (Wuhan strain) and B.1.617.2 Delta variant through the conjugation of specific, fluorescently modified molecular beacons (sensor module) through a complementary oligonucleotide handle DNA functionalized with the NanoLuc (NLuc) luciferase protein such that the dissolution of the molecular beacon loop upon the binding of the viral oligonucleotide will result in a decrease in BRET efficiency and, thus, a change in the bioluminescence spectra. Following the assembly of the BioODs, we determined their kinetics response, affinity for variant-specific oligonucleotides, and specificity, and found them to be rapid and highly specific. Furthermore, the decrease in BRET efficiency of the BioODs in the presence of viral oligonucleotides can be detected as a change in color in cell phone camera images. We envisage that the BioODs developed here will find application in detecting viral infections with variant specificity in a point-of-care-testing format, thus aiding in large-scale viral infection surveillance.

Turn-on RNA Mango Beacons for trans-acting fluorogenic nucleic acid detection

The Mango I and II RNA aptamers have been widely used in vivo and in vitro as genetically encodable fluorogenic markers that undergo large increases in fluorescence upon binding to their ligand, TO1-Biotin. However, while studying nucleic acid sequences, it is often desirable to have trans-acting probes that induce fluorescence upon binding to a target sequence. Here, we rationally design three types of light-up RNA Mango Beacons based on a minimized Mango core that induces fluorescence upon binding to a target RNA strand. Our first design is bimolecular in nature and uses a DNA inhibition strand to prevent folding of the Mango aptamer core until binding to a target RNA. Our second design is unimolecular in nature, and features hybridization arms flanking the core that inhibit G-quadruplex folding until refolding is triggered by binding to a target RNA strand. Our third design builds upon this structure, and incorporates a self-inhibiting domain into one of the flanking arms that deliberately binds to, and precludes folding of, the aptamer core until a target is bound. This design separates G-quadruplex folding inhibition and RNA target hybridization into separate modules, enabling a more universal unimolecular beacon design. All three Mango Beacons feature high contrasts and low costs when compared to conventional molecular beacons, with excellent potential for in vitro and in vivo applications.

Engineering of a Chimeric Template Triggers RNase H-Based Isothermal Amplification Approach for Sensitive Detection of Pathogen RNA

RNA-based detection of pathogenic organisms is an emerging field of research that is crucial for disease diagnosis and environmental and food safety. By rationally engineering an RNA-DNA tandem (RDT) structural template, we proposed a novel RNase H-based isothermal exponential amplification (RH-IEA) reaction to rapidly identify long-stranded RNA. In this strategy, the rigid and compact RDT template selectively recognized the target RNA and formed a stable hybrid with it. Upon site-specific cleavage of RNase H, the 3' overhang of the target RNA was cut off, and a free hydroxyl end at the hydrolysis site was generated to trigger an exponential amplification reaction (EXPAR). This method maintained the high efficiency and rapid amplification kinetics of EXPAR. As a result, the RH-IEA strategy was able to sensitively and specifically detect the characteristic sequence of Escherichia coli O157:H7 RNA, with a detection sensitivity of 1 fg/μL. Besides, the RDT template can be used as an RNA protector to prevent specific segments of the target RNA from being degraded by RNase enzymes, allowing the sample to be stored at room temperature for a long time. With this advantage, the practicality of RH-IEA will be more flexible than the reverse transcription polymerase chain reaction. It was successfully applied in the identification of E. coli O157:H7 in milk with a minimum detection concentration of 1.0 × 102 CFU/mL. Therefore, the RH-IEA method will serve as a powerful tool for detecting long-stranded RNA and will also shed light on the pathogen detection in food safety and molecular diagnosis.

Rapid Detection and Quantification of Viable Cells of Pectobacterium brasiliense Using Propidium Monoazide Combined with Real-Time PCR

Pectobacterium brasiliense (Pbr) has caused significant economic losses in major vegetable production areas in Northern China by causing bacterial soft rot in cash crops such as potatoes and cucumbers. This study aimed to establish a PMA-qPCR detection method for Pbr by screening specific and sensitive primers based on the glu gene and the conserved region of the 23S rRNA gene. Based on the optimized PMA pretreatment conditions, a standard curve was designed and constructed for PMA-qPCR detection (y = -3.391x + 36.28; R2 = 0.99). The amplification efficiency reached 97%, and the lowest detection limit of viable cells was approximately 2 × 102 CFU·mL-1. The feasibility of the PMA-qPCR method was confirmed through a manually simulated viable/dead cell assay under various concentrations. The analysis of potato tubers and cucumber seeds revealed that nine naturally collected seed samples contained a range from 102 to 104 CFU·g-1 viable Pbr bacteria. Furthermore, the system effectively identified changes in the number of pathogenic bacteria in cucumber and potato leaves affected by soft rot throughout the disease period. Overall, the detection and prevention of bacterial soft rot caused by Pbr is crucial.

Isothermal nucleic acid amplification and its uses in modern diagnostic technologies

Nucleic acids are prominent biomarkers for diagnosing infectious pathogens using nucleic acid amplification techniques (NAATs). PCR, a gold standard technique for amplifying nucleic acids, is widely used in scientific research and diagnosis. Efficient pathogen detection is a key to adequate food safety and hygiene. However, using bulky thermal cyclers and costly laboratory setup limits its uses in developing countries, including India. The isothermal amplification methods are exploited to develop miniaturized sensors against viruses, bacteria, fungi and other pathogenic organisms and have been applied for in situ diagnosis. Isothermal amplification techniques have been found suitable for POC techniques and follow WHO's ASSURED criteria. LAMP, NASBA, SDA, RCA and RPA are some of the isothermal amplification techniques which are preferable for POC diagnostics. Furthermore, methods such as WGA, CPA, HDA, EXPAR, SMART, SPIA and DAMP were introduced for even more accuracy and robustness. Using recombinant polymerases and other nucleic acid-modifying enzymes has dramatically broadened the detection range of target pathogens under the scanner. The coupling of isothermal amplification methods with advanced technologies such as CRISPR/Cas systems, fluorescence-based chemistries, microfluidics and paper-based sensors has significantly influenced the biosensing and diagnosis field. This review comprehensively analyzed isothermal nucleic acid amplification methods, emphasizing their advantages, disadvantages and limitations.

Isothermal nucleic acid amplification technology for rapid detection of virus

While the research field and industrial market of in vitro diagnosis (IVD) thrived during and post the COVID-19 pandemic, the development of isothermal nucleic acid amplification test (INAAT) based rapid diagnosis was engendered in a global wised large measure as a problem-solving exercise. This review systematically analyzed the recent advances of INAAT strategies with practical case for the real-world scenario virus detection applications. With the qualities that make INAAT systems useful for making diagnosis relevant decisions, the key performance indicators and the cost-effectiveness of enzyme-assisted methods and enzyme-free methods were compared. The modularity of nucleic acid amplification reactions that can lead to thresholding signal amplifications using INAAT reagents and their methodology design were examined, alongside the potential application with rapid test platform/device integration. Given that clinical practitioners are, by and large, unaware of many the isothermal nucleic acid test advances. This review could bridge the arcane research field of different INAAT systems and signal output modalities with end-users in clinic when choosing suitable test kits and/or methods for rapid virus detection.

Enzyme-Assisted Nucleic Acid Amplification in Molecular Diagnosis: A Review

Infectious diseases and tumors have become the biggest medical challenges in the 21st century. They are driven by multiple factors such as population growth, aging, climate change, genetic predispositions and more. Nucleic acid amplification technologies (NAATs) are used for rapid and accurate diagnostic testing, providing critical information in order to facilitate better follow-up treatment and prognosis. NAATs are widely used due their high sensitivity, specificity, rapid amplification and detection. It should be noted that different NAATs can be selected according to different environments and research fields; for example, isothermal amplification with a simple operation can be preferred in developing countries or resource-poor areas. In the field of translational medicine, CRISPR has shown great prospects. The core component of NAAT lies in the activity of different enzymes. As the most critical material of nucleic acid amplification, the key role of the enzyme is self-evident, playing the upmost important role in molecular diagnosis. In this review, several common enzymes used in NAATs are compared and described in detail. Furthermore, we summarize both the advances and common issues of NAATs in clinical application.

Visualizing the Nucleome Using the CRISPR–Cas9 System: From in vitro to in vivo

One of the latest methods in modern molecular biology is labeling genomic loci in living cells using fluorescently labeled Cas protein. The NIH Foundation has made the mapping of the 4D nucleome (the three-dimensional nucleome on a timescale) a priority in the studies aimed to improve our understanding of chromatin organization. Fluorescent methods based on CRISPR-Cas are a significant step forward in visualization of genomic loci in living cells. This approach can be used for studying epigenetics, cell cycle, cellular response to external stimuli, rearrangements during malignant cell transformation, such as chromosomal translocations or damage, as well as for genome editing. In this review, we focused on the application of CRISPR-Cas fluorescence technologies as components of multimodal imaging methods for in vivo mapping of chromosomal loci, in particular, attribution of fluorescence signal to morphological and anatomical structures in a living organism. The review discusses the approaches to the highly sensitive, high-precision labeling of CRISPR-Cas components, delivery of genetically engineered constructs into cells and tissues, and promising methods for molecular imaging.

Microfluidic chip and isothermal amplification technologies for the detection of pathogenic nucleic acid

The frequency of outbreaks of newly emerging infectious diseases has increased in recent years. The coronavirus disease 2019 (COVID-19) outbreak in late 2019 has caused a global pandemic, seriously endangering human health and social stability. Rapid detection of infectious disease pathogens is a key prerequisite for the early screening of cases and the reduction in transmission risk. Fluorescence quantitative polymerase chain reaction (qPCR) is currently the most commonly used pathogen detection method, but this method has high requirements in terms of operating staff, instrumentation, venues, and so forth. As a result, its application in the settings such as poorly conditioned communities and grassroots has been limited, and the detection needs of the first-line field cannot be met. The development of point-of-care testing (POCT) technology is of great practical significance for preventing and controlling infectious diseases. Isothermal amplification technology has advantages such as mild reaction conditions and low instrument dependence. It has a promising prospect in the development of POCT, combined with the advantages of high integration and portability of microfluidic chip technology. This study summarized the principles of several representative isothermal amplification techniques, as well as their advantages and disadvantages. Particularly, it reviewed the research progress on microfluidic chip-based recombinase polymerase isothermal amplification technology and highlighted future prospects.

Fluorescent Platforms for RNA Chemical Biology Research

Efficient detection and observation of dynamic RNA changes remain a tremendous challenge. However, the continuous development of fluorescence applications in recent years enhances the efficacy of RNA imaging. Here we summarize some of these developments from different aspects. For example, single-molecule fluorescence in situ hybridization (smFISH) can detect low abundance RNA at the subcellular level. A relatively new aptamer, Mango, is widely applied to label and track RNA activities in living cells. Molecular beacons (MBs) are valid for quantifying both endogenous and exogenous mRNA and microRNA (miRNA). Covalent binding enzyme labeling fluorescent group with RNA of interest (ROI) partially overcomes the RNA length limitation associated with oligonucleotide synthesis. Forced intercalation (FIT) probes are resistant to nuclease degradation upon binding to target RNA and are used to visualize mRNA and messenger ribonucleoprotein (mRNP) activities. We also summarize the importance of some fluorescence spectroscopic techniques in exploring the function and movement of RNA. Single-molecule fluorescence resonance energy transfer (smFRET) has been employed to investigate the dynamic changes of biomolecules by covalently linking biotin to RNA, and a focus on dye selection increases FRET efficiency. Furthermore, the applications of fluorescence assays in drug discovery and drug delivery have been discussed. Fluorescence imaging can also combine with RNA nanotechnology to target tumors. The invention of novel antibacterial drugs targeting non-coding RNAs (ncRNAs) is also possible with steady-state fluorescence-monitored ligand-binding assay and the T-box riboswitch fluorescence anisotropy assay. More recently, COVID-19 tests using fluorescent clustered regularly interspaced short palindromic repeat (CRISPR) technology have been demonstrated to be efficient and clinically useful. In summary, fluorescence assays have significant applications in both fundamental and clinical research and will facilitate the process of RNA-targeted new drug discovery, therefore deserving further development and updating.

A rapid and high sensitivity RNA detection based on NASBA and G4-ThT fluorescent biosensor

In recent years, various newly emerged and re-emerged RNA viruses have seriously threatened the global public health. There is a pressing need for rapid and reliable nucleic acid–based assays for detecting viral RNA. Here, we successfully developed a highly sensitive, easy-to-operate G4-ThT-NASBA system to detect viral RNA that no need for labeled primers and probes. Next, we tested the system for detecting the Classical Swine Fever Virus (CSFV), an RNA virus that causes a highly contagious disease in domestic pigs and wild boar and easily causes huge economic losses. Results showed that the system, integrated the G4-ThT fluorescent biosensor and NASBA (Nuclear acid sequence-based amplification),is capable to detect as little as 2 copies/μL of viral RNA without interfering by other swine viral RNA. Moreover, we were able to detect CSFV RNA within 2 h in serum samples taken from the field in a real-time mode. These findings indicate that the G4-ThT-NASBA system is a rapid, high sensitivity and easy-to-operate technique for RNA detection. The method also has the real-time detection capability which may be easily integrated in a highly automated system such as microfluidic chips.

Isothermal RNA Amplification for the Detection of Viable Pathogenic Bacteria to Estimate the Salmonella Virulence for Causing Enteritis

Viable foodborne pathogens can cause intestinal infection and food poisoning. Herein, we reported an RNA assay allowing for sensitive (close to 1 CFU and 1% viable bacteria detectable) and rapid (within 2.5 h) detection of viable pathogenic bacteria by coupling isothermal RNA amplification (nucleic acid sequence-based amplification, NASBA) with a CRISPR/Cas13a system. NASBA allowed direct amplification of 16S rRNA extracted from viable S. enterica (RNAs degrade rapidly in dead bacteria), and the specificity of amplification was ensured using Cas13a/crRNA to recognize the amplicons. We used the CRISPR/Cas13-based NASBA assay (termed cNASBA assay) to investigate the in vivo colonization and intestinal infection of S. enterica in mice. We found that S. enterica was mainly colonized at the cecum, colon, and rectum, and the severity of enteritis caused by S. enterica was determined by the number of viable S. enterica rather than the total count of S. enterica. The cNASBA assay can quantify viable S. enterica and thus can improve the accuracy of virulence estimation compared to qPCR. It shows promise as a reliable tool for monitoring pathogen contamination and biosafety control.

Aptamers-Diagnostic and Therapeutic Solution in SARS-CoV-2

The SARS-CoV-2 virus is currently the most serious challenge to global public health. Its emergence has severely disrupted the functioning of health services and the economic and social situation worldwide. Therefore, new diagnostic and therapeutic tools are urgently needed to allow for the early detection of the SARS-CoV-2 virus and appropriate treatment, which is crucial for the effective control of the COVID-19 disease. The ideal solution seems to be the use of aptamers—short fragments of nucleic acids, DNA or RNA—that can bind selected proteins with high specificity and affinity. They can be used in methods that base the reading of the test result on fluorescence phenomena, chemiluminescence, and electrochemical changes. Exploiting the properties of aptamers will enable the introduction of rapid, sensitive, specific, and low-cost tests for the routine diagnosis of SARS-CoV-2. Aptamers are excellent candidates for the development of point-of-care diagnostic devices and are potential therapeutic tools for the treatment of COVID-19. They can effectively block coronavirus activity in multiple fields by binding viral proteins and acting as carriers of therapeutic substances. In this review, we present recent developments in the design of various types of aptasensors to detect and treat the SARS-CoV-2 infection.

Engineered RNA biosensors enable ultrasensitive SARS-CoV-2 detection in a simple color and luminescence assay

This work reports engineered toehold RNA–based biosensors for COVID-19 diagnostics, with a simple color or luminescence readout that makes it easily deployable in both well-equipped labs as well as low resource settings.

The continued resurgence of the COVID-19 pandemic with multiple variants underlines the need for diagnostics that are adaptable to the virus. We have developed toehold RNA–based sensors across the SARS-CoV-2 genome for direct and ultrasensitive detection of the virus and its prominent variants. Here, isothermal amplification of a fragment of SARS-CoV-2 RNA coupled with activation of our biosensors leads to a conformational switch in the sensor. This leads to translation of a reporter protein, for example, LacZ or nano-lantern that is easily detected using color/luminescence. By optimizing RNA amplification and biosensor design, we have generated a highly sensitive diagnostic assay that is capable of detecting as low as 100 copies of viral RNA with development of bright color. This is easily visualized by the human eye and quantifiable using spectrophotometry. Finally, this PHAsed NASBA-Translation Optical Method (PHANTOM) using our engineered RNA biosensors efficiently detects viral RNA in patient samples. This work presents a powerful and universally accessible strategy for detecting COVID-19 and variants. This strategy is adaptable to further viral evolution and brings RNA bioengineering center-stage.

Identification and Engineering of Aptamers for Theranostic Application in Human Health and Disorders

An aptamer is a short sequence of synthetic oligonucleotides which bind to their cognate target, specifically while maintaining similar or higher sensitivity compared to an antibody. The in-vitro selection of an aptamer, applying a conjoining approach of chemistry and molecular biology, is referred as Systematic Evolution of Ligands by Exponential enrichment (SELEX). These initial products of SELEX are further modified chemically in an attempt to make them stable in biofluid, avoiding nuclease digestion and renal clearance. While the modification is incorporated, enough care should be taken to maintain its sensitivity and specificity. These modifications and several improvisations have widened the window frame of aptamer applications that are currently not only restricted to in-vitro systems, but have also been used in molecular imaging for disease pathology and treatment. In the food industry, it has been used as sensor for detection of different diseases and fungal infections. In this review, we have discussed a brief history of its journey, along with applications where its role as a therapeutic plus diagnostic (theranostic) tool has been demonstrated. We have also highlighted the potential aptamer-mediated strategies for molecular targeting of COVID-19. Finally, the review focused on its future prospective in immunotherapy, as well as in identification of novel biomarkers in stem cells and also in single cell proteomics (scProteomics) to study intra or inter-tumor heterogeneity at the protein level. Small size, chemical synthesis, low batch variation, cost effectiveness, long shelf life and low immunogenicity provide advantages to the aptamer over the antibody. These physical and chemical properties of aptamers render them as a strong biomedical tool for theranostic purposes over the existing ones. The significance of aptamers in human health was the key finding of this review.

The mechanism and improvements to the isothermal amplification of nucleic acids, at a glance

A comparative review of the most common isothermal methods is provided. In the last two decades, the challenge of using isothermal amplification systems as an alternate to the most extensive and long-standing nucleic acids-amplifying method-the polymerase chain reaction-has arisen. The main advantage of isothermal amplification is no requirement for expensive laboratory equipment for thermal cycling. Considerable efforts have been made to improve the current techniques of nucleic acid amplification and the development of new approaches based on the main drawbacks of each method. The most important and challenging goal was to achieve a low-cost, straightforward system that is rapid, specific, accurate, and sensitive.

Broad Applications of Thiazole Orange in Fluorescent Sensing of Biomolecules and Ions

Fluorescent sensing of biomolecules has served as a revolutionary tool for studying and better understanding various biological systems. Therefore, it has become increasingly important to identify fluorescent building blocks that can be easily converted into sensing probes, which can detect specific targets with increasing sensitivity and accuracy. Over the past 30 years, thiazole orange (TO) has garnered great attention due to its low fluorescence background signal and remarkable 'turn-on' fluorescence response, being controlled only by its intramolecular torsional movement. These features have led to the development of numerous molecular probes that apply TO in order to sense a variety of biomolecules and metal ions. Here, we highlight the tremendous progress made in the field of TO-based sensors and demonstrate the different strategies that have enabled TO to evolve into a versatile dye for monitoring a collection of biomolecules.

Rapid and highly sensitive pathogen detection by real-time DNA monitoring using a nanogap impedimetric sensor with recombinase polymerase amplification

Fast detection of pathogens is important for protecting our health and society. Herein, we present a high-performance nanogap impedimetric sensor for monitoring nucleic acid amplification in real time using isothermal recombinase polymerase amplification (RPA) for rapid pathogen detection. The nanogap electrode chip has two pairs of opposing gold electrodes with a 100 nm gap and was fixed to a PCB. Then, the nanogap impedimetric sensor was immersed in RPA reaction solution for the detection of E. coli O157:H7, and target DNA amplification was evaluated through bulk solution impedance changes using impedance spectroscopy every minute during RPA. In addition, target gene amplification in the sample solution during RPA was confirmed with a 2% DNA agarose gel. Our nanogap impedimetric sensor can detect down to a single copy of the eae A gene in gDNA extracted from E. coli O157:H7 as well as a single cell of pathogenic E. coli O157:H7 strain within 5 min during direct RPA, which was performed with the pathogen itself and without the extraction and purification of target gDNA. The miniaturized nanogap impedimetric sensor has potential as a cost-effective point-of-care device for fast and accurate portable pathogen detection via real-time nucleic acid analysis.

RNA Peach and Mango: Orthogonal two-color fluorogenic aptamers distinguish nearly identical ligands

Two-channel fluorogenic RNA aptamer-based imaging is currently challenging. While we have previously characterized the Mango series of aptamers that bind tightly and specifically to the green fluorophore TO1-Biotin, the next aim was to identify an effective fluorogenic aptamer partner for two-color imaging. A competitive in vitro selection for TO3-Biotin binding aptamers was performed resulting in the Peach I and II aptamers. Remarkably, given that the TO1-Biotin and TO3-Biotin heterocycles differ by only two bridging carbons, these new aptamers exhibit a marked preference for TO3-Biotin binding relative to the iM3 and Mango III A10U aptamers, which preferentially bind TO1-Biotin. Peach I, like Mango I and II, appears to contain a quadruplex core isolated by a GAA^A type tetraloop-like adaptor from its closing stem. Thermal melts of the Peach aptamers reveal that TO3-Biotin binding is cooperative, while TO1-Biotin binding is not, suggesting a unique and currently uncharacterized mode of ligand differentiation. Using only fluorescent measurements, the concentrations of Peach and Mango aptamers could be reliably determined in vitro. The utility of this orthogonal pair provides a possible route to in vivo two-color RNA imaging.

Extraction-free RT-LAMP to detect SARS-CoV-2 is less sensitive but highly specific compared to standard RT-PCR in 101 samples

The current scale of public and private testing cannot be expected to meet the emerging need for higher levels of community-level and repeated screening of asymptomatic Canadians for SARS-CoV-2. Rapid point-of-care techniques are increasingly being offered to fill the gap in screening levels required to identify undiagnosed individuals with high viral loads. However, rapid, point-of-care tests often have lower sensitivity in practice. Reverse transcription loop-mediated isothermal amplification (RT-LAMP) for SARS-CoV-2 has proven sensitive and specific and provides visual results in minutes. Using a commercially available kit for RT-LAMP and primer set targetting nucleocapsid (N), we tested a blinded set of 101 archived nasopharyngeal (NP) swab samples with known RT-PCR results. RT-LAMP reactions were incubated at 65 °C for 30 min, using heat-inactivated nasopharyngeal swab sample in viral transport medium, diluted tenfold in water, as input. RT-LAMP agreed with all RT-PCR defined negatives (N = 51), and all positives with cycle threshold (Ct) less than 20 (N = 24), 65% of positives with Ct between 20−30 (N = 17), and no positives with Ct greater than 30 (N = 9). RT-LAMP requires fewer and different core components, so may not compete directly with the mainline testing workflow, preserving precious central laboratory resources for those with the greatest need. Careful messaging must be provided when using less-sensitive tests, so that people are not falsely reassured by negative results, but this caveat must be weighed against the clear benefits of reliably identifying those with high levels of virus in prioritized samples at the point of care.

Developments in integrating nucleic acid isothermal amplification and detection systems for point-of-care diagnostics

Early disease detection through point-of-care (POC) testing is vital for quickly treating patients and preventing the spread of harmful pathogens. Disease diagnosis is generally accomplished using quantitative polymerase chain reaction (qPCR) to amplify nucleic acids in patient samples, permitting detection even at low target concentrations. However, qPCR requires expensive equipment, trained personnel, and significant time. These resources are not available in POC settings, driving researchers to instead utilize isothermal amplification, conducted at a single temperature, as an alternative. Common isothermal amplification methods include loop-mediated isothermal amplification, recombinase polymerase amplification, rolling circle amplification, nucleic acid sequence-based amplification, and helicase-dependent amplification. There has been a growing interest in combining such amplification methods with POC detection methods to enable the development of diagnostic tests that are well suited for resource-limited settings as well as developed countries performing mass screenings. Exciting developments have been made in the integration of these two research areas due to the significant impact that such approaches can have on healthcare. This review will primarily focus on advances made by North American research groups between 2015 and June 2020, and will emphasize integrated approaches that reduce user steps, reliance on expensive equipment, and the system's time-to-result.

Isothermal SARS-CoV-2 Diagnostics: Tools for Enabling Distributed Pandemic Testing as a Means of Supporting Safe Reopenings

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, poses grave threats to both the global economy and health. The predominant diagnostic screens in use for SARS-CoV-2 detection are molecular techniques such as nucleic acid amplification tests. In this Review, we compare current and emerging isothermal diagnostic methods for COVID-19. We outline the molecular and serological techniques currently being used to detect SARS-CoV-2 infection, past or present, in patients. We also discuss ongoing research on isothermal techniques, CRISPR-mediated detection assays, and point-of-care diagnostics that have potential for use in SARS-CoV-2 detection. Large-scale viral testing during a global pandemic presents unique challenges, chief among them the simultaneous need for testing supplies, durable equipment, and personnel in many regions worldwide, with each of these regions possessing testing needs that vary as the pandemic progresses. The low-cost isothermal technologies described in this Review provide a promising means by which to address these needs and meet the global need for testing of symptomatic individuals as well as provide a possible means for routine testing of asymptomatic individuals, providing a potential means of safely enabling reopenings and early monitoring of outbreaks.

Highly specific, multiplexed isothermal pathogen detection with fluorescent aptamer readout

Isothermal, cell-free, synthetic biology-based approaches to pathogen detection leverage the power of tools available in biological systems, such as highly active polymerases compatible with lyophilization, without the complexity inherent to live-cell systems, of which nucleic acid sequence based amplification (NASBA) is well known. Despite the reduced complexity associated with cell-free systems, side reactions are a common characteristic of these systems. As a result, these systems often exhibit false positives from reactions lacking an amplicon. Here we show that the inclusion of a DNA duplex lacking a promoter and unassociated with the amplicon fully suppresses false positives, enabling a suite of fluorescent aptamers to be used as NASBA tags (Apta-NASBA). Apta-NASBA has a 1 pM detection limit and can provide multiplexed, multicolor fluorescent readout. Furthermore, Apta-NASBA can be performed using a variety of equipment, for example, a fluorescence microplate reader, a qPCR instrument, or an ultra-low-cost Raspberry Pi-based 3D-printed detection platform using a cell phone camera module, compatible with field detection.