DNA aptamer identification and characterization for E. coli O157 detection using cell based SELEX method

DNA-modified Prussian blue nanozymes for enhanced electrochemical biosensing

Prussian blue nanoparticles exhibit the potential to be employed in bioanalytical applications due to their robust stability, peroxidase-like catalytic functionality, straightforward synthesis, and biocompatibility. An efficient approach is presented for the synthesis of nucleic acid-modified Prussian blue nanoparticles (DNA-PBNPs), utilizing nanoparticle porosity to adsorb nucleic acids (polyT). This strategic adsorption leads to the exposure of nucleic acid sequences on the particle surface while retaining catalytic activity. DNA-PBNPs further couple with functional nucleic acid sequences and aptamers through complementary base pairing to act as transducers in biosensors and amplify signal acquisition. Subsequently, we integrated a copper ion-dependent DNAzyme (Cu2+-DNAzyme) and a vascular endothelial growth factor aptamer (VEGF aptamer) onto screen-printed electrodes to serve as recognition elements for analytes. Significantly, our approach leverages DNA-PBNPs as a superior alternative to traditional enzyme-linked antibodies in electrochemical biosensors, thereby enhancing both the efficiency and adaptability of these devices. Our study conclusively demonstrates the application of DNA-PBNPs in two different biosensing paradigms: the sensitive detection of copper ions and vascular endothelial growth factor (VEGF). These results indicate the promising potential of DNA-modified Prussian blue nanoparticles in advancing bioanalytical sensing technologies.

Aptamer decorated PDA@magnetic silica microparticles for bacteria purification

One significant constraint in the advancement of biosensors is the signal-to-noise ratio, which is adversely affected by the presence of interfering factors such as blood in the sample matrix. In the present investigation, a specific aptamer binding was chosen for its affinity, while exhibiting no binding affinity towards non-target bacterial cells. This selective binding property was leveraged to facilitate the production of magnetic microparticles decorated with aptamers. A novel assay was developed to effectively isolate S. pneumoniae from PBS or directly from blood samples using an aptamer with an affinity constant of 72.8 nM. The capture experiments demonstrated efficiencies up to 87% and 66% are achievable for isolating spiked S. pneumoniae in 1 mL PBS and blood samples, respectively.

Nucleic Acid Aptamer-Based Biosensors: A Review

Aptamers, short strands of either DNA, RNA, or peptides, known for their exceptional specificity and high binding affinity to target molecules, are providing significant advancements in the field of health. When seamlessly integrated into biosensor platforms, aptamers give rise to aptasensors, unlocking a new dimension in point-of-care diagnostics with rapid response times and remarkable versatility. As such, this review aims to present an overview of the distinct advantages conferred by aptamers over traditional antibodies as the molecular recognition element in biosensors. Additionally, it delves into the realm of specific aptamers made for the detection of biomarkers associated with infectious diseases, cancer, cardiovascular diseases, and metabolomic and neurological disorders. The review further elucidates the varying binding assays and transducer techniques that support the development of aptasensors. Ultimately, this review discusses the current state of point-of-care diagnostics facilitated by aptasensors and underscores the immense potential of these technologies in advancing the landscape of healthcare delivery.

Aptamer-based assay for rapid detection, surveillance, and screening of pathogenic Leptospira in water samples

Leptospirosis is a potentially fatal waterborne infection caused by Leptospira interrogans, impacting both humans and animals in tropical regions. However, current diagnostic methods for detecting pathogenic Leptospira have sensitivity, cost, and time limitations. Therefore, there is a critical need for a rapid, sensitive, and cost-effective detection method. This study presents the development of an aptamer-based assay for pathogenic Leptospira detection. Aptamers targeting Leptospira were generated using the SELEX method and screened for binding affinity with major Leptospiral outer membrane proteins through in silico analysis. The aptamer with the highest binding affinity was selected for further evaluation. To enable visual detection, the aptamer was conjugated to gold nanoparticles (AuNPs), resulting in a colorimetric response in the presence of L. interrogans. The aptamer-AuNP-based colorimetric assay exhibited a detection limit of 57 CFU/mL and demonstrated high specificity and reproducibility in detecting pathogenic Leptospira in water samples. This aptamer-based assay represents a significant advancement in leptospirosis diagnostics, offering a rapid, sensitive, and cost-effective approach for detecting pathogenic Leptospira. Its potential for epidemiological applications, such as outbreak source identification and improved prevention, diagnosis, and treatment, particularly in resource-limited settings, highlights its importance in addressing the challenges associated with leptospirosis.

Molecular Targets for Foodborne Pathogenic Bacteria Detection

The detection of foodborne pathogenic bacteria currently relies on their ability to grow on chemically defined liquid and solid media, which is the essence of the classical microbiological approach. Such procedures are time-consuming and the quality of the result is affected by the selectivity of the media employed. Several alternative strategies based on the detection of molecular markers have been proposed. These markers may be cell constituents, may reside on the cell envelope or may be specific metabolites. Each marker provides specific advantages and, at the same time, suffers from specific limitations. The food matrix and chemical composition, as well as the accompanying microbiota, may also severely compromise detection. The aim of the present review article is to present and critically discuss all available information regarding the molecular targets that have been employed as markers for the detection of foodborne pathogens. Their strengths and limitations, as well as the proposed alleviation strategies, are presented, with particular emphasis on their applicability in real food systems and the challenges that are yet to be effectively addressed.

Capture-SELEX: Selection Strategy, Aptamer Identification, and Biosensing Application

Small-molecule contaminants, such as antibiotics, pesticides, and plasticizers, have emerged as one of the substances most detrimental to human health and the environment. Therefore, it is crucial to develop low-cost, user-friendly, and portable biosensors capable of rapidly detecting these contaminants. Antibodies have traditionally been used as biorecognition elements. However, aptamers have recently been applied as biorecognition elements in aptamer-based biosensors, also known as aptasensors. The systematic evolution of ligands by exponential enrichment (SELEX) is an in vitro technique used to generate aptamers that bind their targets with high affinity and specificity. Over the past decade, a modified SELEX method known as Capture-SELEX has been widely used to generate DNA or RNA aptamers that bind small molecules. In this review, we summarize the recent strategies used for Capture-SELEX, describe the methods commonly used for detecting and characterizing small-molecule-aptamer interactions, and discuss the development of aptamer-based biosensors for various applications. We also discuss the challenges of the Capture-SELEX platform and biosensor development and the possibilities for their future application.

Advances in Aptamer-Based Biosensors and Cell-Internalizing SELEX Technology for Diagnostic and Therapeutic Application

Aptamers are a group of synthetic single-stranded nucleic acids. They are generated from a random library of single-stranded DNA or RNA by a technology named systematic evolution of ligands by exponential enrichment (SELEX). SELEX is a repetitive process to select and identify suitable aptamers that show high affinity and specificity towards target cells. Great strides have been achieved in the design, construction, and use of aptamers up to this point. However, only a small number of aptamer-based applications have achieved widespread commercial and clinical acceptance. Additionally, finding more effective ways to acquire aptamers with high affinity remains a challenge. Therefore, it is crucial to thoroughly examine the existing dearth and advancement in aptamer-related technologies. This review focuses on aptamers that are generated by SELEX to detect pathogenic microorganisms and mammalian cells, as well as in cell-internalizing SELEX for diagnostic and therapeutic purposes. The development of novel aptamer-based biosensors using optical and electrical methods for microbial detection is reported. The applications and limitations of aptamers are also discussed.

Aptamers: A prospective tool for infectious diseases diagnosis

It is well known that people's health is seriously threatened by various pathogens (such as Mycobacterium tuberculosis, Treponema pallidum, Novel coronavirus, HIV, Mucor, etc.), which leads to heavy socioeconomic burdens. Therefore, early and accurate pathogen diagnosis is essential for timely and effective therapies. Up to now, diagnosing human contagious diseases at molecule and nano levels is remarkably difficult owing to insufficient valid probes when it comes to determining the biological markers of pathogens. Aptamers are a set of high‐specificity and high‐sensitivity plastic oligonucleotides screened in vitro via the selective expansion of ligands by exponential enrichment (SELEX). With the advent of aptamer‐based technologies, their merits have aroused mounting academic interest. In recent years, as new detection and treatment tools, nucleic acid aptamers have been extensively utilized in the field of biomedicine, such as pathogen detection, new drug development, clinical diagnosis, nanotechnology, etc. However, the traditional SELEX method is cumbersome and has a long screening cycle, and it takes several months to screen out aptamers with high specificity. With the persistent development of SELEX‐based aptamer screening technologies, the application scenarios of aptamers have become more and more extensive. The present research briefly reviews the research progress of nucleic acid aptamers in the field of biomedicine, especially in the diagnosis of contagious diseases.

A Polyclonal Selex Aptamer Library Directly Allows Specific Labelling of the Human Gut Bacterium Blautia producta without Isolating Individual Aptamers

Recent studies have demonstrated that changes in the abundance of the intestinal bacterium Blautia producta, a potential probiotic, are closely associated with the development of various diseases such as obesity, diabetes, some neurodegenerative diseases, and certain cancers. However, there is still a lack of an effective method to detect the abundance of B. producta in the gut rapidly. Especially, DNA aptamers are now widely used as biometric components for medical testing due to their unique characteristics, including high chemical stability, low production cost, ease of chemical modification, low immunogenicity, and fast reproducibility. We successfully obtained a high-affinity nucleic acid aptamer library (B.p-R14) after 14 SELEX rounds, which efficiently discriminates B. producta in different analysis techniques including fluorometric suspension assays or fluorescence microscopy from other major gut bacteria in complex mixtures and even in human stool samples. These preliminary findings will be the basis towards aptamer-based biosensing applications for the fast and reliable monitoring of B. producta in the human gut microbiome.

Future perspectives on aptamer for application in food authentication

Food fraudulence and food contamination are major concerns, particularly among consumers with specific dietary, cultural, lifestyle, and religious requirements. Current food authentication methods have several drawbacks and limitations, necessitating the development of a simpler, more sensitive, and rapid detection approach for food screening analysis, such as an aptamer-based biosensor system. Although the use of aptamer is growing in various fields, aptamer applications for food authentication are still lacking. In this review, we discuss the limitations of existing food authentication technologies and describe the applications of aptamer in food analyses. We also project several potential targets or marker molecules to be targeted in the SELEX process. Finally, this review highlights the drawbacks of current aptamer technologies and outlines the potential route of aptamer selection and applications for successful food authentication. This review provides an overview of the use of aptamer in food research and its potential application as a molecular reporter for rapid detection in food authentication process. Developing databases to store all biochemical profiles of food and applying machine learning algorithms against the biochemical profiles are urged to accelerate the identification of more reliable biomarker molecules as aptamer targets for food authentication.

Gold nanoparticles (AuNP)-based aptasensor for enteropathogenic Escherichia coli detection

BACKGROUND

Diarrhea is a major cause of severe gastrointestinal illness in the infant especially in many developing countries. Although this molecular technique has been accepted as standard technique to detect Diarrhea-causing EPEC, the practical aspect of this technique for in-site rapid screening purposes is still facing a major challenge. In this study, we characterized EPEC specific aptamers and applied it as an AuNP-based aptasensor for point of care (POC) diagnosis purpose.

METHODS

As many as six selected DNA aptamers was screened using target bacteria and the bound aptamer was measured by qPCR technique. Moreover, Kd value for each optimal bound aptamer was measured by using the same technique. Colorimetry assay was applied to test specificity and LOD of AuNP-based aptasensor.

RESULTS

Two DNA aptamers have been successfully obtained to detect Enteropathogenic Escherichia coli K.1.1. DNA aptamer S8-7 exhibited constant dissociation (Kd) value of 17.08 nM, while DNA aptamer S10-5 exhibited Kd value of 34.14 nM. AuNP-based aptasensor showed high selectivity and specificity for EPEC K.1.1 with a limit of detection (LOD) value of 10⁵ CFU/mL. Truncation study on DNA aptamer S8-7 showed that elimination of primer binding sequence only slightly increased both performance of detection and LOD value of AuNP-based aptasensor.

CONCLUSION

Further study is necessary to improve AuNP-aptasensor performance such as through mutagenesis approach on targeted DNA aptamers before AuNP-based aptasensor can be applied as a biosensor in point of care (POC) diagnosis.

SELEX against whole-cell bacteria resulted in lipopolysaccharide binding aptamers

Nucleic acid aptamers are target-specific oligonucleotides selected from combinatorial libraries through an iterative in vitro screening process known as Systemic Evolution of Ligands by Exponential Enrichment (SELEX). In this report, the selection of bacteria differentiating ssDNA aptamer candidates from a combinatorial library through the whole-cell SELEX method was performed. The enriched SELEX pool was sequenced using Illumina Next-Generation Sequencing (NGS) technology and analyzed for the most abundant sequences using CLC Genomics Workbench. The sequencing data resulted in several oligonucleotide families from which three individual sequences were chosen per SELEX based on the copy numbers. The binding performance of the selected aptamers was assessed by flow cytometry and fluorescence spectroscopy, and the binding constants were estimated using binding saturation curves. Varying results were obtained from two independent SELEX procedures where the SELEX against the model gram-negative bacterium Escherichia coli provided more selective sequences while the SELEX library used against gram-positive bacterium Listeria monocytogenes did not evolve as expected. The sequences that emerged from E. coli SELEX were shown to bind Lipopolysaccharide residues (LPS) and inhibit LPS-induced macrophage polarization. Thus, it can be said that, performed whole-cell SELEX could be resulted as the selection of aptamers which can bind LPS and inhibit LPS induced inflammation response and thus can be candidates for the inhibition of bacterial infections. In future studies, the selected aptamer sequences could be structurally and chemically modified and exploited as potential diagnostic tools and therapeutic agents as LPS antagonists.

Recent Advances in Aptasensors For Rapid and Sensitive Detection of Staphylococcus Aureus

The infection of Staphylococcus aureus (S.aureus) and the spread of drug-resistant bacteria pose a serious threat to global public health. Therefore, timely, rapid and accurate detection of S. aureus is of great significance for food safety, environmental monitoring, clinical diagnosis and treatment, and prevention of drug-resistant bacteria dissemination. Traditional S. aureus detection methods such as culture identification, ELISA, PCR, MALDI-TOF-MS and sequencing, etc., have good sensitivity and specificity, but they are complex to operate, requiring professionals and expensive and complex machines. Therefore, it is still challenging to develop a fast, simple, low-cost, specific and sensitive S. aureus detection method. Recent studies have demonstrated that fast, specific, low-cost, low sample volume, automated, and portable aptasensors have been widely used for S. aureus detection and have been proposed as the most attractive alternatives to their traditional detection methods. In this review, recent advances of aptasensors based on different transducer (optical and electrochemical) for S. aureus detection have been discussed in details. Furthermore, the applications of aptasensors in point-of-care testing (POCT) have also been discussed. More and more aptasensors are combined with nanomaterials as efficient transducers and amplifiers, which appears to be the development trend in aptasensors. Finally, some significant challenges for the development and application of aptasensors are outlined.

Aptamers against Pathogenic Bacteria: Selection Strategies and Apta-assay/Aptasensor Application for Food Safety

Pathogenic bacteria are primarily kinds of detrimental agents that cause mankind illness via contaminated food with traits of multiple types, universality, and low content. In view of the detection demands for rapidity, aptamer recognition factors emerged as a substitution for antibodies, which are short single strands of nucleic acid selected via in vitro. They display certain superiorities over antibodies, such as preferable stability, liable modification, and cost-efficiency. Taking advantage of the situation, numerous aptamers against pathogenic bacteria have been successfully selected and applied, yet there are still restrictions on commercial availability. In this review, the strategies/approaches to key sections in pathogen aptamers SELEX and post-SELEX are summarized and sorted out. Recently, optical, electrochemical, and piezoelectric aptamer-based assays or sensors dedicated to pathogen detection have been critically reviewed. Ultimately, the existing challenges and future trends in this field are proposed to further promote development prospects.

Research progress of whole-cell-SELEX selection and the application of cell-targeting aptamer

BACKGROUND

Aptamers refer to the artificially synthesized nucleic acid sequences (DNA/RNA) that can bind to a wide range of targets with high affinity and specificity, which are generally generated from systematic evolution of ligands by exponential enrichment (SELEX). As a novel method of aptamers screening, whole-cell-SELEX (WC-SELEX) has gained more and more attention in many fields such as biomedicine, analytical chemistry, and molecular diagnostics due to its ability to screen multiple potential aptamers without knowing the detailed structural information of target molecules.

METHODS AND RESULTS

In recent years, with the deepening of research on application of aptamers, the traditional WC-SELEX cannot meet the practical application because of long experimental period, complicated operation process and low specificity, etc. Therefore, the development of more efficient methods for screening aptamer is always on the road. This paper summarizes the current research status of WC-SELEX for bacteria, parasites and animal cells, and reviews the latest advances of WC-SELEX techniques that are dependent on novel instruments, materials and microelectronics, including fluorescence-activated cell sorting-assisted SELEX, three-dimensional assisted WC-SELEX, and microfluidic chip system-assisted WC-SELEX. In addition, the application of aptamers targeting cells was discussed.

CONCLUSION

Taken together, this review is aimed at providing a reference for WC-SELEX selection and application of aptamer targeting cells.

In silico screening of ssDNA aptamer against Escherichia coli O157:H7: A machine learning and the Pseudo K-tuple nucleotide composition based approach

This study was planned to in silico screening of ssDNA aptamer against Escherichia coli O157:H7 by combination of machine learning and the PseKNC approach. For this, firstly a total numbers of 47 validated ssDNA aptamers as well as 498 random DNA sequences were considered as positive and negative training data respectively. The sequences then converted to numerical vectors using PseKNC method through Pse-in-one 2.0 web server. After that, the numerical vectors were subjected to classification by the SVM, ANN and RF algorithms available in Orange 3.2.0 software. The performances of the tested models were evaluated using cross-validation, random sampling and ROC curve analyzes. The primary results demonstrated that the ANN and RF algorithms have appropriate performances for the data classification. To improve the performances of mentioned classifiers the positive training data was triplicated and re-training process was also performed. The results confirmed that data size improvement had significant effect on the accuracy of data classification especially about RF model. Subsequently, the RF algorithm with accuracy of 98% was selected for aptamer screening. The thermodynamics details of folding process as well as secondary structures of the screened aptamers were also considered as final evaluations. The results confirmed that the selected aptamers by the proposed method had appropriate structure properties and there is no thermodynamics limit for the aptamers folding.

Aptamer-based lateral flow assay on-site biosensors

The lateral flow assay (LFA) is a widely used paper-based on-site biosensor that can detect target analytes and obtain test results in several minutes. Generally, antibodies are utilized as the biorecognition molecules in the LFA. However, antibodies selected using an in vivo process not only may risk killing the animal hosts and causing errors between different batches but also their range is restricted by the refrigerated conditions used to store them. To avoid these limitations, aptamers screened by an in vitro process have been studied as biorecognition molecules in LFAs. Based on the sandwich or competitive format, the aptamer-based LFA can accomplish on-site detection of target analytes. Since aptamers have a distinctive ability to undergo conformational changes, the adsorption-desorption format has also been exploited to detect target analytes in aptamer-based LFAs. This paper reviews developments in aptamer-based LFAs in the last three years for the detection of target analytes. Three formats of aptamer-based LFAs, i.e., sandwich, competitive, and adsorption-desorption, are described in detail. Based on these formats, signal amplification strategies and multiplexed detection are discussed in order to provide an overview of aptamer-based LFAs for on-site detection of target analytes. In addition, the potential commercialization and future perspectives of aptamer-based LFAs for rapid detection of SARS-CoV-2 are given to support the COVID-19 pandemic.

Electrochemical aptasensor for Escherichia coli O157:H7 bacteria detection using a nanocomposite of reduced graphene oxide, gold nanoparticles and polyvinyl alcohol

In recent years, public attention has drawn to food safety due to the constant outbreaks of foodborne diseases; subsequently, to control and prevent this group of diseases, early screening of foodborne pathogens has become significant. In this study, a new aptamer-based electrochemical sensor was proposed to detect Escherichia coli O157:H7 (E. coli), one of the most threatening bacterial pathogens, using nanoparticles-modified glassy carbon electrode. Firstly, the electrode was coated with a reduced graphene oxide-poly(vinyl alcohol) and gold nanoparticles nanocomposite (AuNPs/rGO-PVA/GCE) to increase the electrode surface area and consequently raise the sensor sensitivity. Afterwards, to enhance the selectivity of the modified electrode, aptamers were attached to the surface of the prepared electrode. The prepared electrode was characterized using energy-dispersive spectroscopy, field-emission scanning electron microscopy, atomic force microscopy, Fourier-transform infrared spectroscopy, and electrochemical impedance spectroscopy. The relationship of the E. coli concentration and the peak current in the range from 9.2 CFU mL^-1 to 9.2 × 10⁸ CFU mL^-1 was linear, and the limit of detection was calculated as 9.34 CFU mL^-1. The suitability of the proposed sensor for real sample measurements was investigated by recovery studies in tap water, milk, and meat samples. The results showed that the biosensor and traditional culture counting methods are equally sensitive for detecting E. coli.

BipD of Burkholderia pseudomallei: Structure, Functions, and Detection Methods

Melioidosis is a severe disease caused by Burkholderia pseudomallei (B. pseudomallei), a Gram-negative environmental bacterium. It is endemic in Southeast Asia and Northern Australia, but it is underreported in many other countries. The principal routes of entry for B. pseudomallei are skin penetration, inhalation, and ingestion. It mainly affects immunocompromised populations, especially patients with type 2 diabetes mellitus. The laboratory diagnosis of melioidosis is challenging due to its non-specific clinical manifestations, which mimic other severe infections. The culture method is considered an imperfect gold standard for the diagnosis of melioidosis due to its low sensitivity. Antibody detection has low sensitivity and specificity due to the high seropositivity among healthy people in endemic regions. Antigen detection using various proteins has been tested for the rapid determination of B. pseudomallei; however, it presents certain limitations in terms of its sensitivity and specificity. Therefore, this review aims to frame the present knowledge of a potential target known as the Burkholderia invasion protein D (BipD), including future directions for its detection using an aptamer-based sensor (aptasensor).

Aptamers and Aptamer-Coupled Biosensors to Detect Water-Borne Pathogens

Aptamers can serve as efficient bioreceptors for the development of biosensing detection platforms. Aptamers are short DNA or RNA oligonucleotides that fold into specific structures, which enable them to selectively bind to target analytes. The method used to identify aptamers is Systematic Evolution of Ligands through Exponential Enrichment (SELEX). Target properties can have an impact on aptamer efficiencies. Therefore, characteristics of water-borne microbial targets must be carefully considered during SELEX for optimal aptamer development. Several aptamers have been described for key water-borne pathogens. Here, we provide an exhaustive overview of these aptamers and discuss important microbial aspects to consider when developing such aptamers.

Enlarging the Toolbox Against Antimicrobial Resistance: Aptamers and CRISPR-Cas

In the post-genomic era, molecular treatments and diagnostics have been envisioned as powerful techniques to tackle the antimicrobial resistance (AMR) crisis. Among the molecular approaches, aptamers and CRISPR-Cas have gained support due to their practicality, sensibility, and flexibility to interact with a variety of extra- and intracellular targets. Those characteristics enabled the development of quick and onsite diagnostic tools as well as alternative treatments for pan-resistant bacterial infections. Even with such potential, more studies are necessary to pave the way for their successful use against AMR. In this review, we highlight those two robust techniques and encourage researchers to refine them toward AMR. Also, we describe how aptamers and CRISPR-Cas can work together with the current diagnostic and treatment toolbox.

Selection and applications of functional nucleic acids for infectious disease detection and prevention

Infectious diseases caused by pathogenic microorganisms such as viruses and bacteria pose a great threat to human health. Although a significant progress has been obtained in the diagnosis and prevention of infectious diseases, it still remains challenging to develop rapid and cost-effective detection approaches and overcome the side effects of therapeutic agents and pathogen resistance. Functional nucleic acids (FNAs), especially the most widely used aptamers and DNAzymes, hold the advantages of high stability and flexible design, which make them ideal molecular recognition tools for bacteria and viruses, as well as potential therapeutic drugs for infectious diseases. This review summarizes important advances in the selection and detection of bacterial- and virus-associated FNAs, along with their potential prevention ability of infectious disease in recent years. Finally, the challenges and future development directions are concluded.

Binding Characteristics Study of DNA based Aptamers for E. coli O157:H7

E. coli O157:H7 is a pathogenic bacterium producing verotoxins that could lead to serious complications such as hemolytic uremia syndrome. Fast detection of such pathogens is important. For rapid detection, aptamers are quickly gaining traction as alternative biorecognition molecules besides conventional antibodies. Several DNA aptamers have been selected for E. coli O157:H7. Nonetheless, there has not been a comparative study of the binding characteristics of these aptamers. In this work, we present a comprehensive analysis of binding characteristics including binding affinity (K_d) and binding capacity (B_max) of DNA-based aptamers for E. coli O157:H7 using qPCR. Our results show that aptamer E18R has the highest binding capacity to E. coli 157:H7 and the highest specificity over non-pathogenic E. coli strains K12 and DH5α. Our study also finds that the common biotin-tag modification at 5' end typically changes the binding capacity significantly. For most of the selected aptamers, the binding capacity after a biotin-tag modification decreases. There exists a discrepancy in the binding capability between the selected aptamer and the aptamer used for detection. Our study also shows that a lower concentration of Mg²⁺ ions in the binding buffer leads to a decrease in the binding capacity of E17F and E18R, while it does not affect the binding capacity of S1 and EcoR1.

Prospective Application of Aptamer-based Assays and Therapeutics in Bloodstream Infections

Sepsis is still a severe health problem worldwide with high morbidity and mortality. Blood bacterial culture remains the gold standard for the detection of pathogenic bacteria in bloodstream infections, but it is time-consuming, and both the sophisticated equipment and well-trained personnel are required. Immunoassays and genetic diagnosis are expensive and limited to specificity and sensitivity. Aptamers are single-stranded deoxyribonucleic acid (ssDNA) and ribonucleic acid (RNA) oligonucleotide or peptide sequence generated in vitro based on the binding affinity of aptamer-target by a process known as Systematic Evolution of Ligands by Exponential Enrichment (SELEX). By taking several advantages over monoclonal antibodies and other conventional small-molecule therapeutics, such as high specificity and affinity, negligible batch-to-batch variation, flexible modification and production, thermal stability, low immunogenicity and lack of toxicity, aptamers are presently becoming promising novel diagnostic and therapeutic agents. This review describes the prospective application of aptamerbased laboratory diagnostic assays and therapeutics for pathogenic bacteria and toxins in bloodstream infections.

Novel Detection of Nasty Bugs, Prevention Is Better than Cure

Hospital-acquired infections (HAIs) are a growing concern around the world. They contribute to increasing mortality and morbidity rates and are an economic threat. All hospital patients have the potential to contract an HAI, but those with weakened or inferior immune systems are at highest risk. Most hospital patients will contract at least one HAI, but many will contract multiple ones. Bacteria are the most common cause of HAIs and contribute to 80-90% of all HAIs, with Staphylococcus aureus, Clostridium difficile, Escherichia coli, Acinetobacter baumannii, Pseudomonas aeruginosa and Klebsiella pneumoniae accounting for the majority. Each of these bacteria are highly resistant to antibiotics and can produce a protective film, known as a biofilm, to further prevent their eradication. It has been shown that by detecting and eradicating bacteria in the environment, infection rates can be reduced. The current methods for detecting bacteria are time consuming, non-specific, and prone to false negatives or false positives. Aptamer-based biosensors have demonstrated specific, time-efficient and simple detection, highlighting the likelihood that they could be used in a similar way to detect HAI-causing bacteria.

Inhibition of enteropathogenic Escherichia coli biofilm formation by DNA aptamer

Enteropathogenic Escherichia coli (EPEC) is a bioagent that causes diarrhea through the formation of biofilm. The recalcitrant of EPEC to the current conventional antibiotic treatment has grown a big concern in a way to find effective alternative inhibitors. Aptamers have been demonstrated to show the ability to kill the pathogenic bacteria through inhibition of biofilm formation. Therefore, this study aimed to investigate antibiofilm activities of six types of aptamers against EPEC K1.1 which was isolated from patients with diarrhea. Environmental conditions such as temperatures and pH which impacted on biofilm formation of EPEC K1.1 and also biofilm inhibition of aptamer on EPEC K1.1 were performed by counting the crystal violet formation in 96-well polystyrene microplates at OD₅₇₀. The motility examination combined with qPCR were applied to prove the mechanism of aptamers inhibition on biofilm by targeting essential genes that involve biofilm formation. The result showed that by applying cut off value at 0.399, aptamer SELEX 10 Colony 5 exhibited the highest biofilm inhibition against EPEC K1.1 with an absorbance value of 0.126. Further analysis showed that this aptamer also was able to reduce the motility diameter of EPEC K1.1. The effect of this aptamer on EPEC K1.1 motility was confirmed by qPCR where the mRNA level of motB, csgA and lsrA gene reduced significantly compared to the untreated group. Aptamer SELEX 10 Colony 5 was able to inhibit biofilm formation through interfering the motility ability of EPEC K1.1 and also by reducing the mRNA level of biofilm formation-related genes. This study provides evidences that aptamer is effective and promising for both antibiofilm of EPEC K1.1 and alternative treatment of diarrhea.

Recent advances on functional nucleic acid-based biosensors for detection of food contaminants

It has seen increasing development of reliable, robust, and flexible biosensors for rapid food-safety analysis in the past few decades. Recently, functional nucleic acid-based biosensors have attracted attention because of their programmability, bottom-up characteristics, and structural switches. However, few systematic reviews devoted to categorizing the potential of DNA nanostructures and devices were found for detecting food contaminants. Hence, the applications of functional nucleic acid-based biosensors were reviewed for analyzing food contaminants, including foodborne pathogen bacteria, biotoxins, heavy metals, and et al. In addition to categorizing the various biosensors, multiple signal readout strategies, such as optical, electrochemical, and mass-based signals were also examined. Finally, the future changes and potential opportunities, as well as practical applications of functional nucleic acid-based biosensors were discussed.

Recent Progress in the Identification of Aptamers Against Bacterial Origins and Their Diagnostic Applications

Aptamers have gained an increasing role as the molecular recognition element (MRE) in diagnostic assay development, since their first conception thirty years ago. The process to screen for nucleic acid-based binding elements (aptamers) was first described in 1990 by the Gold Laboratory. In the last three decades, many aptamers have been identified for a wide array of targets. In particular, the number of reports on investigating single-stranded DNA (ssDNA) aptamer applications in biosensing and diagnostic platforms have increased significantly in recent years. This review article summarizes the recent (2015 to 2020) progress of ssDNA aptamer research on bacteria, proteins, and lipids of bacterial origins that have implications for human infections. The basic process of aptamer selection, the principles of aptamer-based biosensors, and future perspectives will also be discussed.

An overview and future prospects on aptamers for food safety

Introduction

Many bacteria are responsible for infections in humans and plants, being found in vegetables, water, and medical devices. Most bacterial detection methods are time-consuming and take days to give the result. Aptamers are a promising alternative for a quick and reliable measurement technique to detect bacteria present in food products. Selected aptamers are DNA or RNA oligonucleotides that can bind with bacteria or other molecules with affinity and specificity for the target cells by the SELEX or cell-SELEX technique. This method is based on some rounds to remove the non-ligand oligonucleotides, leaving the aptamers specific to bind to the selected bacteria. Compared with conventional methodologies, the detection approach using aptamers is a rapid, low-cost form of analysis.

Objective

This review summarizes obtention methods and applications of aptamers in the food industry and biotechnology. Besides, different techniques with aptamers are presented, which enable more effective target detection.

Conclusion

Applications of aptamers as biosensors, or the association of aptamers with nanomaterials, may be employed in analyses by colorimetric, fluorescence, or electrical devices. Additionally, more efficient ways of sample preparation are presented, which can support food safety to provide human health, with a low-cost method for contaminant detection.

Key points • Aptamers are promising for detecting contaminants outbreaks. • Studies are needed to identify aptamers for different targets.

Research advances of DNA aptasensors for foodborne pathogen detection

Aptamers, referring to single-stranded DNA or RNA molecules can specifically recognize and bind to their targets. Based on their excellent specificity, sensitivity, high affinity, and simplicity of modification, aptamers offer great potential for pathogen detection and biomolecular screening. This article reviews aptamer screening technologies and aptamer application technologies, including gold-nanoparticle lateral flow assays, fluorescence assays, electrochemical assays, colorimetric assays, and surface-enhanced Raman assays, in the detection of foodborne pathogens. Although notable progress (more rapid, sensitive, and accurate) has been achieved in the field, challenges and drawbacks in their applications still remain to be overcome.

Recognition of Helicobacter pylori by protein-targeting aptamers

BACKGROUND

Helicobacter pylori (H pylori) is a disease-causing pathogen capable of surviving under acidic conditions of the human stomach. Almost half of the world's population is infected with H pylori, with gastric cancer being the most unsatisfactory prognosis. Although H pylori has been discovered 30 years ago, the effective treatment and elimination of H pylori continue to be problematic.

MATERIALS AND METHODS

In our study, we screened nucleic acid aptamers using H pylori surface recombinant antigens as targets. Trypsin was used for separating aptamers that were bound to proteins. Following nine rounds of screening, we performed sequence similarity analyses to assess whether the aptamers can recognize the target protein. Two sequences with desirable recognition ability were selected for affinity detection. Aptamer Hp4 with the strongest binding ability to the H pylori surface recombinant antigen was chosen. After optimization of the binding conditions, we conducted specificity tests for Hp4 using Escherichia coli, Staphylococcus aureus, Vibrioanguillarum, and H pylori.

RESULTS

The data indicated that the aptamer Hp4 had an equilibrium dissociation constant (K_d ) of 26.48 ± 5.72 nmol/L to the target protein. This aptamer was capable of exclusively detecting H pylori cells, without displaying any specificity for other bacteria.

CONCLUSIONS

We obtained a high-affinity aptamer for H pylori, which is expected to serve as a new molecular probe for detection of H pylori.

CD30 aptamer-functionalized PEG-PLGA nanoparticles for the superior delivery of doxorubicin to anaplastic large cell lymphoma cells

Nanoparticles (NPs) conjugated with aptamers have been extensively in recent years, which can efficiently target cancer cells that improve the therapeutic effect. Aptamers (Apt) are small oligonucleotide molecule ligands have specific high-affinity. In this work, we developed a PEG-PLGA nanoparticles (NPs) encapsulated with doxorubicin. The NPs were modified with C2NP, a ssDNA aptamer specifically binding to CD30 protein which was over expressed in anaplastic large cell lymphoma (ALCL) cells. PEG-PLGA nanoparticles (NPs) were formed by nanoprecipitation and loaded with doxorubicin, further conjugated C2NP aptamer via an EDC/NHS technique. Obtained results demonstrated that the targeted agent was successfully conjugated confirming by Urea PAGE and XPS. The physicochemical properties of Apt-DOX-NPs like particle size at 168.07 ± 2.72 nm and zeta potential at -30.76 ± 0.153 mV. The time of the release drugs was efficiently increased in targeted formulations and showed higher accumulation in ALCL cells than non-targeted system. Findings from this work demonstrated the potential efficacy of C2NP-functionalized nanoparticles for a therapy in ALCL.

Investigations on the interface of nucleic acid aptamers and binding targets

Nucleic acid aptamers are single-stranded DNA or RNA of 20-100 nucleotides in length that have attracted substantial scientific interest due to their ability to specifically bind to target molecules via the formation of three-dimensional structures. Compared to traditional protein antibodies, aptamers have several advantages, such as their small size, high binding affinity, specificity, flexible structure, being chemical synthesizable and modifiable, good biocompatibility, high stability and low immunogenicity, which all contribute to their widely applications in the biomedical field. To date, much progress has been made in the study and applications of aptamers, however, detailed information on how aptamers bind to their targets is still scarce. Over the past few decades, many methods have been introduced to investigate the aptamer-target binding process, such as measuring the main kinetic or thermodynamic parameters, detecting the structural changes of the binding complexes, etc. Apart from traditional physicochemical methods, various types of molecular docking programs have been applied to simulate the aptamer-target interactions, while these simulations also have limitations. To facilitate the further research on the interactions, herein, we provide a brief review to illustrate the recent advances in the study of aptamer-target interactions. We summarize the binding targets of aptamers, such as small molecules, macromolecules, and even cells. Their binding constants (KD) are also summarized. Methods to probe the aptamer-target binding process, such as surface plasmon resonance (SPR), circular dichroism spectroscopy (CD), isothermal titration calorimetry (ITC), footprinting assay, truncation and mutation assay, nuclear magnetic resonance spectroscopy (NMR), X-ray crystallography and molecular docking simulation are indicated. The binding forces mediating the aptamer-target interactions, such as hydrogen bonding, electrostatic interaction, the hydrophobic effect, π-π stacking and van der Waals forces are summarized. The challenges and future perspectives are also discussed.

Application of Aptamer-based Hybrid Molecules in Early Diagnosis and Treatment of Diabetes Mellitus: From the Concepts Towards the Future

Aptamers have several positive advantages that made them eminent as a potential factor in diagnosing and treating diseases such as their application in prevention and treatment of diabetes. In this opinion-based mini-review article, we aimed to investigate the DNA and RNA-based hybrid molecules specifically aptamers and had a logical conclusion as a promising future perspective in early diagnosis and treatment of diabetes.

Development of duplex PCR-ELISA for simultaneous detection of Salmonella spp. and Escherichia coli O157: H7 in food

In the current study, a duplex PCR-ELISA method was developed targeting the specific genes, invA of Salmonella spp. and rfbE of Escherichia coli O157: H7, to detect one or both bacteria in food. In brief, PCR product amplified by PCR primer labeled with digoxin at the 5'-end and a probe labeled with biotin at the 3'-end can form dimer by nucleic acid hybridization which can be captured by binding of biotin to streptomycin coated in ELISA plate before using enzyme-labeled anti-digoxin antibody and substrate to develop color. Also, evaluation of the duplex PCR-ELISA method was conducted in different food samples including milk, juice, cabbage, shrimp, chicken, pork and beef. Results indicated that the duplex PCR-ELISA developed here was specific when using 25 non-target bacteria strains as controls and was sensitive with a limit of detection (LOD) of 1 CFU/mL, 1, 000 times higher than that of the duplex PCR method and was repeatable regardless of inter- and intra-batch variations. The duplex PCR-ELISA method established in the present study has proven to be highly specific, sensitive and repeatable. It has the potential to be applied in such fields as clinical diagnosis of food-borne diseases, food hygiene monitoring and pathogen detection in food.

Application of Aptamer-Based Biosensor for Rapid Detection of Pathogenic Escherichia coli

Pathogenic Escherichia coli (E. coli) widely exist in Nature and have always been a serious threat to the human health. Conventional colony forming units counting-based methods are quite time consuming and not fit for rapid detection for E. coli. Therefore, novel strategies for improving detection efficiency and sensitivity are in great demand. Aptamers have been widely used in various sensors due to their extremely high affinity and specificity. Successful applications of aptamers have been found in the rapid detection of pathogenic E. coli. Herein, we present the latest advances in screening of aptamers for E. coli, and review the preparation and application of aptamer-based biosensors in rapid detection of E. coli. Furthermore, the problems and new trends in these aptamer-based biosensors for rapid detection of pathogenic microorganism are also discussed.

Selection, Identification, and Binding Mechanism Studies of an ssDNA Aptamer Targeted to Different Stages of E. coli O157:H7

Enterohemorrhagic Escherichia coli O157:H7 ( E. coli O157:H7) is known as an important food-borne pathogen related to public health. In this study, aptamers which could bind to different stages of E. coli O157:H7 (adjustment phase, log phase, and stationary phase) with high affinity and specificity were obtained by the whole cell-SELEX method through 14 selection rounds including three counter-selection rounds. Altogether, 32 sequences were obtained, and nine families were classified to select the optimal aptamer. To analyze affinity and specificity by flow cytometer, an ssDNA aptamer named Apt-5 was picked out as the optimal aptamer that recognizes different stages of E. coli O157:H7 specifically with the K_d value of 9.04 ± 2.80 nM. In addition, in order to study the binding mechanism, target bacteria were treated by proteinase K and trypsin, indicating that the specific binding site is not protein on the cell membrane. Furthermore, when we treated E. coli O157:H7 with EDTA, the result showed that the binding site might be lipopolysaccharide (LPS) on the outer membrane of E. coli O157:H7.

Oligonucleotide aptamers: promising and powerful diagnostic and therapeutic tools for infectious diseases

The entire human population is at risk of infectious diseases worldwide. Thus far, the diagnosis and treatment of human infectious diseases at the molecular and nanoscale levels have been extremely challenging tasks because of the lack of effective probes to identify and recognize biomarkers of pathogens. Oligonucleotide aptamers are a class of small nucleic acid ligands that are composed of single-stranded DNA (ssDNA) or RNA and act as affinity probes or molecular recognition elements for a variety of targets. These aptamers have an exciting potential for diagnose and/or treatment of specific diseases. In this review, we highlight areas where aptamers have been developed as diagnostic and therapeutic agents for both bacterial and viral infectious diseases as well as aptamer-based detection.

Selection and characterization, application of a DNA aptamer targeted to Streptococcus pyogenes in cooked chicken

An aptamer against Streptococcus pyogenes was selected and identified, and a fluorescent method based on the reported aptamer was established to detect S. pyogenes in the cooked chicken. Through a twelve rounds of whole-bacterium SELEX (systematic evolution of ligands by exponential enrichment) selection in vitro, a set of aptamers binding to the whole cell of S. pyogenes were generated, harvesting a low-level dissociation constant (K_d) value of 44 ± 5 nmol L^-1 of aptamer S-12. Aptamer-based quantification of S. pyogenes in the cooked chicken sample was implemented in a fluorescence resonance energy transfer-based assay by using graphene oxide, resulting in a limit of detection of 70 cfu mL^-1. The selected aptamer showed affinity and selectivity recognizing S. pyogenes; besides, more biosensors based on the selected aptamer as a molecular recognition element could be developed in the innovative determinations of S. pyogenes.

A fluorescent dual aptasensor for the rapid and sensitive onsite detection ofE. coliO157:H7 and its validation in various food matrices

The speedy analysis of food products remains a keen area of concern; thus, rapid, highly efficient and robust on-site detection platforms are essential.