Selection of aptamers against inactive Vibrio alginolyticus and application in a qualitative detection assay

Aptamers: A Cutting-Edge Approach for Gram-Negative Bacterial Pathogen Identification

Early and accurate diagnoses of pathogenic microorganisms is essential to correctly identify diseases, treating infections, and tracking disease outbreaks associated with microbial infections, to develop precautionary measures that allow a fast and effective response in epidemics and pandemics, thus improving public health. Aptamers are a class of synthetic nucleic acid molecules with the potential to be used for medical purposes, since they can be directed towards any target molecule. Currently, the use of aptamers has increased because they are a useful tool in the detection of specific targets. We present a brief review of the use of aptamers to detect and identify bacteria or even some toxins with clinical importance. This work describes the advances in the technology of aptamers, with the purpose of providing knowledge to develop new aptamers for diagnoses and treatment of different diseases caused by infectious microorganisms.

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.

DNA aptamers specific for Legionella pneumophila: systematic evolution of ligands by exponential enrichment in whole bacterial cells

Legionella pneumophila is the major causative agent of Legionnaires' disease and Pontiac fever, which pose major public health problems. Rapid detection of L. pneumophila is important for global control of these diseases. Aptamers, short oligonucleotides that bind to targets with high affinity and specificity, have great potential for use in pathogenic bacterium detection, diagnostics, and therapy. Here, we used a whole-cell SELEX (systematic evolution of ligands by exponential enrichment) method to isolate and characterize single-stranded DNA (ssDNA) aptamers against L. pneumophila. A total of 60 ssDNA sequences were identified after 17 rounds of selection. Other bacterial species (Escherichia coli, Bacillus subtilis, Pseudomonas syringae, Staphylococcus aureus, Legionella quateirensis, and Legionella adelaidensis) were used for counterselection to enhance the specificity of ssDNA aptamers against L. pneumophila. Four ssDNA aptamers showed strong affinity and high selectivity for L. pneumophila, with K_d values in the nanomolar range. Bioinformatic analysis of the most specific aptamers revealed predicted conserved secondary structures that might bind to L. pneumophila cell walls. In addition, the binding of these four fluorescently labeled aptamers to the surface of L. pneumophila was observed directly by fluorescence microscopy. These aptamers identified in this study could be used in the future to develop medical diagnostic tools and public environmental detection assays for L. pneumophila.

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.

Generation and characterization of aptamers against grass carp reovirus infection for the development of rapid detection assay

Grass carp reovirus (GCRV) causes devastating viral haemorrhagic disease in farmed grass carp (Ctenopharyngon idellus). As novel molecular probes, aptamers have been widely applied in rapid diagnosis and efficient therapies against virus or diseases. In this study, three single-stranded DNA (ssDNA) aptamers were selected against GCRV-infected CIK cells via SELEX (systematic evolution of ligands by exponential enrichment technology). Secondary structures predicted by MFOLD indicated that aptamers formed stem-loop structures, and GVI-11 had the lowest ΔG value of -30.84 KJ/mol. Three aptamers could specifically recognize GCRV-infected CIK cells, with calculated dissociation constants (Kd) of 220.86, 176.63 and 278.66 nM for aptamers GVI-1, GVI-7 and GVI-11, respectively, which indicated that they could serve as specific delivery system for antiviral therapies. The targets of aptamers GVI-1, GVI-7 and GVI-11 on the surface of GCRV-infected cells could be membrane proteins, which were trypsin-sensitive. Furthermore, FAM-labelled aptamer GVI-7 could be applied to detect GCRV infection in vivo. It is the first time to generate and characterize aptamers against GCRV-infected cells. These aptamers have great potentials in development of rapid diagnosis technology and antiviral agents against GCRV infection in aquaculture.

Isolation ssDNA aptamers specific for both live and viable but nonculturable stateVibrio vulnificususing whole bacteria-SEILEX technology

Vibrio vulnificus is a ubiquitous marine bacterium that may cause rapid and deadly infection, threatening lives of people living around natural bodies of water, especially in coastal regions. However, traditional culture-based methods are time-consuming and unable to detect Viable But Non-Culturable (VBNC) V. vulnificus cells. In this work, we isolated a batch of detection aptamers specifically binding to V. vulnificus in all culture status. With traditional whole bacteria-SELEX (Systematic Evolution of Ligands by EXponential enrichment), flow cytometer analysis and imaging, we identify 18 candidates and validated two of them (V8 and V13) as applicable aptamers. Their truncated sequences also showed comparable performance. The dissociation constant (KD) value of V8 is shown to be as low as 11.22 ± 1.32 nM. Optimal aptamers V8 and V13 are also validated to be effective to detect different Vibrio vulnificus strains under different binding environments using flow cytometry. As for detection parameters, the LOD of the V8 from cytometry is 29.96 CFU mL⁻¹, and the linear range is 10²–5 × 10⁵ CFU mL⁻¹. This is the first case demonstrating that aptamers can detect the existence of VBNC bacteria as well as live bacteria.

With whole-bacteria SELEX, we got aptamers that can bind to V. vulnificus in VBNC Status for the first time.

Development of novel aptamer-based enzyme-linked apta-sorbent assay (ELASA) for rapid detection of mariculture pathogen Vibrio alginolyticus

As the major opportunistic pathogen to both marine animals and humans, Vibrio alginolyticus (V. alginolyticus) has caused heavy economic losses to mariculture. ssDNA aptamer VA2 targeting live V. alginolyticus was generated by systematic evolution of ligands by exponential enrichment (SELEX) technology in our previous study. In this study, we first developed aptamer (VA2)-based enzyme-linked apta-sorbent assay (VA2-ELASA) for rapid detection of mariculture pathogen V. alginolyticus. The VA2-ELASA could achieve the rapid detection for V. alginolyticus infection with high specificity and sensitivity. The VA2-ELASA could specifically identify V. alginolyticus, but not other non-target bacterial strains. VA2-ELASA could detect V. alginolyticus at the concentration of 5 × 10⁴ /ml, the incubation time short to 1 min and the incubation temperature as high as 45°C, which proved sensitivity and stability of the novel VA2-ELASA in this study. It took less than one hour to accomplish the detection process by VA2-ELASA. The characteristics of specificity, sensitivity and easy operation make VA2-ELASA a novel useful technology for the rapid diagnosis of pathogen V. alginolyticus in mariculture.

Selection and characterization of ssDNA aptamers specifically recognizing pathogenic Vibrio alginolyticus

Vibrio alginolyticus (V. alginolyticus) is a major opportunistic pathogen to both marine animals and humans, which has also caused heavy economic losses to mariculture. The aim of this study was to develop highly specific aptamers for V. alginolyticus. Single-stranded DNA (ssDNA) aptamers with high binding affinity to viable V. alginolyticus were generated by Systematic Evolution of Ligands by Exponential Enrichment (SELEX) and identified by flow cytometric analysis in this study. The selected aptamers showed high specificity for V. alginolyticus and low apparent binding for other bacteria. The aptamers formed distinct stem-loop structures, which could form the basis of aptamers' specific binding to the target V. alginolyticus. Aptamer VA2 and VA8 showed particularly high binding affinity constant (Kd) of 14.31 ± 4.26 and 90.00 ± 13.51 nM, respectively. The aptamers produced no cytotoxic effects in vitro and in vivo. ssDNA aptamers were successfully selected against the viable bacteria pathogen V. alginolyticus by SELEX. The aptamers selected in this study could be not only applied as specific chemical molecular probes for studying V. alginolyticus pathogenesis to Trachinotus ovatus, but also developing rapid convenient diagnosis assay for V. alginolyticus infection, even when applied to the complex sample matrix, such as food and environment samples.

Aptamer-assisted novel technologies for detecting bacterial pathogens

Nowadays, all people are at risk of infectious diseases that are mainly caused by bacteria causing infection. There is a permanent demand for an appropriate detection method that is affordable, practical, careful, rapid, sensitive, efficient and economical. Aptamers are single stranded DNA or RNA oligonucleotides, which can be recognized specifically and bind to their target molecules and also, be exploited in diagnostic applications. Recently, aptamer-based systems have offered great potentials in applications for the recognition of several important bacterial pathogens from clinical and food specimens. There are several reports appraising the diagnostic applicability of aptamer-based systems for the detection of pathogens. As for its excellent sensitivity, as well as its rapid and efficient detectability, this technique may be practical to indicate bacterial targets with less sample size and may consume less time than traditional methods These systems offer a promising approach for the sensitive and quick detection of food-borne and clinical agents. This review provides an overview of aptamer-based methods as a novel approach for detecting bacterial pathogens.

Aptamer-nanobody based ELASA for specific detection of Acinetobacter baumannii isolates

Acinetobacter baumannii has turned into an important threat in nosocomial outbreak infections and multidrug resistance leading to high mortality rates in the 21st century. In recent years its mortality has increased by 15% which in part could be due to lack of a rapid and sensitive diagnostic test. In this work we introduced a new detection test for A. baumannii with two highly specific aptamer and nanobody molecules. High binding affinity DNA oligonucleotide aptamers toward A. baumannii were selected through 12 rounds of whole cell System Evolution of Ligands by EXponential enrichment process (SELEX). The SELEX procedures was monitored by flow cytometry. The dissociation constant and binding efficiency of the selected aptamer Aci49 was 7.547±1:353pM and 47.50%, respectively. A sandwich enzyme linked aptamer sorbent assay (ELASA) was designed with the biotinylated Aci49 aptamer and our previously developed nanobody against biofilm associated protein (Bap). The assay system was optimized with A. baumannii (ATCC 19606) and 47 clinical isolates of A. baumannii were tested. The threshold of detection in sandwich ELASA process was10(3) CFU/ml. The sensitivity of test toward the clinical isolates was 95.47%. Our results reveal that the sandwich ELASA is sensitive and specific enough for the rapid detection of A. baumannii from clinical isolates.

Isolation of an Aptamer that Binds Specifically to E. coli

Escherichia coli is a bacterial species found ubiquitously in the intestinal flora of animals, although pathogenic variants cause major public health problems. Aptamers are short oligonucleotides that bind to targets with high affinity and specificity, and have great potential for use in diagnostics and therapy. We used cell-based Systematic Evolution of Ligands by EXponential enrichment (cell-SELEX) to isolate four single stranded DNA (ssDNA) aptamers that bind strongly to E. coli cells (ATCC generic strain 25922), with Kd values in the nanomolar range. Fluorescently labeled aptamers label the surface of E. coli cells, as viewed by fluorescent microscopy. Specificity tests with twelve different bacterial species showed that one of the aptamers–called P12-31—is highly specific for E. coli. Importantly, this aptamer binds to Meningitis/sepsis associated E. coli (MNEC) clinical isolates, and is the first aptamer described with potential for use in the diagnosis of MNEC-borne pathologies.

Single-Stranded DNA Aptamers against Pathogens and Toxins: Identification and Biosensing Applications

Molecular recognition elements (MREs) can be short sequences of single-stranded DNA, RNA, small peptides, or antibody fragments. They can bind to user-defined targets with high affinity and specificity. There has been an increasing interest in the identification and application of nucleic acid molecular recognition elements, commonly known as aptamers, since they were first described in 1990 by the Gold and Szostak laboratories. A large number of target specific nucleic acids MREs and their applications are currently in the literature. This review first describes the general methodologies used in identifying single-stranded DNA (ssDNA) aptamers. It then summarizes advancements in the identification and biosensing application of ssDNA aptamers specific for bacteria, viruses, their associated molecules, and selected chemical toxins. Lastly, an overview of the basic principles of ssDNA aptamer-based biosensors is discussed.

Aptasensor and genosensor methods for detection of microbes in real world samples

The increasing concerns about food and environmental safety have prompted the desire to develop rapid, specific, robust and highly sensitive methods for the detection of microorganisms to ensure public health. Although traditional microbiological methods are available, they are labor intensive, unsuitable for on-site and high throughput analysis, and need well-trained personnel. To circumvent these drawbacks, many efforts have been devoted towards the development of biosensors, using nucleic acid as bio-recognition element. In this review, we will focus on recent significant advances made in two types of DNA-based biosensors, namely genosensors, and aptasensors. In genosensor approach, DNA or RNA target is detected through the hybridization reaction between DNA or RNA and ssDNA sensing element, while in aptasensor method, DNA or RNA aptamer, capable of binding to a target molecule with high affinity and specificity, plays the role of receptor. The goal of this article is to review the innovative methods that have been emerged in genosensor and aptasensor during recent years. Particular attention is given to recent advances and trends in selection of biorecognition element, DNA immobilization strategies and sensing formats.