Investigation and validation of labelling loop mediated isothermal amplification (LAMP) products with different nucleotide modifications for various downstream analysis

Both aerosol and primer dimer breakdown for straightforward genotyping based on an integrated immunochromatographic biosensor

Straightforward genotyping can provide timely diagnostic information for diseases prevention and treatment. Taking advantages of speediness and convenience, although numerous genotyping strategies combined loop-mediated isothermal amplification (LAMP) and lateral flow have been reported to satisfy the demand of point-of-care test, the false positive result caused by aerosol and primer dimer as an innate conflict seriously limits their practical application. In this study, both aerosol and primer dimer as extrinsic and intrinsic inducements respectively are first broken through at one stroke based on an integrated immunochromatographic biosensor. By introducing digoxigenin labeled dUTP into LAMP, not only the amplicon can be analyzed through naked eye, but also the aerosol contamination can be eliminated thoroughly by uracil DNA glycosylase ignoring the open vessel. Primer dimer, the significant drawback in lateral flow-based strategies, has been overcome due to the bio-labeled deoxyribonucleotide and oligonucleotide cannot couple for signal generation even under the high primer concentration. Instead of colloidal gold, the gold magnetic nanoparticle is synthesized and assembled into this biosensor as a nanoprobe, which enables the result to be quantified by the magnetic signal for subjective bias elimination. The polymorphism of C677T in methylenetetrahydrofolate reductase, a crucial genetic code related to folate metabolism, is genotyped using saliva as noninvasive specimen dispense with DNA purification. Only 1 ng genomic DNA can provide accurate result within 25 min by a simple heater, which proves the potential of this biosensor to facilitate precision medicine.

Evaluation of amplicons by AF4 as assistant for deep comprehension of loop-mediated isothermal amplification combined with lateral flow assay

Loop-mediated isothermal amplification (LAMP) is a rapid and efficient method for DNA amplification, producing concatemers of varying lengths (amplicons). This study explores the characterization of LAMP amplicons using asymmetric flow field-flow fractionation (AF4) and their realization in LAMP - lateral flow assay (LFA) for point-of-care diagnostics. We examined LAMP products from the invA gene of Salmonella enterica using two specific primer sets and three methods: fluorescent staining with SYBR Green, electrophoretic detection, and AF4. LAMP products labeled with biotin and fluorescein (introduced through primers, 12 combinations) were detected by LFA due to binding the labels. The efficiency of amplicon detection by test strips varied depending on the primer combination: the color intensities for the best and worst combinations differed by more than 7 times. AF4 enabled precise detection of components from primers to amplicons up to 500 bp and revealed differences influenced by labeling and primer combinations. An important advantage of AF4, which was demonstrated for the first time, was the characterization of native complexes of amplicons with receptors. The analysis of amplicons in complexes with streptavidin, anti-fluorescein antibodies, or both receptors together using AF4 revealed significant differences in the fractograms. The shape and maxima of the absorption peaks at 260 nm depended on retention time in the range from 20 to 35 min, indicating the formation of complexes of different compositions. AF4 and LFA showed that smaller amplicons (200-300 bp) primarily contribute to the binding of detected labels, while longer amplicons (1000-1500 bp) do not enhance the signal due to structural complexities. Our findings demonstrate the potential of AF4 as a tool for deep comprehension of LAMP processes and the composition of the formed amplicon-receptor complexes, offering valuable insights for enhancing LAMP-LFA systems. This work provides a foundation for the advanced application of AF4 for nucleic acid amplification technologies.

A DAMP-Based Assay for Rapid and Affordable Diagnosis of Bacterial Meningitis Agents: Haemophilus influenzae, Neisseria meningitidis, and Streptococcus pneumoniae

The rapid and accurate diagnosis of meningitis is critical for preventing severe complications and fatalities. This study addresses the need for accessible diagnostics in the absence of specialized equipment by developing a novel diagnostic assay. The assay utilizes dual-priming isothermal amplification (DAMP) with unique internal primers to significantly reduce non-specificity. For fluorescence detection, the dye was selected among Brilliant Green, Thioflavin T, and dsGreen. Brilliant Green is preferred for this assay due to its availability, high fluorescence level, and optimal sample-to-background (S/B) ratio. The assay was developed for the detection of the primary causative agents of meningitis (Haemophilus influenzae, Neisseria meningitidis, and Streptococcus pneumoniae), and tested on clinical samples. The developed method demonstrated high specificity, no false positives, sensitivity comparable to that of loop-mediated isothermal amplification (LAMP), and a high S/B ratio. This versatile assay can be utilized as a standalone test or an integrated assay into point-of-care systems for rapid and reliable pathogen detection.

Direct TAMRA-dUTP labeling of M. tuberculosis genes using loop-mediated isothermal amplification (LAMP)

Fluorescent molecule-based direct labeling of amplified DNA is a sensitive method employed across diverse DNA detection and diagnostics systems. However, using pre-labeled primers only allows for the attachment of a single fluorophore to each DNA strand and any modifications of the system are less flexible, requiring new sets of primers. As an alternative, direct labeling of amplified products with modified nucleotides is available, but still poorly characterized. To address these limitations, we sought a direct and adaptable approach to label amplicons produced through Loop-mediated isothermal amplification (LAMP), using labeled nucleotides (dUTPs) rather than primers. The focus of this study was the development and examination of a direct labeling technique of specific genes, including those associated with drug resistance in Mycobacterium tuberculosis. We used 5-(3-Aminoallyl)-2'-deoxyuridine-5'triphosphate, tagged with 5/6-TAMRA (TAMRA-dUTP) for labeling LAMP amplicons during the amplification process and characterized amplification and incorporation efficiency. The optimal TAMRA-dUTP concentration was first determined based on amplification efficiency (0.5% to total dNTPs). Higher concentrations of modified nucleotides reduced or completely inhibited the amplification yield. Target size also showed to be determinant to the success of amplification, as longer sequences showed lower amplification rates, thus less TAMRA incorporated amplicons. Finally, we were able to successfully amplify all four M. tuberculosis target genes using LAMP and TAMRA-modified dUTPs.

Comparison of pre-labelled primers and nucleotides as DNA labelling method for lateral flow detection of Legionella pneumophila amplicons

Labelling of nucleic acid amplicons during polymerase chain reaction (PCR) or isothermal techniques is possible by using both labelled primers and labelled nucleotides. While the former is the widely used method, the latter can offer significant advantages in terms of signal enhancement and improving the detection limit of an assay. Advantages and disadvantages of both methods depend on different factors, including amplification method, detection method and amplicon length. In this study, both methods for labelling PCR products for lateral flow assay (LFA) analysis (LFA-PCR) were analysed and compared. It was shown that labelling by means of nucleotides results in an increase in label incorporation rates. Nonetheless, this advantage is negated by the need for post-processing and competitive interactions. In the end, it was possible to achieve a detection limit of 3 cell equivalents for the detection of the Legionella-DNA used here via primer labelling. Labelling via nucleotides required genomic DNA of at least 3000 cell equivalents as starting material as well as an increased personnel and experimental effort.

Malaria diagnosis using a combined system of a simple and fast extraction method with a lyophilised Dual-LAMP assay in a non-endemic setting

Malaria is a parasitic disease distributed in tropical areas but with a high number of imported cases in non-endemic countries. The most specific and sensitive malaria diagnostic methods are PCR and LAMP. However, both require specific equipment, extraction procedures and a cold chain. This study aims to solve some limitations of LAMP method with the optimization and validation of six LAMP assays, genus and species-specific, using an easy and fast extraction method, the incorporation of a reaction control assay, two ways (Dual) of result reading and reagent lyophilization. The Dual-LAMP assays were validated against the Nested-Multiplex Malaria PCR. A conventional column and saline extraction methods, and the use of lyophilized reaction tubes were also assessed. A new reaction control Dual-LAMP-RC assay was designed. Dual-LAMP-Pspp assay showed no cross-reactivity with other parasites, repeatability and reproducibility of 100%, a significant correlation between parasite concentration and time to amplification and a LoD of 1.22 parasites/µl and 5.82 parasites/µl using column and saline extraction methods, respectively. Sensitivity and specificity of the six Dual-LAMP assays reach values of 100% or close to this, being lower for the Dual-LAMP-Pm. The Dual-LAMP-RC assay worked as expected. Lyophilized Dual-LAMP results were concordant with the reference method. Dual-LAMP malaria assays with the addition of a new reaction control LAMP assay and the use of a fast and easy saline extraction method, provided low limit of detection, no cross-reactivity, and good sensitivity and specificity. Furthermore, the reagent lyophilization and the dual result reading allow their use in most settings.

Rapid detection of Mucorales in human blood and urine samples by functionalized Heusler magnetic nanoparticle assisted customized loop-mediated isothermal amplification

Mucormycosis is a rare disease with scarce diagnostic methods for early intervention. Available strategies employing direct microscopy using calcofluor white-KOH, culture, radiologic, and histopathologic testing often are time-intensive and demand intricate protocols. Nucleic Acid Amplification Test holds promise due to its high sensitivity combined with rapid detection. Loop-mediated isothermal amplification (LAMP) based detection offers an ultrasensitive technique that does not require complicated thermocyclers like in polymerase chain reaction, offering a straightforward means for improving diagnoses as a near-point-of-care test. The study introduces a novel magnetic nanoparticle-based LAMP assay for carryover contaminant capture to reduce false positives. Solving the main drawback of LAMP-based diagnosis techniques. The assay targets the cotH gene, which is invariably specific to Mucorales. The assay was tested with various species of Mucorales, and the limit of detections for Rhizopus microsporus, Lichtheimia corymbifera, Rhizopus arrhizus, Rhizopus homothallicus, and Cunninghamella bertholletiae were 1 fg, 1 fg, 0.1 pg, 0.1 pg, and 0.01 ng, respectively. This was followed by a clinical blindfolded study using whole blood and urine samples from 30 patients diagnosed with Mucormycosis. The assay has a high degree of repeatability and had an overall sensitivity of > 83%. Early Mucormycosis detection is crucial, as current lab tests from blood and urine lack sensitivity and take days for confirmation despite rapid progression and severe complications. Our developed technique enables the confirmation of Mucormycosis infection in < 45 min, focusing specifically on the RT-LAMP process. Consequently, this research offers a viable technique for quickly identifying Mucormycosis from isolated DNA of blood and urine samples instead of invasive tissue samples.

Detection of Reverse Transcriptase LAMP-Amplified Nucleic Acid from Oropharyngeal Viral Swab Samples Using Biotinylated DNA Probes through a Lateral Flow Assay

This study focuses on three key aspects: (a) crude throat swab samples in a viral transport medium (VTM) as templates for RT-LAMP reactions; (b) a biotinylated DNA probe with enhanced specificity for LFA readouts; and (c) a digital semi-quantification of LFA readouts. Throat swab samples from SARS-CoV-2 positive and negative patients were used in their crude (no cleaning or pre-treatment) forms for the RT-LAMP reaction. The samples were heat-inactivated but not treated for any kind of nucleic acid extraction or purification. The RT-LAMP (20 min processing time) product was read out by an LFA approach using two labels: FITC and biotin. FITC was enzymatically incorporated into the RT-LAMP amplicon with the LF-LAMP primer, and biotin was introduced using biotinylated DNA probes, specifically for the amplicon region after RT-LAMP amplification. This assay setup with biotinylated DNA probe-based LFA readouts of the RT-LAMP amplicon was 98.11% sensitive and 96.15% specific. The LFA result was further analysed by a smartphone-based IVD device, wherein the T-line intensity was recorded. The LFA T-line intensity was then correlated with the qRT-PCR Ct value of the positive swab samples. A digital semi-quantification of RT-LAMP-LFA was reported with a correlation coefficient of R2 = 0.702. The overall RT-LAMP-LFA assay time was recorded to be 35 min with a LoD of three RNA copies/µL (Ct-33). With these three advancements, the nucleic acid testing-point of care technique (NAT-POCT) is exemplified as a versatile biosensor platform with great potential and applicability for the detection of pathogens without the need for sample storage, transportation, or pre-processing.

Diverse methods of reducing and confirming false-positive results of loop-mediated isothermal amplification assays: A review

Loop-mediated isothermal amplification (LAMP), a rapid and sensitive isothermal nucleic acid amplification method, is a promising alternative to other molecular amplification techniques due to its superior specificity and sensitivity. However, due to primer dimerization, LAMP results in nonspecific and nontemplate amplification. And during the amplification confirmation process, there is carry-over contamination. These factors can result in false-positive results that overestimate the amount of DNA, preventing accurate detection. This review outlined several techniques for reducing false-positive LAMP results before amplification and confirming false-positive results after amplification. Before the amplification step, DNA polymerase activity can be decreased with organic additives such as dimethyl sulfoxide, betaine, and pullulan to prevent nonspecific amplification. The enzyme uracil-DNA-glycosylase (UDG) can eliminate false-positive results caused by carry-over contamination, and the hot-start effect with gold nanoparticles can reduce nonspecific amplification. When confirming false-positive results using clustered regularly interspaced short palindromic repeats, guide RNA accurately detects LAMP amplification, allowing differentiation from nonspecific amplification. By confirming amplification, the colorimetric change in the deoxyribozyme (DNAzyme) formed by the reaction of the G-quadruplex sequence of the LAMP amplicon and hemin can distinguish false-positive results. Lateral flow immunoassay can distinguish false-positive results by accurately recognizing hybridized probes to LAMP amplicons.

An experimental comparison between primer and nucleotide labelling to produce RPA-amplicons used for multiplex detection of antibiotic resistance genes

Direct labelling of amplification products using isothermal amplification is currently done most frequently by incorporating previously labelled primer. Although this method is well proven and widely used, it is not a universal solution due to some weaknesses. Alternatively, labelled nucleotides could be used, whose application and functionality have been already partially demonstrated. It remains to be determined how this method performs in comparison to traditional labelling, in particular combined with isothermal amplification methods. In this work, we show a detailed analysis of the labelling efficiency under different conditions and compare the results with the traditional primer-labelling method in the context of RPA amplification. Impressively, our results showed that using Cy5-labelled dUTPs can achieve much more efficient labelling for fragments above 200 bp, while using them for smaller fragments does not bring any relevant disadvantages, but also no major benefit. Furthermore, this work successfully demonstrate for the first time a quadruplex microarray for the detection of resistance genes using RPA and direct labelling with Cy5-dUTP as a potential application scenario. The sensitivities achieved here extend to SNP discovery for the detection of the proper blaKPC variant.

The SHERLOCK Platform: An Insight into Advances in Viral Disease Diagnosis

Persistence and prevalence of microbial diseases (pandemics, epidemics) is the most alarming threats to the human resulting in huge health and economic losses. Rapid detection and understanding of the disease dynamics by molecular biotechnology tools allow for robust reporting, treatment and control of diseases. As per WHO, the optimal diagnostic approach should be quick, specific, sensitive, without a stringed instrument, and low cost. The drawbacks of traditional detection techniques promote the use of CRISPR-mediated nucleic acid detection methods such as SHERLOCK as detection method. It takes advantage of the unexpected in vitro features of CRISPR-Cas system to develop field-deployable sensitive detection tools. Previously, CRISPR-mediated diagnostic methods have extensively been reviewed particularly for SARS-COV-2 detection, but it fails to provide the insight into advances of this technique. This study is the first attempt to review the advances of SHERLOCK approach as diagnostic tool for viral diseases detection. Variations of SHERLOCK mechanism for improved efficiency are discussed. Particularly integrated SHERLOCK approaches in terms of extraction-free assay and Bluetooth-enabled detection are reviewed to access their feasibility for the development of simpler and cost-effective diagnostic toolkits. Insight in to perks and limitations of diagnostic methods indicates its potential as ultimate diagnostic instrument for disease management.