Real-time Reverse Transcription Loop-mediated Isothermal Amplification Assay Research Articles

Rapid detection of tomato spotted wilt virus by real-time RT-LAMP and in-field application

Tomato spotted wilt virus (TSWV) is considered one of the most threatening viruses worldwide for different economically important agricultural crops. In this scenario, it is important to perform an early detection by laboratory tests to prevent TSWV spread. A rapid and sensitive TSWV detection protocol based on real time reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed in this work, also using cost-effective and simplified sample preparation procedure, to assess the suitability of the RT-LAMP assay in field conditions on tomato and pepper samples. A set of six primers was designed within the nucleotide sequence region coding for the nucleocapsid protein (N) of segment S, targeting a 220-nucleotide sequence. Sensitivity, specificity, accuracy, and in-field application of the real-time RT-LAMP assay were evaluated. The developed real-time RT-LAMP assay proved to be one thousand and one hundred times more sensitive than end-point RT-PCR and real-time RT-PCR methods, respectively, detecting a total of 9.191 × 101 genome copies as minimum target, and no cross-reactivity were detected with other viruses belonging to Tospoviridae and Bromoviridae families used as outgroup. In addition, the in-field application of the assay using the rapid sample preparation gave adequate and reliable results within 60 minutes, with an acceptable reaction delay when compared to canonical RNA extraction. The in-field analyses showed an increase of TSWV-positive samples (37%) detection compared with end-point RT-PCR and real-time RT-PCR (32% and 29%, respectively), particularly on asymptomatic samples, confirming that the real-time RT-LAMP assay can be implemented as a routine test both in-field and laboratory conditions as a rapid and sensitive technique for TSWV detection.

Comparison of dye-based and probe-based RT-LAMP in detection of canine astrovirus.

A rapid and sensitive assay is essential for reliable surveillance and diagnosis of canine astrovirus (CaAstV). In this study, two real-time reverse transcription loop-mediated isothermal amplification (RT-LAMP) assays with high sensitivity, rapidity, and reliability were developed using fluorescence dye and FRET-based assimilating probes for real-time detection of CaAstV. These assays specifically amplified the ORF2 gene of CaAstV and did not amplify any sequences from canine enterovirus. The limit of detection (LOD) of both the probe-based and dye-based RT-LAMPs was 100 copies/μL. Fluorescence signals were generated within 30 min for the lowest concentration of a standard RNA sample, which was significantly faster than that achieved by real-time fluorescence quantitative PCR (qRT-PCR) assay. When clinical samples were tested, the positive and negative agreement of the dye-based RT-LAMP assay with qRT-PCR was 87.5% (14/16) and 93.55% (29/31), respectively. The positive and negative agreement of the probe-based RT-LAMP assay with qRT-PCR was 94.11% (16/17) and 96.55% (28/29), respectively. The RT-LAMP assays developed in this study showed strong potential for use as an on-site diagnostic assay for rapid, specific, and reliable detection of CaAstV in clinical samples.

Detection by Sensitive Real-Time Reverse Transcription Loop-Mediated Isothermal Amplification of Olive Leaf Yellowing Associated Virus and Its Incidence in Italy and Spain

Olive trees (Olea europea L.) are constantly threatened by many viruses, such as the olive leaf yellowing-associated virus (OLYaV), that belong to the Olivavirus genus, family Closteroviridae. In this work, the OLYaV incidence in different regions of Italy and Spain, which represent the two most important European areas for olive production, was evaluated through the development of a real-time reverse transcription-loop-mediated isothermal amplification (RT-LAMP) for reliable and sensitive OLYaV detection. The specificity and accuracy of the developed real-time RT-LAMP assay were determined; the assay showed that potential cross-reactivity with other viruses belonging to the Closteroviridae family was excluded. The LAMP assay detected OLYaV with a higher sensitivity than conventional end-point RT-PCR, detecting a total of 1.34 × 10−2 genome copies. A total of 80 and 120 plants of different olive cultivars from Spain (Comunitat Valenciana, Andalusia) and Italy (Sicily, Calabria, Apulia, Lazio, and Umbria) regions were tested, respectively. The percentage of infected plants was 46.25% and 30% for Spain and Italy, respectively, while the most susceptible cultivars were “Serrana Espadán” and “Villalonga” from Comunitat Valenciana and Andalusia regions (Spain) and “Ogliarola barese” from Apulia region (Italy). In addition, the survey demonstrated that the real-time RT-LAMP showed good sensitivity for OLYaV-positive sample detection, especially on asymptomatic olive trees. For this reason, the developed assay could be very suitable for phytopathological laboratories as a reliable and efficient method for a rapid and sensitive routine test on olive samples.

Simultaneous detection of SARS-CoV-2 and pandemic (H1N1) 2009 virus with real-time isothermal platform

The recent ongoing outbreak of novel coronavirus SARS-CoV-2 (known as COVID-19) is a severe threat to human health worldwide. By press time, more than 3.3 million people have died from COVID-19, with many countries experiencing peaks in infections and hospitalizations. The main symptoms of infection with SARS-CoV-2 include fever, chills, coughing, shortness of breath or difficulty breathing, fatigue, muscle or body aches and pains. While the symptoms of the pandemic (H1N1) 2009 virus have many similarities to the signs and transmission routes of the novel coronavirus, e.g., fever, cough, sore throat, body aches, headache, chills and fatigue. And a few cases of serious illness, rapid progress, can appear viral pneumonia, combined with respiratory failure, multiple organ function damage, serious people can die. Therefore, there is an urgent need to develop a rapid and accurate field diagnostic method to effectively identify the two viruses and treat these early infections on time, thus helping to control the spread of the disease. Among molecular detection methods, RT-LAMP (real-time reverse transcription-loop-mediated isothermal amplification) has some advantages in pathogen detection due to its rapid, accurate and effective detection characteristics. Here, we combined the primers of the two viruses with the fluorescent probes on the RT-LAMP detection platform to detect the two viruses simultaneously. Firstly, RT-LAMP method was used respectively to detect the two viruses at different concentrations to determine the effectiveness and sensitivity of probe primers to the RNA samples. And then, the two virus samples were detected simultaneously in the same reaction tube to validate if testing for the two viruses together had an impact on the results compared to detecting alone.We verified the detection efficiency of three highly active BST variants during RT-LAMP assay. We expect that this assay can effectively and accurately distinguish COVID-19 from the pandemic (H1N1) 2009, so that these two diseases with similar symptoms can be appropriately differentiated and treated.

Comparison of clinical diagnostic performance between commercial RRT-LAMP and RT-qPCR assays for SARS-CoV-2 detection

The rapid and reliable detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) plays a key role in isolating infected patients and preventing further viral transmission. In this study, we evaluated the clinical diagnostic performances of a commercial real-time reverse transcription loop-mediated isothermal amplification (RRT-LAMP) assay (Isopollo® COVID-2 assay, M-monitor, Daegu, Korea) using eighty COVID-19 suspected clinical samples and compared these with the results of a commercial real-time reverse transcription polymerase chain reaction (RT-qPCR) assay (AllplexTM 2019-nCoV rRT-QPCR Assay, SeeGene, Seoul, Korea). The results of the RRT-LAMP assay targeting the N or RdRp gene of SARS-CoV-2 showed perfect agreement with the RT-qPCR assay results in terms of detection. Furthermore, the RRT-LAMP assay was completed in just within a 20-min reaction time, which is significantly faster than about the 2 h currently required for the RT- qPCR assay, thus enabling prompt decision making regarding the isolation of infected patients. The RRT-LAMP assay will be a valuable tool for rapid, sensitive, and specific detection of SARS-CoV-2 in human or unexpected animal clinical cases.

Development of a Real-Time Loop-Mediated Isothermal Amplification Method for the Detection of West Nile Virus

Background: The West Nile Virus (WNV), discovered in New York, USA in 1999 after it was first isolated in Uganda in 1937, has since spread not only in the United States but also around the world. Africa, Eurasia, Australia, and the Middle East have sporadic cases of the disease. Objectives: We aimed to find real-time reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay to be more sensitive than conventional RT-PCR, and more rapid and efficient than conventional RT-PCR and real-time RT-PCR for WNV detection. Methods: A total of 32 genomic sequences from different strains of WNV were analyzed to identify conserved nucleotide sequence regions. Six WNV specific RT-LAMP primers targeting the E gene were designed. Results: The novel primer for the real-time RT-LAMP assay can detect WNV with high specificity. The efficiency of the real-time RT-LAMP assay is higher than the conventional RT-PCR and real-time RT-PCR. Real-time RT-PCR and conventional PCR require at least 30 – 40 min and 2 h, respectively, to yield results, whereas real-time RT-LAMP provides positive results in only 10 – 20 min. Conclusions: The novel primers were developed by analyzing of 32 genomic sequences of WNV strains. The primers were designed from the most conserved region of the E gene for real-time RT-LAMP. The LAMP assay is a rapid, efficient, highly sensitive, and specific tool for the identification of WNV.

Rapid discriminative detection of dengue viruses via loop mediated isothermal amplification

Dengue virus (DENV) is one of the life-threatening viruses to the human. In this study, we have designed specific novel primers for rapid discriminative detection of DENV-1, DENV-2, and DENV-4 by real-time reverse transcription loop-mediated isothermal amplification (RT-LAMP) reaction. The effect of parameters such as reaction temperature and magnesium sulfate was investigated on the RT-LAMP reaction for detection of DENV RNA. Under the optimal conditions, this method is able to differentiate and to detect DENV within 25 min, exhibiting detection limit of 3.5 copies/μL. Importantly, the novel specific primers-based RT-LAMP assay did not react with other viruses, suggesting the selectivity of the method towards DENV RNA. The RT-LAMP reaction products are easily visualized with naked-eye when irradiated them under UV light at 365 nm. Amplification products could be visualized directly for color changes. This method provides a facile, and accurate molecular amplication technique for the rapid discriminative detection of dengue viruses. The RT-LAMP platform can be used as a promissing diagnostic tool for discriminative detection of DENV without aid of complicated protocols or sophisticated equipment.

Evaluation of real-time reverse-transcription loop-mediated isothermal amplification assay for clinical diagnosis of West Nile virus in patients.

Background & objectives:West Nile virus (WNV) is a mosquito-borne flavivirus. The disease can be diagnosed by isolation followed by fluorescent antibody tests, enzyme-linked immunosorbent assay and polymerase chain reaction (PCR) assay. These diagnostic methods are laborious and time-consuming. The present study was aimed to evaluate the real-time reverse-transcription loop-mediated isothermal amplification (RT-LAMP) method for rapid, early and accurate diagnosis of WNV.Methods:A one-step single tube accelerated quantitative RT-LAMP assay was evaluated by targeting the Env gene of WNV. The gene amplification was accomplished by incubating the reaction mixture at 63°C for 60 min in both real time turbidimeter as well as routine laboratory water bath/dry heating bath. To rule out contamination issues, proper negative controls, including no template, no primer; and no enzyme, were always kept alongside each run. The RT-LAMP assay was evaluated on 105 clinical samples from individuals having ocular infection.Results:Of the 105 samples tested, 27 were positive for WNV by RT-LAMP assay. The comparative evaluation with conventional RT-PCR revealed 100 per cent accordance with sensitivity and specificity of 100 and 95 per cent, respectively. The specificity of this assay was confirmed with serum samples obtained from patients with dengue and chikungunya.Interpretation & conclusions:The RT-LAMP test seemed to be a sensitive and specific method for rapid detection of WNV infection and would be useful for rapid screening of a large number of clinical samples in endemic areas during outbreaks.

Reverse transcription loop-mediated isothermal amplification for rapid and quantitative assay of covert mortality nodavirus in shrimp.

A disease known as covert mortality disease has become an increasing problem in the shrimp farming industry in recent years in China and several countries of Southeast Asia, leading to serious losses in production. Litopenaeus vannamei (also known as Pacific white shrimp) is affected by this disease that leads to a range of clinical symptoms including hepatopancreas atrophy and necrosis, soft shell, slow growth, and abdominal muscle whitening and necrosis in the acute stage of disease. A new nodavirus, termed covert mortality nodavirus (CMNV), has been shown to be the etiological agent. In this study, we report a sensitive and specific real-time reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay for the rapid and quantitative detection of CMNV. The optimal conditions for this newly developed RT-LAMP reaction were found to be 6mM MgCl2 and 1.6mM dNTPs, an incubation temperature of 65°C and a reaction time of 50min. The analytical sensitivity of the RT-LAMP assay was estimated to be 6.3pg total RNA of CMNV-infected shrimp and 27 copies of the target plasmid. The diagnostic sensitivity and specificity of the newly developed assay versus the standard nested reverse transcription PCR (RT-PCR) assay was 96.4% and 94.4%, respectively. The reaction products were detected by visual inspection after staining with an in-tube DNA fluorescent dye, a measure taken to eliminate the risk of contamination. The quantitative RT-LAMP assay for CMNV showed high correlation coefficient (r2=0.9953) when the initial templates were above 1000 copies, however the correlation coefficient decreased when the initial templates were lower than 1000 copies. Test of viral load in shrimp indicated that the viral loads varied from 1.5×102 to 6.7×106 copies per mg of cephalothorax tissue. Thus, the CMNV RT-LAMP assay is a sensitive and specific new tool for the field detection and quantification of CMNV in the diagnosis and surveillance of covert mortality disease.

Rapid and real-time detection of Porcine Sapelovirus by reverse transcription loop-mediated isothermal amplification assay

The present study describes the development and validation of a one-step, single-tube, and real-time reverse transcription loop-mediated isothermal amplification (RT-LAMP) detecting Porcine Sapelovirus. RT-LAMP characterized by one strand displacement reaction with the specific stem-loop structure and Bst DNA polymerase could be finished in 60min under isothermal condition at 63°C. RT-LAMP assay showed higher sensitivity with 101 copies/μL than RT-PCR for the detection of Sapelovirus. The specificity of RT-LAMP assay was validated by the absence of any cross-reaction with other closely related virus in Picornaviridae group and other common virus causing porcine diarrhea. 7 positive Sapelovirus infection out of 63 fecal samples were identified using RT-LAMP, while 5 positive samples were determined by a conventional RT-PCR. A cost-effective method for Saplovirus detection with high sensitivity and specificity was developed and evaluated.

Improved Detection Limit in Rapid Detection of Human Enterovirus 71 and Coxsackievirus A16 by a Novel Reverse Transcription–Isothermal Multiple-Self-Matching-Initiated Amplification Assay

Rapid detection of human enterovirus 71 (EV71) and coxsackievirus A16 (CVA16) is important in the early phase of hand-foot-and-mouth disease (HFMD). In this study, we developed and evaluated a novel reverse transcription-isothermal multiple-self-matching-initiated amplification (RT-IMSA) assay for the rapid detection of EV71 and CVA16 by use of reverse transcriptase, together with a strand displacement DNA polymerase. Real-time RT-IMSA assays using a turbidimeter and visual RT-IMSA assays to detect EV71 and CVA16 were established and completed in 1 h, and the reported corresponding real-time reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assays targeting the same regions of the VP1 gene were adopted as parallel tests. Through testing VP1 RNAs transcribed in vitro, the real-time RT-IMSA assays exhibited better linearity of quantification, with R(2) values of 0.952 (for EV71) and 0.967 (for CVA16), than the real-time RT-LAMP assays, which had R(2) values of 0.803 (for EV71) and 0.904 (for CVA16). Additionally, the detection limits of the real-time RT-IMSA assays (approximately 937 for EV71 and 67 for CVA16 copies/reaction) were higher than those of real-time RT-LAMP assays (approximately 3,266 for EV71 and 430 for CVA16 copies/reaction), and similar results were observed in the visual RT-IMSA assays. The new approaches also possess high specificities for the corresponding targets, with no cross-reactivity observed. In clinical assessment, compared to commercial reverse transcription-quantitative PCR (qRT-PCR) kits, the diagnostic sensitivities of the real-time RT-IMSA assays (96.4% for EV71 and 94.6% for CVA16) were higher than those of the real-time RT-LAMP assays (91.1% for EV71 and 90.8% for CVA16). The visual RT-IMSA assays also exhibited the same results. In conclusion, this proof-of-concept study suggests that the novel RT-IMSA assay is superior to the RT-LAMP assay in terms of detection limit and has the potential to rapidly detect EV71 and CVA16 viruses.

Development of a Reverse Transcription-Loop-Mediated Isothermal Amplification Assay for Detection of Pandemic (H1N1) 2009 Virus as a Novel Molecular Method for Diagnosis of Pandemic Influenza in Resource-Limited Settings

This paper reports on the development of a one-step, real-time reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assay targeting the hemagglutinin (HA) gene for the rapid molecular-based detection of pandemic (H1N1) 2009 virus. The detection limit of the pandemic (H1N1) 2009 virus HA-specific RT-LAMP assay was same as that of the currently used real-time reverse transcription-PCR method. The assay detected the pandemic (H1N1) 2009 virus HA gene in 136 RNA samples extracted from nasopharyngeal swab specimens from Japanese and Vietnamese patients. No cross-reactive amplification with the RNA of other seasonal influenza viruses was observed, and the detection of specific viral genome targets in clinical specimens was achieved in less than 40 min. The sensitivity and specificity of the pandemic (H1N1) 2009 virus HA-specific RT-LAMP assay obtained in this study were 97.8% and 100%, respectively. Use of the (H1N1) 2009 virus HA-specific RT-LAMP assay will enable the faster and easier diagnosis of pandemic (H1N1) 2009 virus infection, especially in resource-limited situations in developing countries.

Rapid and real-time detection of hepatitis A virus by reverse transcription loop-mediated isothermal amplification assay

A one-step, single tube, real-time reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed for detecting sequences of the untranslated region in the genome of hepatitis A virus (HAV). The RT-LAMP assay reported in this study was very simple and rapid; the HAV-specific amplification was obtained in 50 min under isothermal conditions at 62.5 °C by employing a set of seven primers. The RNAs of three cell-adapted HAV strains belonging to different subgenotypes (IA, IB and IIIB) were equally well amplified. The detection limits of the RT-LAMP assay for these HAV strains were 0.4–0.8 focus forming units (FFU)/reaction. The results of the calibration using the WHO international standard indicated that the RT-LAMP assay had similar sensitivity to the conventional RT-PCR method. A comparison of the results from the RT-LAMP and the LightCycler PCR assay using clinical samples in feces revealed that the findings were similar between the two methods. Although several genotypes remain to be tested, it is concluded that the new real-time RT-LAMP assay is very suitable for detection and quantitation of most prevalent genotypes of HAV in diagnostic laboratories.

Rapid Detection and Quantification of Japanese Encephalitis Virus by Real‐Time Reverse Transcription Loop‐Mediated Isothermal Amplification

We established a rapid, quantitative real-time reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay targeting the envelope gene of Japanese encephalitis virus. The RT-LAMP enabled us to detect the target product within 1 hr by only reacting reverse transcriptase and Bst DNA polymerase in a single tube at an isothermal temperature. The detection sensitivity of the RT-LAMP for Japanese encephalitis virus was 1 PFU, similar to that of conventional reverse transcription-polymerase chain reaction (RT-PCR). Flaviviruses of the Japanese encephalitis virus group, such as Dengue virus and West Nile virus, could not be detected. This confirmed the specificity of the RT-LAMP assay for Japanese encephalitis virus. A standard curve was constructed by plotting viral titer against the time for virus detection by the RT-LAMP, validating the quantitative accuracy of the assay. In addition, the amount of virus estimated by RT-LAMP was strongly correlated (r = 0.902) with that determined by plaque assay, a conventional method for virus quantification. These results indicate that the RT-LAMP assay established in this study is specific for Japanese encephalitis virus, and allows more rapid detection and quantification of the virus.