Evaluation of a loop-mediated isothermal amplification assay for detecting Vibrio vulnificus in raw oysters

Direct or DNA Extraction-Free Amplification and Quantification of Foodborne Pathogens

The use of direct nucleic acid amplification of pathogens from food matrices has the potential to reduce time to results over DNA extraction-based approaches as well as traditional culture-based approaches. Here we describe protocols for assay design and experiments for direct amplification of foodborne pathogens in food sample matrices using loop-mediated isothermal amplification (LAMP) and polymerase chain reaction (PCR). The examples provided include the detection of Escherichia coli in milk samples and Salmonella in pork meat samples. This protocol includes relevant reagents and methods including obtaining target sequences, assay design, sample processing, and amplification. These methods, though used for specific example matrices, could be applied to many other foodborne pathogens and sample types.

Evaluation of a novel lysis-based sample processing method to optimize Vibrio vulnificus detecting by loop-mediated isothermal amplification assay

BACKGROUND

Vibrio vulnificus exists as one of the most serious foodborne pathogens for humans, and rapid and sensitive detection methods are needed to control its infections. As an emerging method, The Loop-Mediated Isothermal Amplification (LAMP) assay has been applied to the early detection of various foodborne pathogens due to its high efficiency, but sample preprocessing still prolongs the complete detection. To optimize the detection process, our study established a novel sample preprocessing method that was more efficient compared to common methods.

RESULT

Using V. vulnificus as the detecting pathogen, the water-lysis-based detecting LAMP method shortened the preprocessing time to ≤ 1 min with 100% LAMP specificity; the detection limits of the LAMP assay were decreased to 1.20 × 102 CFU/mL and 1.47 × 103 CFU/g in pure culture and in oyster, respectively. Furthermore, the 100% LAMP specificity and high sensitivity of the water-lysis method were also obtained on detecting V. parahaemolyticus, V. alginolyticus, and P. mirabilis, revealing its excellent LAMP adaption with improvement in sensitivity and efficiency.

CONCLUSION

Our study provided a novel LAMP preprocessing method that was more efficient compared to common methods and possessed the practical potential for LAMP application in the future.

Rapid visual detection of Aeromonas salmonicida by loop-mediated isothermal amplification with hydroxynaphthol blue dye

To make crucial prevention, reduce fish losses and minimize the economic damage of diseases on the fish farm owners, a rapid detection of fish pathogens is mandatory. In this study, a loop-mediated isothermal amplification assay combined with hydroxynaphthol blue dye (LAMP-HNB) was developed and used for the rapid detection of Aeromonas salmonicida that caused significant economic losses in fish farming. Firstly, a pair of outer and inner primers specific for conserved fragment of vapA gene in A. salmonicida were designed and synthesized. Secondly, by optimizing the reaction conditions including reaction temperature, time, Mg²⁺ concentration, dNTP concentration and primer ratio, a LAMP-HNB assay was successfully established for the detection of A. salmoncida. Thirdly, the assay showed good specificity with no false-positive and false-negative results, and good sensitivity with the detection limit of 3.077 × 10^-6  ng/μl, which was 10² times more sensitive than the conventional PCR. Finally, the LAMP-HNB assay was validated by the fish samples inoculated with different concentrations of A. salmoncida. This is the first development of rapid visual detection of A. salmonicida based on LAMP-HNB assay, which has great application prospect and market for diagnostic testing, health certification and active surveillance programmers.

Rapid single-cell detection of pathogenic bacteria for in situ determination of food safety

A highly sensitive in situ method to detect bacterial pathogens is of utmost importance in preventing the outbreak of foodborne diseases. In this study, a simple method enabling the detection of a single bacterial cell in a sample was developed based on magnetic capture particles (CPs), and europium-fluorescent labeling particles (LPs) functionalized with antibodies. After mixing the sample with the particles in a sample tube, the sample tube was connected to an assay chip, where the CP-bacteria-LP complex was transported from the sample chamber to a detection chamber using a simple assay device. The number of bacteria was quantitatively determined by measuring the fluorescence emitted from the detection chamber. This assay method enabled the detection of a single cell of Vibrio parahaemolyticus from 0.1 mL pure broth culture samples within 30 min. A simple enrichment method that can be performed using only the vibrating action of the assay device without any additional instruments was also developed for the analysis of food samples. By analyzing the enriched sample using the assay method, we could detect V. parahaemolyticus quantitatively with a detection limit of 1 colony forming unit from oyster samples within 130 min. Due to simplicity of this methodology and the instrumentation involved, and its capability of rapid single-cell detection, it may be considered as an in situ method for the determination of food safety.

Visual, sensitive and rapid event-specific detection of genetically modified potato EH92-527-1 by loop-mediated isothermal amplification method

To date, studies on the application of loop-mediated isothermal amplification (LAMP) in the detection of genetically modified organisms (GMOs) are stably increasing and demonstrates LAMP is a potential and promising method for on spot identification of GMOs. However, little information is known for detection of GM potato events by LAMP. In this report, we developed an optimized and visual LAMP assay with high specificity and sensitivity to rapidly amplify genomic DNA of potato EH92-527-1 within 45 min. The limit of detection of LAMP in our study is 10-fold higher than the conventional PCR. Furthermore, LAMP products can be directly observed via naked eyes by addition of SYBR Green I without gel electrophoresis analysis and PCR-based equipment. Therefore, the LAMP assay developed in this paper provides an efficient, convenient and cost-effective tool for the detection of GM potato EH92-527-1.

Loop-mediated isothermal amplification assay as a point-of-care diagnostic tool for Vibrio parahaemolyticus: recent developments and improvements

INTRODUCTION

A number of DNA-based diagnostic tools have been developed for the detection of Vibrio parahaemolyticus in seafood. However, the loop-mediated isothermal amplification (LAMP) has distinct advantages with regards to its simplicity, speed and the ease of performing without any need for sophisticated equipment. Over the last decade, LAMP has emerged as a potential tool for the detection of V. parahaemolyticus. Area covered: The literature search was restricted to LAMP assay and its variants for the detection of V. parahaemolyticus. The focus in this review is to enlist the various techniques that have been developed using the principle of the LAMP towards improved simplicity, sensitivity and specificity of the assay. Expert commentary: LAMP assay and its variants are significantly faster and require minimum accessories compared to other DNA based molecular techniques such as PCR and their types. Despite the availability of several versions, LAMP-based diagnostics is not the first choice for the detection of V. parahaemolyticus in the seafood sector. Our recommendation would be to explore the possibilities of developing cost-effective LAMP kits and implementing these kits as point-of-care diagnostic tools for rapid and sensitive detection of pathogenic V. parahaemolyticus.

Direct or DNA Extraction-Free Amplification and Quantification of Foodborne Pathogens

The use of direct nucleic acid amplification of pathogens from food matrices has the potential to reduce time to results over DNA extraction-based approaches as well as traditional culture-based approaches. Here we describe protocols for assay design and experiments for direct amplification of foodborne pathogens in food sample matrices using loop-mediated isothermal amplification (LAMP) and polymerase chain reaction (PCR). The examples provided include the detection Escherichia coli in milk samples and Salmonella in pork meat samples. This protocol includes relevant reagents and methods including obtaining target sequences, assay design, sample processing, and amplification. These methods, though used for specific example matrices, could be applied to many other foodborne pathogens and sample types.

Recombinase polymerase amplification combined with lateral flow dipstick for equipment-free detection of Salmonella in shellfish

Salmonella is a major pathogen that causes acute foodborne outbreaks worldwide. Seafood, particularly shellfish, is a proven source of Salmonella spp. infection because many people prefer to eat it raw or lightly cooked. However, traditional identification methods are too time-consuming and complex to detect contamination of bacteria in the food chain in a timely manner, and few studies have aimed to identify Salmonella in shellfish early in the supply chain. We herein developed a method for rapid detection of Salmonella in shellfish based on the method of recombinase polymerase amplification (RPA) combined with lateral flow dipstick (LFD), which targets the invasion gene A (invA). The RPA-LFD was able to function at 30-45 °C, and at the temperature of 40 °C, it only took 8 min of amplification to reach the test threshold of amplicons. The established method had both a good specificity and a sensitivity of 100 fg DNA per reaction (20 µL). Regarding practical performance, RPA-LFD performed better than real-time PCR. Another advantage of RPA-LFD is that it was capable of being performed without expensive equipments. Thus, RPA-LFD has potential for further development as a detection kit for Salmonella in shellfish and other foods under field conditions.

An Innovative Method for Rapid Identification and Detection of Vibrio alginolyticus in Different Infection Models

Vibrio alginolyticus is one of the most common pathogenic marine Vibrio species, and has been found to cause serious seafood-poisoning or fatal extra-intestinal infections in humans, such as necrotizing soft-tissue infections, bacteremia, septic shock, and multiple organ failures. Delayed accurate diagnosis and treatment of most Vibrio infections usually result to high mortality rates. The objective of this study was to establish a rapid diagnostic method to detect and identify the presence of V. alginolyticus in different samples, so as to facilitate timely treatment. The widely employed conventional methods for detection of V. alginolyticus include biochemical identification and a variety of PCR methods. The former is of low specificity and time-consuming (2–3 days), while the latter has improved accuracy and processing time. Despite such advancements, these methods are still complicated, time-consuming, expensive, require expertise and advanced laboratory systems, and are not optimal for field use. With the goal of providing a simple and efficient way to detect V. alginolyticus, we established a rapid diagnostic method based on loop-mediated Isothermal amplification (LAMP) technology that is feasible to use in both experimental and field environments. Three primer pairs targeting the toxR gene of V. alginolyticus were designed, and amplification was carried out in an ESE tube scanner and Real-Time PCR device. We successfully identified 93 V. alginolyticus strains from a total of 105 different bacterial isolates and confirmed their identity by 16s rDNA sequencing. We also applied this method on infected mouse blood and contaminated scallop samples, and accurate results were both easily and rapidly (20–60 min) obtained. Therefore, the RT-LAMP assay we developed can be conveniently used to detect the presence of V. alginolyticus in different samples. Furthermore, this method will also fulfill the gap for real-time screening of V. alginolyticus infections especially while on field.

Genome Wide Analysis for Rapid Identification of Vibrio Species

The highly conserved 16S rRNA (rrs) gene is generally used for bacterial identification. In organisms possessing multiple copies of rrs, high intra-genomic heterogeneity does not allow easy distinction among different species. In order to identify Vibrio species, a wide range of genes have been employed. There is an urgent requirement of a consensus gene, which can be used as biomarker for rapid identification. Eight sequenced genomes of Vibrio species were screened for selecting genes which were common among all the genomes. Out of 108 common genes, 24 genes of sizes varying from 0.11 to 3.94 kb were subjected to in silico digestion with 10 type II restriction endonucleases (RE). A few unique genes-dapF, fadA, hisD, ilvH, lpxC, recF, recR, rph and ruvB in combination with certain REs provided unique digestion patterns, which can be used as biomarkers. This protocol can be exploited for rapid diagnosis of Vibrio species.

Development of a real-time loop-mediated isothermal amplification assay for the sensitive and rapid detection of Listeria monocytogenes

A real-time loop-mediated isothermal amplification (RealAmp) assay for the detection of Listeria was developed. The RealAmp assay, using primers specific for the hemolysin-encoding hlyA gene, was verified using Listeria monocytogenes strains (n = 58) from different regions of the world. Both the sensitivity and specificity of the RealAmp assay were high. The RealAmp assay could detect 10(3) CFU ml(-1) within 30 min. A comparative evaluation of the RealAmp assay, the API Listeria assay, and the real-time PCR assay revealed that the RealAmp assay is simpler, faster, and has a higher specificity than the other two assays.

SIGNIFICANCE AND IMPACT OF THE STUDY

Conventional culture and molecular detection methods are always time consuming and require a specific laboratory infrastructure, thereby restricting their use for the rapid detection and diagnosis of pathogens. A real-time loop-mediated isothermal amplification (RealAmp) assay performed by ESEtube scanner to rapidly detect Listeria monocytogenes isolated from food was developed. The results showed that the RealAmp assay using the tube scanner was more efficient and precise than the conventional API Listeria assay and the real-time PCR assay.

Loop-mediated isothermal amplification (LAMP) method for detection of genetically modified maize T25

The loop-mediated isothermal amplification (LAMP) assay indicates a potential and valuable means for genetically modified organism (GMO) detection especially for its rapidity, simplicity, and low cost. We developed and evaluated the specificity and sensitivity of the LAMP method for rapid detection of the genetically modified (GM) maize T25. A set of six specific primers was successfully designed to recognize six distinct sequences on the target gene, including a pair of inner primers, a pair of outer primers, and a pair of loop primers. The optimum reaction temperature and time were verified to be 65°C and 45 min, respectively. The detection limit of this LAMP assay was 5 g kg(-1) GMO component. Comparative experiments showed that the LAMP assay was a simple, rapid, accurate, and specific method for detecting the GM maize T25.

A pentaplex PCR assay for detection and characterization of Vibrio vulnificus and Vibrio parahaemolyticus isolates

Vibrio parahaemolyticus and Vibrio vulnificus are the leading causes of seafood-related illnesses and also can cause wound infections. These bacteria often co-exist in marine and estuarine environments. However, there have been no reported protocols that can detect and characterize (i.e. pathogenic or nonpathogenic) them in a single PCR. In this study, we developed a pPCR assay with a combination of two species-specific and three pathogenic-specific PCR primers to simultaneously detect virulent (viuB in V. vulnificus and tdh/trh in V. parahaemolyticus) and nonvirulent (vvhA in V. vulnificus and tlh in V. parahaemolyticus) markers of the two species in bacterial isolates. The assay was validated by three methods. First, the pPCR was used to characterize 300 bacterial isolates consisting of seven reference strains and 293 environmental strains isolated from the Gulf of Mexico water. Results were compared with characterizations based on single-gene PCR amplifications and previously published multiplex PCR protocols. Second, 51 isolates characterized with the pPCR were analysed by 16S rRNA sequencing to confirm any false-negative/positive reaction. Finally, the effectiveness of the assay for heterogeneous bacterial samples was validated. The pPCR correctly characterized isolates from the Gulf with an efficiency of 96·6-98·7%.

Development of a sensitive and specific LAMP PCR assay for detection of fish pathogen Lactococcus garvieae

Based on use of a loop-mediated isothermal amplification (LAMP) identification protocol, this study attempted to detect Lactococcus garvieae in fish by using primer sets designed from an L. garvieae alpha/beta fold family hydrolase gene. Reaction time and temperatures were optimized for 60 min at 60°C with the resulting amplicons visualized by adding SYBR Green I to the reaction tube. The assay specificity was assessed using 45 different bacterial strains. Positive results were observed in all 30 L. garvieae isolates from various aquatic animals. No false-positive results were observed in 15 non-L. garvieae strains. The detection limit of the LAMP assay was 10-fold more sensitive than the conventional polymerase chain reaction (PCR) targeting 16S rDNA when using purified L. garvieae DNA. The detection limit of the LAMP assay was approximately 300 colony-forming units (CFU) using crude bacterial lysates, 100-fold more sensitive than PCR. Furthermore, L. garvieae in spleen, kidney and brain of experimentally challenged tilapia and grey mullet were detected using this optimized LAMP assay. Results of this study demonstrate the effectiveness of LAMP in providing a rapid yet simple test for detecting L. garvieae in fish.

Development of a loop-mediated isothermal amplification assay for sensitive and rapid detection of Cronobacter sakazakii

Cronobacter sakazakii is an emerging pathogen associated with the ingestion of contaminated reconstituted formula, which causes necrotizing enterocolitis, sepsis, and meningitis in low-birth-weight preterm neonatal infants. Sensitive and specific detection methods are needed to better control C. sakazakii infections. This study aims to develop a highly specific and sensitive loop-mediated isothermal amplification (LAMP) assay for detecting C. sakazakii in powdered infant formula (PIF). A set of four LAMP primers were designed based on the published C. sakazakii ompA gene sequence. Specificity of the assay was evaluated using a panel of 22 C. sakazakii, 27 Enterobacteriaceae family except C. sakazakii, and 25 other strains. Assay sensitivity was determined using serial dilutions of C. sakazakii American Type Culture Collection 51329 culture ranging from 10(6) colony-forming units (CFU)/mL to extinction. The assay was also tested in experimentally inoculated PIF samples. The ompA-based LAMP assay was able to detect specifically all of the 22 C. sakazakii strains without amplification from 52 non-C. sakazakii strains. The detection limit was 10(1) CFU/mL in pure culture, up to 10-fold more sensitive than that of the ompA-polymerase chain reaction (PCR). When applied to PIF, sensitivity was 10(2) CFU/mL, up to 10-fold that of the ompA-PCR. The ompA-based LAMP assay developed in this study was sensitive, specific, and low cost with great potential for future field detection of C. sakazakii in PIF.

Loop-mediated isothermal amplification assays for detecting shiga toxin-producing Escherichia coli in ground beef and human stools

Shiga toxin-producing Escherichia coli (STEC), encompassing E. coli O157 and non-O157 STEC, is a significant cause of food-borne illnesses and deaths in the United States and worldwide. Shiga toxins (encoded by stx) and intimin (encoded by eae) are important virulence factors for STEC strains linked to severe human illnesses such as hemorrhagic colitis and hemolytic-uremic syndrome. In this study, the stx(1), stx(2), and eae genes were chosen as targets to design loop-mediated isothermal amplification (LAMP) assays for the rapid, specific, sensitive, and quantitative detection of STEC strains. The assay performances in pure culture and spiked ground beef and human stools were evaluated and compared with those of quantitative PCR (qPCR). No false-positive or false-negative results were observed among 90 bacterial strains used to evaluate assay specificity. The limits of detection for seven STEC strains of various serogroups (O26, O45, O103, O111, O121, O145, and O157) were approximately 1 to 20 CFU/reaction in pure culture and 10(3) to 10(4) CFU/g in spiked ground beef, which were comparable to the results of qPCR. Standard curves generated suggested good linear relationships between STEC cell numbers and LAMP turbidity signals. When applied in ground beef samples spiked with two low levels (1 to 2 and 10 to 20 CFU/25 g) of STEC cultures, the LAMP assays achieved accurate detection after 6 to 8 h enrichment. The assays also consistently detected STEC in human stool specimens spiked with 10(3) or 10(4) CFU/0.5 g stool after 4 h enrichment, while qPCR required 4 to 6 h. In conclusion, the LAMP assays developed in this study may facilitate rapid and reliable identification of STEC contaminations in high-risk food commodities and also facilitate prompt diagnosis of STEC infections in clinical laboratories.

Rapid and sensitive detection of Vibrio vulnificus by loop-mediated isothermal amplification combined with lateral flow dipstick targeted to rpoS gene

A novel loop-mediated isothermal amplification (LAMP) combined with amplicon detection by chromatographic lateral flow dipstick (LFD) assay was developed and evaluated for the detection of Vibrio vulnificus. Biotinylated LAMP amplicons were produced by a set of six designed primers that recognized the V. vulnificus RNA polymerase subunit sigma factor S (rpoS) gene followed by hybridization with an FITC-labeled probe and LFD detection. The optimized time and temperature conditions for the LAMP assay were 90 min at 65 °C. The LAMP-LFD method accurately identified 14 isolates of V. vulnificus but did not detect 25 non-vulnificus Vibrio isolates and 37 non-Vibrio isolates. The sensitivity of LAMP-LFD for V. vulnificus detection in pure culture was 1.5 × 10(3) CFU ml(-1) or equivalent to 2.8 CFU per reaction. In the case of spiked oyster samples without enrichment, the detection limit for V. vulnificus was 1.2 × 10(4) CFU g(-1) or equivalent to 11 CFU per reaction. The results show that this method appears to be accurate, precise and valuable tool for identification of V. vulnificus and can be used efficiently for detection of V. vulnificus in contaminated food sample.

Rapid detection of viable salmonellae in produce by coupling propidium monoazide with loop-mediated isothermal amplification

Recent outbreaks linked to Salmonella-contaminated produce heightened the need to develop simple, rapid, and accurate detection methods, particularly those capable of determining cell viability. In this study, we examined a novel strategy for the rapid detection and quantification of viable salmonellae in produce by coupling a simple propidium monoazide sample treatment with loop-mediated isothermal amplification (PMA-LAMP). We first designed and optimized a LAMP assay targeting Salmonella. Second, the performance of PMA-LAMP for detecting and quantifying viable salmonellae was determined. Finally, the assay was evaluated in experimentally contaminated produce items (cantaloupe, spinach, and tomato). Under the optimized condition, PMA-LAMP consistently gave negative results for heat-killed Salmonella cells with concentrations up to 10(8) CFU/ml (or CFU/g in produce). The detection limits of PMA-LAMP were 3.4 to 34 viable Salmonella cells in pure culture and 6.1 × 10(3) to 6.1 × 10(4) CFU/g in spiked produce samples. In comparison, PMA-PCR was up to 100-fold less sensitive. The correlation between LAMP time threshold (T(T)) values and viable Salmonella cell numbers was high (R(2) = 0.949 to 0.993), with a quantification range (10(2) to 10(5) CFU/reaction in pure culture and 10(4) to 10(7) CFU/g in produce) comparable to that of PMA in combination with quantitative real-time PCR (PMA-qPCR). The complete PMA-LAMP assay took about 3 h to complete when testing produce samples. In conclusion, this rapid, accurate, and simple method to detect and quantify viable Salmonella cells in produce may present a useful tool for the produce industry to better control potential microbial hazards in produce.

Detecting potentially virulent Vibrio vulnificus strains in raw oysters by quantitative loop-mediated isothermal amplification

Vibrio vulnificus is a leading cause of seafood-related deaths in the United States. Sequence variations in the virulence-correlated gene (vcg) have been used to distinguish between clinical and environmental V. vulnificus strains, with a strong association between clinical ones and the C sequence variant (vcgC). In this study, vcgC was selected as the target to design a loop-mediated isothermal amplification (LAMP) assay for the rapid, sensitive, specific, and quantitative detection of potentially virulent V. vulnificus strains in raw oysters. No false-positive or false-negative results were generated among the 125 bacterial strains used to evaluate assay specificity. The detection limit was 5.4 CFU per reaction for a virulent V. vulnificus strain (ATCC 33815) in pure culture, 100-fold more sensitive than that of PCR. In spiked raw oysters, the assay was capable of detecting 2.5 × 10(3) CFU/g of V. vulnificus ATCC 33815, while showing negative results for a nonvirulent V. vulnificus strain (515-4c2) spiked at 10(7) CFU/g. After 6 h of enrichment, the LAMP assay could detect 1 CFU/g of the virulent V. vulnificus strain ATCC 33815. Standard curves generated in pure culture and spiked oysters suggested a good linear relationship between cell numbers of the virulent V. vulnificus strain and turbidity signals. In conclusion, the LAMP assay developed in this study could quantitatively detect potentially virulent V. vulnificus in raw oysters with high speed, specificity, and sensitivity, which may facilitate better control of V. vulnificus risks associated with raw oyster consumption.

The development of loop-mediated isothermal amplification combined with lateral flow dipstick for detection of Vibrio parahaemolyticus

AIMS

The current study was aimed to develop a loop-mediated isothermal amplification (LAMP) combined with amplicon detection by chromatographic lateral flow dipstick (LFD) assay for rapid and specific detection of Vibrio parahaemolyticus.

METHODS AND RESULTS

Biotinylated LAMP amplicons were produced by a set of four designed primers that recognized specifically the V. parahaemolyticus thermolabile haemolysin (tlh) gene followed by hybridization with an FITC-labelled probe and LFD detection. The optimized time and temperature conditions for the LAMP assay were 90 min at 65 °C. The LAMP-LFD method accurately identified 28 isolates of V. parahaemolyticus but did not detect 24 non-parahaemolyticus Vibrio isolates and 35 non-Vibrio bacterial isolates. The sensitivity of LAMP-LFD for V. parahaemolyticus detection in pure cultures was 120 CFU ml⁻¹. In the case of spiked shrimp samples without enrichment, the detection limit for V. parahaemolyticus was 1·8 x 10³ CFU g⁻¹ or equivalent to 3 CFU per reaction while that of conventional PCR was 30 CFU per reaction.

CONCLUSIONS

The established LAMP-LFD assay targeting tlh gene was specific, rapid and sensitive for identification of V. parahaemolyticus.

SIGNIFICANCE AND IMPACT OF THE STUDY

The developed LAMP-LFD assay provided a valuable tool for detection of V. parahaemolyticus and can be used effectively for identification of V. parahaemolyticus in contaminated food sample.

Sensitive and rapid detection of Vibrio corallilyticus by loop-mediated isothermal amplification targeted to the alpha subunit gene of RNA polymerase

AIMS

A diagnostic protocol was developed for rapid detection of Vibrio corallilyticus by method of loop-mediated isothermal amplification (LAMP).

METHODS AND RESULTS

For cloning and sequencing of rpo A gene of V. corallilyticus, a set of four LAMP primers were designed by targeting the rpoA gene. With Bst DNA polymerase, the reaction time and temperature were optimized for 70 min at 65 degrees C, respectively. The amplification products were detected by electrophoresis. The detection limit of V. corallilyticus by LAMP was 3.6 x 10(3) CFU ml(-1) (8 CFU per reaction), but PCR could detect up to 3.6 x 10(4) CFU ml(-1) (72 CFU per reaction). The LAMP method was ninefold more sensitive than conventional PCR. The results also indicated that the LAMP reaction was highly specific to V. corallilyticus.

CONCLUSIONS

The LAMP assay was a sensitive, specific and cost-effective method for the rapid detection of V. corallilyticus.

SIGNIFICANCE AND IMPACT OF THE STUDY

This LAMP method provides an important diagnostic tool for the detection of V. corallilyticus infection. It can replace laborious biochemical tests for the identification of V. corallilyticus.

Development of a toxR-based loop-mediated isothermal amplification assay for detecting Vibrio parahaemolyticus

BACKGROUND

Vibrio parahaemolyticus is a leading cause of seafood-related bacterial gastroenteritis and outbreaks worldwide. Sensitive and specific detection methods are needed to better control V. parahaemolyticus infections. This study aimed at developing a highly specific and sensitive loop-mediated isothermal amplification (LAMP) assay for detecting V. parahaemolyticus in oysters. A set of five LAMP primers, two outer, two inner, and one loop were designed based on the published V. parahaemolyticus toxR sequence. Specificity of the assay was evaluated using a panel of 36 V. parahaemolyticus and 39 other strains. The assay sensitivity was determined using serial dilutions of V. parahaemolyticus ATCC 27969 culture ranging from 10(8) CFU/ml to extinction. The assay was also tested in experimentally inoculated oyster samples.

RESULTS

The toxR-based LAMP assay was able to specifically detect all of the 36 V. parahaemolyticus strains without amplification from 39 other strains. The detection limit was 47-470 cells per reaction in pure culture, up to 100-fold more sensitive than that of toxR-PCR. When applied in spiked oysters, the assay was able to detect 1.1 x 10(5) V. parahaemolyticus cells per gram of oyster without enrichment, up to 100-fold more sensitive than that of toxR-PCR. Standard curves generated for detecting V. parahaemolyticus in both pure culture and spiked oyster samples showed good linear relationship between cell numbers and the fluorescence or turbidity signals.

CONCLUSIONS

The toxR-based LAMP assay developed in this study was sensitive, specific, and quantitative, holding great potential for future field detection of V. parahaemolyticus in raw oysters.

Advanced Technologies for Pathogen and Toxin Detection in Foods: Current Applications and Future Directions

Despite great strides made in the past decades, the detection of microbial pathogens and their toxins in foods remains a challenging task. This is due primarily to several inherent difficulties associated with food analysis, that is, the complexities of food matrices (inhibitors and normal flora), the attributes of target analytes in foods (low level, heterogeneous distribution, and cell injury during processing), and the ratio between the amount of food samples and the detection assay volume. This review aims to provide an overview and a better understanding of the limitations, current applications, and future perspectives in terms of pathogen and toxin detection in foods.