Absolute quantification of Vibrio parahaemolyticus by multiplex droplet digital PCR for simultaneous detection of tlh, tdh and ureR based on single intact cell

Recent Progress in Nanomaterial-Based Fluorescence Assays for the Detection of Food-Borne Pathogens

Food safety is of great concern, and food-borne bacterial infections and diseases are a major crisis for health. Therefore, it is necessary to develop rapid detection techniques for the prevention and recognition of food safety hazards caused by food-borne pathogens. In recent years, the fluorescence assay has become a widely utilized detection method due to its good signal amplification effect, high detection sensitivity, high stability, and short detection time. In this review, the different kinds of fluorescence materials were concentrated, including quantum dots (QDs), carbon dots (CDs), metal-organic frameworks (MOFs), and upconversion nanoparticles (UCNPs). The optical properties and applications of different kinds of fluorescent materials were analyzed and compared. Furthermore, according to the biosensing components, different fluorescence biosensors are reviewed, including label-free based fluorescence probes, aptamer-based biosensors, and antibody-based biosensors. Finally, we focused our attention on the discussion of fluorescent detection techniques combined with other techniques and their applications. The review presents future trends in fluorescence sensors, providing new sights for the detection of food-borne pathogens.

Mutation-Selected Amplification droplet digital PCR: A new single nucleotide variant detection assay for TP53R249S mutant in tumor and plasma samples

The early detection of gene mutations in physiological and pathological processes is a powerful approach to guide decisions in precision medicine. However, detecting low-copy mutant DNA from clinical samples poses a challenge due to the enrichment of wild-type DNA backgrounds. In this study, we devised a novel strategy, named Mutation-Selected Amplification droplet digital PCR (MSA-ddPCR), to quantitatively analyze single nucleotide variants (SNVs) at low variant allele frequencies (VAFs). Using TP53R249S (a hotspot mutation associated with hepatocellular carcinoma) as a model, we optimized the concentration ratio of primers, the annealing temperature and nucleic acid amplification modifiers. Subsequently, we evaluated the linear range and precision of MSA-ddPCR by detecting TP53R249S and TP53wild-type (TP53WT) plasmid DNA, respectively. MSA-ddPCR demonstrated superior ability to discriminate between mutant DNA and wild-type DNA compared to traditional TaqMan-MGB PCR. We further applied MSA-ddPCR to analyze the TP53R249S mutation in 20 plasma samples and 15 formalin-fixed paraffin-embedded (FFPE) tissue samples, and assessed the agreement rates between MSA-ddPCR and amplicon high-throughput sequencing. The results showed that the limit of blanks of MSA-ddPCR are 0.449 copies μL-1 in the FAM channel and 0.452 copies μL-1 in the VIC channel. MSA-ddPCR could accurately quantify VAFs as low as 0.01 %, surpassing existing PCR and next-generation sequencing (NGS) methods. In the detection of clinical samples, a high correlation was found between MSA-ddPCR and amplicon high-throughput sequencing. Additionally, MSA-ddPCR outperformed sequencing methods in terms of detection time and simplicity of data analysis. MSA-ddPCR can be easily implemented into clinical practice and serve as a robust tool for detecting mutant genes due to its high sensitivity and accuracy.

A novel phagomagnetic separation-ATP bioluminescence (PhMS-BL) for rapid and sensitive detection of viable Vibrio parahaemolyticus in aquatic product

Detection of viable Vibrio parahaemolyticus (V. parahaemolyticus) is a major challenge due to its significant risk to food safety and human health. Herein, we developed a phagomagnetic separation-ATP bioluminescence (PhMS-BL) assay based on phage VPHZ6 for rapid and sensitive detection of viable V. parahaemolyticus. Phage as a recognition element was coupled to magnetic beads to capture and enrich V. parahaemolyticus, shortening detection time and improving method sensitivity. The intracellular ATP released by chemical lysis using CTAB was quantified using firefly fluorescein-adenosine triphosphate bioluminescence system to detect viable bacteria. So, PhMS-BL method was able to detect V. parahaemolyticus in a linear range of 2.3 × 102 to 1.3 × 107 CFU mL-1, with a detection limit of 78 CFU mL-1 within 15 min. It is successfully applied to detect V. parahaemolyticus in spiked lake water, lobster tail meat, and clam meat. The developed detection strategy can rapidly and sensitively detect viable V. parahaemolyticus in food matrixes.

Detection of non-pathogenic and pathogenic populations of Vibrio parahaemolyticus in various samples by the conventional, quantitative and droplet digital PCRs

In this study, three generations of polymerase chain reaction (PCR) assays: (i) conventional PCR, (ii) qPCR and (iii) droplet digital PCR (ddPCR), were systematically tested for their abilities to detect non-pathogenic and pathogenic populations of Vibrio parahaemolyticus. The limit of detection (LOD) for the ddPCR was 1.1 pg/µL of purified DNA, followed by the qPCR (5.6 pg/µL) and the conventional PCR (8.8 pg/µL). Regarding the LOD for V. parahaemolyticus cells, the ddPCR assay was able to detect 29 cells, followed by the conventional PCR assay (58 cells) and the qPCR assay (115 cells). Regarding the sensitivities to detect this pathogen from PCR inhibition prone samples (naturally contaminated mussels), the ddPCR assay significantly outperformed the conventional PCR and qPCR. The ddPCR assay was able to consistently detect non-pathogenic and pathogenic populations of V. parahaemolyticus from naturally contaminated mussels, indicating its tolerance to various PCR inhibitors. This study also revealed the significant difference between conventional PCR and qPCR. The conventional PCR assay showed significantly greater sensitivity than that of the qPCR assay in detecting V. parahaemolyticus in crude samples, whereas the qPCR assay showed better sensitivity in detecting the presence of V. parahaemolyticus in purified DNA samples.

Biological Function of Prophage-Related Gene Cluster ΔVpaChn25_RS25055~ΔVpaChn25_0714 of Vibrio parahaemolyticus CHN25

Vibrio parahaemolyticus is the primary foodborne pathogen known to cause gastrointestinal infections in humans. Nevertheless, the molecular mechanisms of V. parahaemolyticus pathogenicity are not fully understood. Prophages carry virulence and antibiotic resistance genes commonly found in Vibrio populations, and they facilitate the spread of virulence and the emergence of pathogenic Vibrio strains. In this study, we characterized three such genes, VpaChn25_0713, VpaChn25_0714, and VpaChn25_RS25055, within the largest prophage gene cluster in V. parahaemolyticus CHN25. The deletion mutants ΔVpaChn25_RS25055, ΔVpaChn25_0713, ΔVpaChn25_0714, and ΔVpaChn25_RS25055-0713-0714 were derived with homologous recombination, and the complementary mutants ΔVpaChn25_0713-com, ΔVpaChn25_0714-com, ΔVpaChn25_RS25055-com, ΔVpaChn25_RS25055-0713-0714-com were also constructed. In the absence of the VpaChn25_RS25055, VpaChn25_0713, VpaChn25_0714, and VpaChn25_RS25055-0713-0714 genes, the mutants showed significant reductions in low-temperature survivability and biofilm formation (p < 0.001). The ΔVpaChn25_0713, ΔVpaChn25_RS25055, and ΔVpaChn25_RS25055-0713-0714 mutants were also significantly defective in swimming motility (p < 0.001). In the Caco-2 model, the above four mutants attenuated the cytotoxic effects of V. parahaemolyticus CHN25 on human intestinal epithelial cells (p < 0.01), especially the ΔVpaChn25_RS25055 and ΔVpaChn25_RS25055-0713-0714 mutants. Transcriptomic analysis showed that 15, 14, 8, and 11 metabolic pathways were changed in the ΔVpaChn25_RS25055, ΔVpaChn25_0713, ΔVpaChn25_0714, and ΔVpaChn25_RS25055-0713-0714 mutants, respectively. We labeled the VpaChn25_RS25055 gene with superfolder green fluorescent protein (sfGFP) and found it localized at both poles of the bacteria cell. In addition, we analyzed the evolutionary origins of the above genes. In summary, the prophage genes VpaChn25_0713, VpaChn25_0714, and VpaChn25_RS25055 enhance V. parahaemolyticus CHN25's survival in the environment and host. Our work improves the comprehension of the synergy between prophage-associated genes and the evolutionary process of V. parahaemolyticus.

End-point rapid detection of total and pathogenic Vibrio parahaemolyticus (tdh+ and/or trh1+ and/or trh2+) in raw seafood using a colorimetric loop-mediated isothermal amplification-xylenol orange technique

Background

Vibrio parahaemolyticus is the leading cause of bacterial seafood-borne gastroenteritis in humans worldwide. To ensure seafood safety and to minimize the occurrence of seafood-borne diseases, early detection of total V. parahaemolyticus (pathogenic and non-pathogenic strains) and pathogenic V. parahaemolyticus (tdh+ and/or trh1+ and/or trh2+) is required. This study further improved a loop-mediated isothermal amplification (LAMP) assay using xylenol orange (XO), a pH sensitive dye, to transform conventional LAMP into a one-step colorimetric assay giving visible results to the naked eye. LAMP-XO targeted rpoD for species specificity and tdh, trh1, and trh2 for pathogenic strains. Multiple hybrid inner primers (MHP) of LAMP primers for rpoD detection to complement the main primer set previously reported were designed by our group to maximize sensitivity and speed.

Methods

Following the standard LAMP protocol, LAMP reaction temperature for rpoD, tdh, trh1, and trh2 detection was first determined using a turbidimeter. The acquired optimal temperature was subjected to optimize six parameters including dNTP mix, betaine, MgSO4, Bst 2.0 WarmStart DNA polymerase, reaction time and XO dye. The last parameter was done using a heat block. The color change of the LAMP-XO result from purple (negative) to yellow (positive) was monitored visually. The detection limits (DLs) of LAMP-XO using a 10-fold serial dilution of gDNA and spiked seafood samples were determined and compared with standard LAMP, PCR, and quantitative PCR (qPCR) assays. Subsequently, the LAMP-XO assay was validated with 102 raw seafood samples and the results were compared with PCR and qPCR assays.

Results

Under optimal conditions (65 °C for 75 min), rpoD-LAMP-XO and tdh-LAMP-XO showed detection sensitivity at 102 copies of gDNA/reaction, or 10 folds greater than trh1-LAMP-XO and trh2-LAMP-XO. This level of sensitivity was similar to that of standard LAMP, comparable to that of the gold standard qPCR, and 10-100 times higher than that of PCR. In spiked samples, rpoD-LAMP-XO, tdh-LAMP-XO, and trh2-LAMP-XO could detect V. parahaemolyticus at 1 CFU/2.5 g spiked shrimp. Of 102 seafood samples, LAMP-XO was significantly more sensitive than PCR (P < 0.05) for tdh and trh2 detection and not significantly different from qPCR for all genes determined. The reliability of tdh-LAMP-XO and trh2-LAMP-XO to detect pathogenic V. parahaemolyticus was at 94.4% and 100%, respectively.

Conclusions

To detect total and pathogenic V. parahaemolyticus, at least rpoD-LAMP-XO and trh2-LAMP-XO should be used, as both showed 100% sensitivity, specificity, and accuracy. With short turnaround time, ease, and reliability, LAMP-XO serves as a better alternative to PCR and qPCR for routine detection of V. parahaemolyticus in seafood. The concept of using a one-step LAMP-XO and MHP-LAMP to enhance efficiency of diagnostic performance of LAMP-based assays can be generally applied for detecting any gene of interest.

Engineering of an adaptive tandem CRISPR/Cas12a molecular amplifier permits robust analysis of Vibrio parahaemolyticus

Seeking new molecular diagnostic method for pathogenic bacteria detection is of utmost importance for ensuring food safety and protecting human health. Herein, we have engineered an adaptive tandem CRISPR/Cas12a molecular amplifier specifically designed for robust analysis of vibrio parahaemolyticus (V. parahaemolyticus), one of the most harmful pathogens. Our strategy involves the integration of three crucial processes: recombinase polymerase amplification (RPA) for copy number amplification, terminal deoxynucleotidyl transferase (TdT) for template-free strand elongation, and CRISPR/Cas12a-mediated trans-cleavage of a reporter molecule. By combining these processes, the target genomic DNA extracted from V. parahaemolyticus is able to activate many CRISPR/Cas12a units (CRISPR/Cas12an) simultaneously, resulting in a greatly amplified target signal to indicate the presence and concentration of V. parahaemolyticus. This unique model offers more advantages compared to traditional amplification models that use one RPA amplicon to activate one CRISPR/Cas12a unit. Under optimized conditions, our method enables the detection of target V. parahaemolyticus within a linear range of 1 × 102-1 × 107 CFU/mL, with an impressive limit of detection as low as 12.4 CFU/mL. It is conceivable that the adaptive tandem CRISPR/Cas12a molecular amplifier could be adapted as routine diagnostic kits in future for in-field detection of pathogens.

Rapid detection methods for foodborne pathogens based on nucleic acid amplification: Recent advances, remaining challenges, and possible opportunities

This article presents a review of recent advancements in the utilization of NAA-based techniques for detecting foodborne pathogens in food products, focusing on studies conducted within the past five years. This review revealed that recent research efforts have primarily aimed at enhancing sensitivity and specificity by improving sample pre-treatment/preparation, DNA isolation, and readout methods. Isothermal-based amplification methods, such as LAMP, RPA, RAA, and RCA, have emerged as promising approaches, providing rapid results within one h and often demonstrating comparable or superior sensitivity to conventional or qPCR methods. However, the attention paid to specific pathogens varies, with Salmonella spp., Listeria spp., E. coli, and V. parahaemolyticus receiving more focus than norovirus and other similar pathogens. NAA-based methods have the potential to significantly contribute to food safety and public health protection. However, further advancements are necessary to fully realize their benefits.

Enhancement of loop mediated isothermal amplification's sensitivity and speed by multiple inner primers for more efficient identification of Vibrio parahaemolyticus

The modified loop-mediated isothermal amplification (LAMP), called multiple hybrid, inner primers (MHP)-LAMP, was developed to enhance the efficiency of the existing LAMP-based assay for Vibrio parahaemolyticus detection. The method was built on a conventional LAMP assay by employing 2 newly designed extra sets of primers to increase the initial binding sites of core primers on the V. parahaemolyticus's rpoD gene from 8 to 12. With this strategy, the assay detection sensitivity was increased by 10 folds, with the detection limit (DL) approaching 100 copies of purified target genomic DNA (gDNA) as analyzed by real-time turbidity measurement and gel electrophoresis. The MHP also accelerated the rate of DNA amplification by 30%, rendering the assay faster. The MHP-LAMP assay did not cross- react with other pathogens, indicating that it was highly specific for V. parahaemolyticus detection. Whilst V. parahaemolyticus was used as a study model herein, our idea of using MHP to maximize assay sensitivity and speed is considered as a universal strategy that can be applied to enhance efficiency of LAMP-based assays for detecting any DNA and RNA of interest. •The strategy of using multiple hybrid, inner primers (MHP) to enhance LAMP assay's efficiency was demonstrated with success.•The MHP enhanced the sensitivity and speed of the existing LAMP assay, designed to detect V. parahaemolyticus, by 10 times and 30%, respectively.•The proposed strategy can be applied to boost up any other LAMP-based assay's diagnostic performance.

Droplet digital PCR method for the absolute quantitative detection and monitoring of Lacticaseibacillus casei

Droplet digital polymerase chain reaction (ddPCR) is an emerging molecular detection assay that provides an absolute quantification of targets. Despite its emerging applications in the detection of food microorganisms, there are limited reports of its use for the monitoring of microorganisms utilized as starters in the dairy industry. This study investigated the applicability of ddPCR as a detection platform for Lacticaseibacillus casei, a probiotic found in fermented foods and exerts beneficial effects on human health. In addition, this study compared the performance of ddPCR with that of real-time PCR. The ddPCR targeting the haloacid dehalogenase-like hydrolase (LBCZ_1793) exhibited high specificity against 102 nontarget bacteria, including Lacticaseibacillus species that is very closely related to L. casei. The ddPCR exhibited high linearity and efficiency within the quantitation range (10⁵-10⁰ CFU/ml), with the limit of detection being 10⁰ CFU/ml. The ddPCR also demonstrated a higher sensitivity than real-time PCR in detecting low bacterial concentration in spiked milk samples. Furthermore, it provided an accurate absolute quantification of the concentration of L. casei, without the need for standard calibration curves. This study demonstrated that ddPCR is a useful method for monitoring starter cultures in dairy fermentations and detecting L. casei in foods.

Ultrasensitive and Rapid Visual Detection of Escherichia coli O157:H7 Based on RAA-CRISPR/Cas12a System

Escherichia coli (E. coli) O157:H7 is a major foodborne and waterborne pathogen that can threaten human health. Due to its high toxicity at low concentrations, it is crucial to establish a time-saving and highly sensitive in situ detection method. Herein, we developed a rapid, ultrasensitive, and visualized method for detecting E. coli O157:H7 based on a combination of Recombinase-Aided Amplification (RAA) and CRISPR/Cas12a technology. The CRISPR/Cas12a-based system was pre-amplified using the RAA method, which showed high sensitivity and enabled detecting as low as ~1 CFU/mL (fluorescence method) and 1 × 10² CFU/mL (lateral flow assay) of E. coli O157:H7, which was much lower than the detection limit of the traditional real-time PCR technology (10³ CFU/mL) and ELISA (10⁴~10⁷ CFU/mL). In addition, we demonstrated that this method still has good applicability in practical samples by simulating the detection in real milk and drinking water samples. Importantly, our RAA-CRISPR/Cas12a detection system could complete the overall process (including extraction, amplification, and detection) within 55 min under optimized conditions, which is faster than most other reported sensors, which take several hours to several days. The signal readout could also be visualized by fluorescence generated with a handheld UV lamp or a naked-eye-detected lateral flow assay depending on the DNA reporters used. Because of the advantages of being fast, having high sensitivity, and not requiring sophisticated equipment, this method has a promising application prospect for in situ detection of trace amounts of pathogens.

Surveillance of Vibrio parahaemolyticus pathogens recovered from ready-to-eat foods

This study examined the occurrence of V. parahaemolyticus from ready-to-eat (RTE) food in Delta State, Nigeria. It also characterized antibiotic resistance and virulence gene profile patterns to determine the associated health risk hazard. Food samples total of 380 were collected randomly and assessed for V. parahaemolyticus. V. parahaemolyticus isolates were characterized for their virulence and antibiogram potentials using a phenotypic and polymerase chain reaction (PCR) approach. A total of 42 (11.1%) samples were contaminated with V. parahaemolyticus. In 17/42 (40.5%) of the V. parahaemolyticus-positive samples, the densities were < 10 MPN/g. However, 19/42 (45.2%) and 6/42 (14.3%) of the samples had densities of 10 - 10² and > 10² MPN/g, respectively. A total of 67 V. parahaemolyticus isolates were identified using PCR; 54(80.6%) isolates were multidrug resistant. A total of 22 (32.8%), 39 (58.2%), and 67 (100%) of the V. parahaemolyticus harbored the tdh, trh, and tlh toxin genes, respectively. The T3SS1 gene (vcrD1) was detected in 67 (100%) of the isolates. The T3SS2α genes which were vcrD2, vopB2, and vopT were detected in 21 (31.3%), 11 (16.4%) and 30 (44.8%) of the isolates respectively. Some of the V. parahaemolytics strains harbored the orf8 gene 20 (29.9%), and a combination of orf8 + tdh genes 12 (17.9%), categorized as pandemic strains. The antibiotic resistance genes detected in this study include bla_TEM 33 (49.3), tetM 19 (28.4), cmlA 32(47.8) and sul1 14 (20.9). The concentration levels and prevalence of V. parahaemolyticus in RTE foods indicate contamination of ready-to-eat foods, particularly street foods consumed in the Delta State of Nigeria, threatening public health and consumer safety.

Comparison of RT-qPCR and RT-ddPCR with Rift valley fever virus (RVFV) RNA

Rift valley fever (RVF) is an important zoonotic disease caused by the Rift valley fever virus (RVFV) which can affect ruminants and humans. In this study, a comparison was done of the reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and reverse transcription-droplet digital PCR (RT-ddPCR) assays with synthesized RVFV RNA, cultured viral RNA, and mock clinical RVFV RNA samples. The genomic segments (L, M, and S) of three RVFV strains (BIME01, Kenya56, and ZH548) were synthesized and used as templates for in vitro transcription (IVT). Both the RT-qPCR and RT-ddPCR assays for RVFV did not react with any of the negative reference viral genomes. Thus, both the RT-qPCR and RT-ddPCR assays are specific to RVFV. The comparison of both the RT-qPCR and RT-ddPCR assays with serially diluted templates showed that the LoD of both assays are similar, and a concordant of the results was observed. The LoD of both assays reached the practical measurable minimum concentration. Taken altogether, the sensitivity of the RT-qPCR and RT-ddPCR assays is similar, and the material measured by RT-ddPCR can be used as a reference material for RT-qPCR.

Development of the Droplet Digital PCR Method for the Detection and Quantification of Erwinia pyrifoliae

Black shoot blight disease caused by Erwinia pyrifoliae has serious impacts on quality and yield in pear production in Korea; therefore, rapid and accurate methods for its detection are needed. However, traditional detection methods require a great deal of time and fail to achieve absolute quantification. In the present study, we developed a droplet digital polymerase chain reaction (ddPCR) method for the detection and absolute quantification of E. pyrifoliae using a pair of species-specific primers. The detection range was 103 - 107 copies/ml (DNA templates) and cfu/ml (cell culture templates). This new method exhibited good linearity and repeatability and was validated by absolute quantification of E. pyrifoliae DNA copies from samples of artificially inoculated immature pear fruits. Here, we present the first study of ddPCR assay for the detection and quantification of E. pyrifoliae. This method has potential applications in epidemiology and for the early prediction of black shoot blight outbreaks.

Application of digital PCR for public health-related water quality monitoring

Digital polymerase chain reaction (dPCR) is emerging as a reliable platform for quantifying microorganisms in the field of water microbiology. This paper reviews the fundamental principles of dPCR and its application for health-related water microbiology. The relevant literature indicates increasing adoption of dPCR for measuring fecal indicator bacteria, microbial source tracking marker genes, and pathogens in various aquatic environments. The adoption of dPCR has accelerated recently due to increasing use for wastewater surveillance of Severe Acute Respiratory Coronavirus 2 (SARS-CoV-2) - the virus that causes Coronavirus Disease 2019 (COVID-19). The collective experience in the scientific literature indicates that well-optimized dPCR assays can quantify genetic material from microorganisms without the need for a calibration curve and often with superior analytical performance (i.e., greater sensitivity, precision, and reproducibility) than quantitative polymerase chain reaction (qPCR). Nonetheless, dPCR should not be viewed as a panacea for the fundamental uncertainties and limitations associated with measuring microorganisms in water microbiology. With dPCR platforms, the sample analysis cost and processing time are typically greater than qPCR. However, if improved analytical performance (i.e., sensitivity and accuracy) is critical, dPCR can be an alternative option for quantifying microorganisms, including pathogens, in aquatic environments.

Research progress on detection techniques for point-of-care testing of foodborne pathogens

The global burden of foodborne disease is enormous and foodborne pathogens are the leading cause of human illnesses. The detection of foodborne pathogenic bacteria has become a research hotspot in recent years. Rapid detection methods based on immunoassay, molecular biology, microfluidic chip, metabolism, biosensor, and mass spectrometry have developed rapidly and become the main methods for the detection of foodborne pathogens. This study reviewed a variety of rapid detection methods in recent years. The research advances are introduced based on the above technical methods for the rapid detection of foodborne pathogenic bacteria. The study also discusses the limitations of existing methods and their advantages and future development direction, to form an overall understanding of the detection methods, and for point-of-care testing (POCT) applications to accurately and rapidly diagnose and control diseases.

An Assay Combining Droplet Digital PCR With Propidium Monoazide Treatment for the Accurate Detection of Live Cells of Vibrio vulnificus in Plasma Samples

Vibrio vulnificus (V. vulnificus) is one of the most common pathogenic Vibrio species to humans; therefore, the establishment of timely and credible detection methods has become an urgent requirement for V. vulnificus illness surveillance. In this study, an assay combining droplet digital PCR (ddPCR) with propidium monoazide (PMA) treatment was developed for detecting V. vulnificus. The primers/probes targeting the V. vulnificus hemolysin A (vvhA) gene, amplification procedures, and PMA processing conditions involved in the assay were optimized. Then, we analyzed the specificity, sensitivity, and ability to detect live cell DNA while testing the performance of PMA-ddPCR in clinical samples. The optimal concentrations of primers and probes were 1.0 and 0.3 μM, respectively. The annealing temperature achieving the highest accuracy in ddPCR assay was 60°C. With an initial V. vulnificus cell concentration of 10⁸ CFU/mL (colony-forming units per milliliter), the optimal strategy to distinguish live cells from dead cells was to treat samples with 100 μM PMA for 15 min in the dark and expose them to LED light with an output wavelength of 465 nm for 10 min. The specificity of the PMA-ddPCR assay was tested on 27 strains, including seven V. vulnificus strains and 20 other bacterial strains. Only the seven V. vulnificus strains were observed with positive signals in specificity analysis. Comparative experiments on the detection ability of PMA-ddPCR and PMA-qPCR in pure cultures and plasma samples were performed. The limit of detection (LOD) and the limit of quantitation (LOQ) in pure culture solutions of V. vulnificus were 29.33 and 53.64 CFU/mL in PMA-ddPCR, respectively. For artificially clinical sample tests in PMA-ddPCR, V. vulnificus could be detected at concentrations as low as 65.20 CFU/mL. The sensitivity of the PMA-ddPCR assay was 15- to 40-fold more sensitive than the PMA-qPCR in this study. The PMA-ddPCR assay we developed provides a new insight to accurately detect live cells of V. vulnificus in clinical samples, which is of great significance to enhance public health safety and security capability and improve the emergency response level for V. vulnificus infection.

CE-RAA-CRISPR Assay: A Rapid and Sensitive Method for Detecting Vibrio parahaemolyticus in Seafood

Vibrio parahaemolyticus is one of the major pathogenic Vibrio species that contaminate seafood. Rapid and accurate detection is crucial for avoiding foodborne diseases caused by pathogens and is important for food safety management and mariculture. In this study, we established a system that combines chemically enhanced clustered regularly interspaced short palindromic repeats (CRISPR) and recombinase-aided amplification (RAA) (CE–RAA–CRISPR) for detecting V. parahaemolyticus in seafood. The method combines RAA with CRISPR-associated protein 12a (Cas12a) for rapid detection in a one-pot reaction, effectively reducing the risk of aerosol contamination during DNA amplifier transfer. We optimized the primers for V. parahaemolyticus, determined the optimal crRNA/Cas12a ratio, and demonstrated that chemical additives (bovine serum albumin and L-proline) could enhance the detection capacity of Cas12a. The limit of detection (at optimal conditions) was as low as 6.7 × 10¹ CFU/mL in pure cultures and 7.3 × 10¹ CFU/g in shrimp. Moreover, this method exhibited no cross-reactivity with other microbial pathogens. The CE–RAA–CRISPR assay was compared with the quantitative polymerase chain reaction assay using actual food samples, and it showed 100% diagnostic agreement.

Comparison of Real-Time PCR and Droplet Digital PCR for the Quantitative Detection of Lactiplantibacillus plantarum subsp. plantarum

Droplet digital polymerase chain reaction (ddPCR) is one of the newest and most promising tools providing absolute quantification of target DNA molecules. Despite its emerging applications in microorganisms, few studies reported its use for detecting lactic acid bacteria. This study evaluated the applicability of a ddPCR assay targeting molecular genes obtained from in silico analysis for detecting Lactiplantibacillus plantarum subsp. plantarum, a bacterium mainly used as a starter or responsible for fermentation in food. The performance characteristics of a ddPCR were compared to those of a quantitative real-time PCR (qPCR). To compare the linearity and sensitivity of a qPCR and ddPCR, the calibration curve for a qPCR and the regression curve for a ddPCR were obtained using genomic DNA [102−108 colony-forming units (CFU)/mL] extracted from a pure culture and spiked food sample. Both the qPCR and ddPCR assays exhibited good linearity with a high coefficient of determination in the pure culture and spiked food sample (R2 ≥ 0.996). The ddPCR showed a 10-fold lower limit of detection, suggesting that a ddPCR is more sensitive than a qPCR. However, a ddPCR has limitations in the absolute quantitation of high bacterial concentrations (>106 CFU/mL). In conclusion, a ddPCR can be a reliable method for detecting and quantifying lactic acid bacteria in food.

Virtual-Partition Digital PCR for High-Precision Chromosomal Counting Applications

Digital PCR (dPCR) is the gold-standard analytical platform for rapid high-precision quantification of genomic fragments. However, current dPCR assays are generally limited to monitoring 1-2 analytes per sample, thereby limiting the platform's ability to address some clinical applications that require the simultaneous monitoring of 20-50 analytes per sample. Here, we present virtual-partition dPCR (VPdPCR), a novel analysis methodology enabling the detection of 10 or more target regions per color channel using conventional dPCR hardware and workflow. Furthermore, VPdPCR enables dPCR instruments to overcome upper quantitation limits caused by partitioning error. While traditional dPCR analysis establishes a single threshold to separate negative and positive partitions, VPdPCR establishes multiple thresholds to identify the number of unique targets present in each positive droplet based on fluorescence intensity. Each physical partition is then divided into a series of virtual partitions, and the resulting increase in partition count substantially decreases partitioning error. We present both a theoretical analysis of the advantages of VPdPCR and an experimental demonstration in the form of a 20-plex assay for noninvasive fetal aneuploidy testing. This demonstration assay─tested on 432 samples contrived from sheared cell-line DNA at multiple input concentrations and simulated fractions of euploid or trisomy-21 "fetal" DNA─is analyzed using both traditional dPCR thresholding and VPdPCR. VPdPCR analysis significantly lowers the variance of the chromosomal ratio across replicates and increases the accuracy of trisomy identification when compared to traditional dPCR, yielding > 98% single-well sensitivity and specificity. VPdPCR has substantial promise for increasing the utility of dPCR in applications requiring ultrahigh-precision quantitation.

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.

A rapid nucleic acid concentration measurement system with large field of view for a droplet digital PCR microfluidic chip

Droplet digital polymerase chain reaction (ddPCR) is an effective technique, with unparalleled sensitivity, for the absolute quantification of target nucleic acids. However, current commercial ddPCR devices for detecting the gene chip are time consuming due to complex image stitching. To address this issue, we propose a universal concentration determination system and realize one-time gene chip imaging with high resolution. All the functional units are controlled by self-developed software using the PyQt5 module in Python. Without stitching technology, images of the ddPCR chip (28 mm × 18 mm) containing 20 000 independent 0.81 nL micro chambers can be obtained in less than 15 seconds, which saves about 165 seconds. A white laser light source (2 mW cm^-2) was employed as a substitute for the mercury lamp. Its wavelength matches well with typical fluorescent dyes (e.g., HEX, ROX and Cy5), and thus it can strengthen the fluorescence intensity for weak signals. The results also demonstrated that the correlation coefficient for the measured concentration and theoretical value was above 99%, by testing the ddPCR products with COVID-19 virus. Such a system can greatly reduce the time required for image acquisition and DNA concentration determination, and thus is able to speed up the lab-to-application process for ddPCR technology.

Multiplexed digital polymerase chain reaction as a powerful diagnostic tool

The digital polymerase chain reaction (dPCR) multiplexing method can simultaneously detect and quantify closely related deoxyribonucleic acid sequences in complex mixtures. The dPCR concept is continuously improved by the development of microfluidics and micro- and nanofabrication, and different complex techniques are introduced. In this review, we introduce dPCR techniques based on sample compartmentalization, droplet- and chip-based systems, and their combinations. We then discuss dPCR multiplexing methods in both laboratory research settings and advanced or routine clinical applications. We focus on their strengths and weaknesses with regard to the character of biological samples and to the required precision of such analysis, as well as showing recently published work based on those methods. Finally, we envisage possible future achievements in this field.

Quantitative detection of trace VBNC Cronobacter sakazakii by immunomagnetic separation in combination with PMAxx-ddPCR in dairy products

One immunomagnetic separation (IMS) assay based on immunomagnetic beads (IMBs) has been evaluated as a potential pretreatment tool for the separation and enrichment of target bacteria. In this study, we successfully immobilized antibodies onto magnetic bead surfaces to form IMBs through biotin and a streptavidin (SA) system to capture viable but nonculturable (VBNC) Cronobacter sakazakii (C. sakazakii) from dairy products. Various parameters that affected the capture efficiency (CE) of IMS, including the number of antibodies, IMBs dose, incubation time, magnetic separation time, and immunoreaction temperature, were systematically investigated. We further determined the optimal enrichment conditions for different dairy substrates to ensure maximum enrichment of target pathogens in the system. An IMS technique combining improved propidium monoazide (PMAxx) and droplet digital PCR (ddPCR) was established to detect the pathogenic VBNC C. sakazakii. The IMS-PMAxx-ddPCR method after IMBs enrichment showed higher accuracy when the VBNC C. sakazakii was under 1 Log₁₀ copies/g. The detection limit for this method in a background of powdered infant formula (PIF) was 5.6 copies/g. In summary, the developed IMS-PMAxx-ddPCR method has great potential for the analysis and detection of VBNC bacteria in food.

Evaluation of loop-mediated isothermal amplification assay along with conventional and real-time PCR assay for sensitive detection of pathogenic Vibrio parahaemolyticus from seafood sample without enrichment

The primary reason for foodborne illness is improper seafood safety testing, and hence, an appropriate tool for testing is the key to control the outbreaks. The current study aimed to develop a loop-mediated isothermal amplification (LAMP) assay to detect pathogenic Vibrio parahaemolyticus, important foodborne pathogen, targeting tdh, and trh genes. The specificity of the LAMP assay was good without any false-positive and false-negative results. The assay was highly sensitive and could detect the pathogenic V. parahaemolyticus as low as 1 CFU/reaction in spiked seafood samples and 1 pg of extracted DNA. Out of 62 seafood samples from India’s southwest coastal region tested with LAMP assay, eight (12.9%) were positive for trh, and seven (11.29%) samples were positive tdh gene. LAMP-based on tdh and trh was found to be significantly more sensitive (p < 0.05) than conventional PCR and nearly equal sensitive as real-time PCR (RT-PCR) for the detection of pathogenic V. parahaemolyticus. Our study shows that LAMP assay can be a better approach as a point-of-care (POC) diagnostic tool and could detect pathogenic V. parahaemolyticus on seafood samples directly without enrichment and isolation. The high sensitivity and simplicity make LAMP assay a better alternative method than the conventional method and RT-PCR for the detection of pathogens. LAMP assay can be considered as a good alternative to PCR for the routine detection of pathogenic V. parahaemolyticus in seafood.

A 4-plex Droplet Digital PCR Method for Simultaneous Quantification and Differentiation of Pathogenic and Non-pathogenic Vibrio parahaemolyticus Based on Single Intact Cells

Vibrio parahaemolyticus is a significant seafood-borne pathogen, leading to serious acute gastrointestinal diseases worldwide. In this study, a reliable 4-plex droplet digital PCR (ddPCR) was successfully established and evaluated for the simultaneous detection of V. parahaemolyticus based on tlh, tdh, ureR, and orf8 in food samples using single intact cells. The targets tlh and ureR were labeled with 6-Carboxyfluorescein (FAM), and the targets tdh and orf8 were labeled with 5’-Hexachlorofluorescein (HEX). Due to reasonable proration of primers and probes corresponding into the two fluorescence channels of the ddPCR detecting platforms, the clearly separated 16 (2⁴) clusters based on fluorescence amplitude were obtained. For better results, the sample hot lysis time and the cycle number were optimized. The results showed that the minimum number of “rain” and maximum fluorescence amplification were presented for precise detection in the condition of 25 min of the sample hot lysis time and 55 cycles. The sensitivity of this 4-plex ddPCR assay was 39 CFU/mL, which was in accordance with that of the conventional plate counting and was 10-fold sensitive than that of qPCR. In conclusion, the 4-plex ddPCR assay presented in this paper was a rapid, specific, sensitive, and accurate tool for the detection of V. parahaemolyticus including pandemic group strains and could be applied in the differentiation of V. parahaemolyticus in a wide variety of samples.

Rapid visual detection of Vibrio parahaemolyticus in seafood samples by loop-mediated isothermal amplification with hydroxynaphthol blue dye

The detection and monitoring of Vibrio parahaemolyticus pathogen in aquatic foods have become essential for preventing outbreaks. In this study, loop-mediated isothermal amplification (LAMP) assay with the azo dye, hydroxynaphthol blue (HNB) was developed targeting species-specific tlh gene. The assay was carried out on 62 seafood samples that included clam and shrimp and compared with conventional LAMP assay performed with the commonly used fluorescent dye, conventional PCR, and real-time PCR (RT-PCR). Of 62 samples studied for tlh gene, 32 (51.61%) gave positive by HNB-LAMP, which comprised 22 (70.96%) clam samples and 10 (32.25%) shrimp samples. The HNB-LAMP assay was found to be highly sensitive, specific, and superior to conventional PCR (p > 0.05). RT-PCR presented higher sensitivity than HNB-LAMP; however, it has the limitation of being cost-intensive and requiring technical expertise to perform. HNB-LAMP is affordable, rapid, simple, and easy to perform, allowing naked eye visualization.