Development of a simple, rapid, and sensitive molecular diagnostic assay for cholera

Cholera continues to inflict high rates of morbidity and mortality. Prompt identification of cholera cases facilitates rapid outbreak responses in the short term while providing reliable surveillance data to guide long-term policies and interventions. Microbiological stool culture, the current recognized gold standard for diagnosing cholera, has significant limitations. Rapid diagnostic tests (RDTs) represent promising alternatives for diagnosing cholera in areas with limited laboratory infrastructure. However, studies conducted with the current cholera RDTs demonstrated wide variations in sensitivity and specificity. To address this gap in the diagnosis of cholera, we developed a simple, rapid, and sensitive diagnostic assay, "Rapid LAMP based Diagnostic Test (RLDT)." With a novel, simple sample preparation method directly from the fecal samples along with lyophilized reaction strips and using established Loop-mediated Isothermal Amplification (LAMP) platform, cholera toxin gene (ctxA) and O1 (O1rfb) gene could be detected in less than an hour. Cholera RLDT assay is cold chain and electricity-free. To avoid any end-user bias, a battery-operated, handheld reader was used to read the RLDT results. The performance specifications of the cholera RLDT assay, including analytical sensitivity and specificity, were evaluated using direct fecal samples, dried fecal samples on filter paper, and environmental water samples spiked with cholera strain. The limit of detection (LOD) was ~104 CFU/gm of stool for both ctxA and O1 genes, corresponding to about 1 CFU of Vibrio cholerae per reaction within 40 minutes. The LOD was 10 bacteria per ml of environmental water when tested with RLDT directly, without enrichment. Being simple, RLDT has the potential to be applied in resource-poor endemic settings for rapid, sensitive, and reliable diagnosis of cholera.

Rapid and Visualized Detection of Virulence-Related Genes of Vibrio cholerae in Water and Aquatic Products by Loop-Mediated Isothermal Amplification

ABSTRACT

Vibrio cholerae can cause pandemic cholera in humans. The bacterium resides in aquatic environments worldwide. Continuous testing of V. cholerae contamination in water and aquatic products is imperative for food safety control and human health. In this study, a rapid and visualized method was developed for the first time based on loop-mediated isothermal amplification (LAMP) for detection of the important virulence-related genes ace, zot, cri, and nanH for toxins and the infectious process of V. cholerae. Three pairs of molecular probes targeting each of these genes were designed and synthesized. The one-step LAMP reaction was conducted at 65°C for 40 min. Positive results were inspected by the production of a light green color under visible light or green fluorescence under UV light (302 nm). Limit of detection of the LAMP method ranged from 1.85 to 2.06 pg per reaction of genomic DNA or 2.50 × 100 to 4.00 × 102 CFU per reaction for target genes of cell culture of V. cholerae, which was more sensitive than standard PCR. Inclusivity and exclusivity of the LAMP method were 100% for all target genes. The method showed similar high efficiency to a certain extent in rapid testing of spiked or collected specimens of water and aquatic products. Target genes were detected by absence from all water samples from various sources. However, high occurrences of the nanH gene were observed in intestinal samples derived from four species of fish and one species of shellfish, indicating a risk of potentially toxic V. cholerae in commonly consumed aquatic products. The results in this study provide a potential tool for rapid and visualized detection of V. cholerae in water and aquatic products.

HIGHLIGHTS

Loop-Mediated Isothermal Amplification (LAMP) Assay for Rapid Detection of viable but non-culturable Vibrio cholerae O1

Vibrio cholerae, an important waterborne pathogen, is a rod-shaped bacterium that naturally exists in aquatic environments. When conditions are unfavorable for growth, the bacterium can undergo morphological and physiological changes to assume a coccoid morphology. This stage in its life cycle is referred to as viable but non-culturable (VBNC) since VBNC cells do not grow on conventional bacteriological culture media. The current study compared polymerase chain reaction (PCR) and loop-mediated isothermal amplification (LAMP) to detect and identify VBNC V. cholerae. Because it is difficult to detect and identify VBNC V. cholerae, the results of the current study are useful in showing LAMP to be more sensitive and rapid than PCR in detecting and identifying non-culturable, coccoid forms of V. cholerae. Furthermore, the LAMP method is effective in detecting and identifying very low numbers of coccoid VBNC V. cholerae in environmental water samples, with the added benefit of being inexpensive to perform.

Duplex On-Site Detection of Vibrio cholerae and Vibrio vulnificus by Recombinase Polymerase Amplification and Three-Segment Lateral Flow Strips

Vibrio cholerae and Vibrio vulnificus are two most reported foodborne Vibrio pathogens related to seafood. Due to global ocean warming and an increase in seafood consumption worldwide, foodborne illnesses related to infection of these two bacteria are growing, leading to food safety issues and economic consequences. Molecular detection methods targeting species-specific genes are effective tools in the fight against bacterial infections for food safety. In this study, a duplex detection biosensor based on isothermal recombinase polymerase amplification (RPA) and a three-segment lateral flow strip (LFS) has been established. The biosensor used lolB gene of Vibrio cholerae and empV gene of Vibrio vulnificus as the detection markers based on previous reports. A duplex RPA reaction for both targets were constructed, and two chemical labels, FITC and DIG, of the amplification products were carefully tested for effective and accurate visualization on the strip. The biosensor demonstrated good specificity and achieved a sensitivity of 10¹ copies per reaction or one colony forming unit (CFU)/10 g of spiked food for both bacteria. Validation with clinical samples showed results consistent with that of real-time polymerase chain reaction. The detection process was simple and fast with a 30-min reaction at 37 °C and visualization on the strip within 5 min. With little dependence on laboratory settings, this biosensor was suitable for on-site detection, and the duplex system enabled simultaneous detection of the two important foodborne bacteria. Moreover, the principle can be extended to healthcare and food safety applications for other pathogens.

Recent methods and biosensors for foodborne pathogen detection in fish: progress and future prospects to sustainable aquaculture systems

The aquaculture industry has advanced toward sustainable recirculating systems, in where parameters of food quality are strictly monitored. Despite that, as in the case of conventional aquaculture practices, the recirculating systems also suffer threats from Aeromonas spp., Vibrio spp., Streptococcus spp., among other foodborne pathogens infecting farmed fish. The aquaculture pathogens are routinely detected by conventional PCR methods or antibody-based tests, with the detection protocols confined to laboratory use. Emerging assay technologies and biosensors recently reported in the literature open new opportunities to the development of sensitive, specific, and portable analytical devices to use in the field. Techniques of DNA/RNA analysis, immunoassays and other nanomolecular technologies have been facing important advances in response time, sensitivity, and enhanced power of discrimination among and within species. Moreover, the recent developments of electrochemical and optical signal transduction have facilitated the incorporation of the innovative assays to practical miniaturized devices. In this work, it is provided a critical review over foodborne pathogen detection by existing and promising methods and biosensors applied to fish samples and extended to other food matrices. While isothermal DNA/RNA amplification methods can be highlighted among the assay methods for their promising analytical performance and suitability for point-of-care testing, the electrochemical transduction provides a way to achieve cost-effective biosensors amenable to use in the aquaculture field. The adoption of new methods and biosensors would constitute a step forward in securing sustainable aquaculture systems.

Simple Visualized Detection Method of Virulence-Associated Genes of Vibrio cholerae by Loop-Mediated Isothermal Amplification

Vibrio cholerae is a leading waterborne pathogenic bacterium worldwide. It can cause human cholera that is still pandemic in developing nations. Detection of V. cholerae contamination in drinking water and aquatic products is imperative for assuring food safety. In this study, a simple, sensitive, specific, and visualized method was developed based on loop-mediated isothermal amplification (LAMP) (designated sssvLAMP) to detect virulence-associated (ctxA, tcpA, hapA, mshA, pilA, and tlh) and species-specific (lolB) genes of V. cholerae. Three pairs of oligonucleotide primers (inner, outer, and loop primers) were designed and or synthesized to target each of these genes. The optimal conditions of the sssvLAMP method was determined, and one-step sssvLAMP reaction was performed at 65°C for 40 min. Positive results were simply read by the naked eye via color change (from orange to light green) under the visible light, or by the production of green fluorescence under the UV light (260 nm). The sssvLAMP method was more efficient in detecting 6.50 × 101-6.45 × 104-fold low number of V. cholerae cells, and more sensitive in V. cholerae genomic DNA (1.36 × 10-2-4.42 × 10-6 ng/reaction) than polymerase chain reaction (PCR) method. Among 52 strains of V. cholerae and 50 strains of non-target species (e.g., other Vibrios and common pathogens) examined, the sensitivity and specificity of the sssvLAMP method were 100% for all the target genes. Similar high efficiency of the method was observed when tested with spiked samples of water and aquatic products, as well as human stool specimens. Water from various sources and commonly consumed fish samples were promptly screened by this simple and efficient visualized method and diversified variation in the occurrence of the target genes was observed. V. cholerae strains could be mostly detected by the presence of hapA and tlh alone or in combination with other genes, indicating a variable risk of potentially pathogenic non-O1/O139 strains in edible food products. This novel LAMP method can be a promising tool to address the increasing need of food safety control of aquatic products.

Development of a loop-mediated isothermal amplification assay for Vibrio cholerae O1 and O139

A loop-mediated isothermal amplification assay was developed. It was designed for recognizing Vibrio cholerae O1/O139, where atpA, rfbN, and wfbR genes were adopted. The assay specifically detected the target with sensitivities of 5-67 copies per reaction in 1 h. The assay will aid rapid detection of the cholera bacterium.

Development of uracil-DNA-glycosylase-supplemented loop-mediated isothermal amplification coupled with nanogold probe (UDG-LAMP-AuNP) for specific detection of Pseudomonas aeruginosa

Pseudomonas aeruginosa (P. aeruginosa) is an important opportunistic pathogen that causes serious infections in humans, including keratitis in contact lens wearers. Therefore, establishing a rapid, specific and sensitive method for the identification of P. aeruginosa is imperative. In the present study, the uracil-DNA-glycosylase-supplemented loop-mediated isothermal amplification combined with nanogold labeled hybridization probe (UDG-LAMP-AuNP) was developed for the detection of P. aeruginosa. UDG-LAMP was performed to prevent carry over contamination and the LAMP reactions can be readily observed using the nanogold probe. A set of 4 primers and a hybridization probe were designed based on the ecfX gene. The UDG-LAMP reactions were performed at 65°C for 60 min using the ratio of 40% deoxyuridine triphosphate to 60% deoxythymidine triphosphate. The detection of UDG-LAMP products using the nanogold labeled hybridization probe, which appeared as a red-purple color, was examined at 65°C for 5 min with 40 mM MgSO₄. The UDG-LAMP-AuNP demonstrated specificity to all tested isolates of P. aeruginosa without cross reaction to other bacteria. The sensitivity for the detection of pure culture was 1.6×10³ colony-forming units (CFU) ml⁻¹ or equivalent to 3 CFU per reaction while that of polymerase chain reaction was 30 CFU per reaction. The detection limit of spiked contact lenses was 1.1×10³ CFU ml⁻¹ or equivalent to 2 CFU per reaction. In conclusion, the UDG-LAMP-AuNP assay was rapid, simple, specific and was effective for the identification of P. aeruginosa in contaminated samples.

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.

Molecular Characterization of Vibrio cholerae Isolated From Clinical Samples in Kurdistan Province, Iran

Background:

Vibrio cholerae causes diarrhoeal disease that afflicts thousands of people annually. V. cholerae is classified on the basis of somatic antigens into serovars or serogroups and there are at least 200 known serogroup. Two serogroups, O1 and O139 have been associated with epidemic diseases. Virulence genes of these bacteria are OmpW, ctxA and tcpA.

Objectives:

Due to the importance of V. cholerae infection and developing molecular diagnostics of this organism in medical and microbiology sciences, this study aimed to describe molecular characterization of V. cholerae isolated from clinical samples using a molecular method.

Materials and Methods:

In this study, 48 samples were provided during summer 2013 (late August and early September) by reference laboratory. Samples were assessed using biochemical tests initially. The primer of OmpW, ctxA and tcpA genes was used in Polymerase Chain Reaction (PCR) protocols. Enterobacterial Repetitive Intergenic Consensus (ERIC)-PCR and Repetitive Extragenic Palindromic (REP)-PCR methods were used to subtype V. cholerae.

Results:

In this study, from a total of 48 clinical stool samples 39 (81.2 %) were positive for V. cholerae in biochemical tests and bacteria culture tests. The PCR results showed that of 39 positive isolates 35 (89.7%), 34 (87.1%) and 37 (94.8%) were positive for ctxA, tcpA and OmpW gene, respectively. Also, in the REP-PCR method with ERIC primer strains were divided into 10 groups. In the REP-PCR method with REP primer, strains were divided into 13 groups.

Conclusions:

Polymerase chain reaction has specificity and accuracy for identification of the organism and is able to differentiate biotypes. Enterobacterial repetitive intergenic consensus sequence is one of the informative and discriminative methods for the analysis of V. cholerae diversity. The REP-PCR is a less informative and discriminative method compared to other methods for the analysis of V. cholerae diversity.

Detection of Vibrio cholerae by isothermal cross-priming amplification combined with nucleic acid detection strip analysis

Vibrio cholerae is a water- and food-borne human pathogen, and V. cholerae serotypes O1 and O139 have attracted attention because of their severe pathogenesis. However, non-O1, non-O139 cholera vibrios (NCVs) were also recently recognized as having virulence properties. In this study, we developed a cross-priming amplification (CPA) method for the detection of all serotypes of V. cholerae. The specificity of the CPA method was tested using a panel of 60 different bacterial strains. All of the V. cholerae strains showed positive results, and 41 other types of bacteria gave negative results. The limit of detection of the CPA method was 79.28 fg of genomic DNA, 4.2 × 10(2) CFU/ml for bacteria in pure culture, and 5.6 CFU per 25 g of sample with pre-enrichment. This method showed a higher sensitivity than the loop-mediated isothermal amplification (LAMP) method did and was more convenient to perform. These results indicate that the CPA method can be used for the rapid preliminary screening of V. cholerae.

Study of aminoglycoside resistance genes in enterococcus and salmonella strains isolated from ilam and milad hospitals, iran

Background:

Aminoglycosides are a group of antibiotics that have been widely used in the treatment of life-threatening infections of Gram-negative bacteria.

Objectives:

This study aimed to evaluate the frequency of aminoglycoside resistance genes in Enterococcus and Salmonella strains isolated from clinical samples by PCR.

Materials and Methods:

In this study, 140 and 79 isolates of Enterococcus and Salmonella were collected, respectively. After phenotypic biochemical confirmation, 117 and 77 isolates were identified as Enterococcus and Salmonella, respectively. After the biochemical identification of the isolates, antibiotic susceptibility for screening of resistance was done using the Kirby-Bauer method for gentamicin, amikacin, kanamycin, tobramycin and netilmycin. DNA was extracted from resistant strains and the presence of acc (3)-Ia, aac (3′)-Ib, acc (6)-IIa ,16SrRNA methylase genes (armA and rat) was detected by PCR amplification using special primers and positive controls.

Results:

Enterococcus isolates have the highest prevalence of resistance to both kanamycin and amikacin (68.4%), and Salmonella isolates have the highest prevalence of resistance against kanamycin (6.9%). Ninety-three and 26 isolates of Enterococcus and Salmonella at least were resistant against one of the aminoglycosides, respectively. Moreover, 72.04%, 66.7%, and 36.6% of the resistant strains of Enterococcus had the aac (3′)-Ia, aac (3′)-IIa, and acc (6′)-Ib genes, respectively. None of the Salmonella isolates have the studied aminoglycoside genes.

Conclusions:

Our results indicate that acetylation genes have an important role in aminoglycoside resistance of the Enterococcus isolates from clinical samples. Moreover, Salmonella strains indicate very low level of aminoglycoside resistance, and aminoglycoside resistance genes were not found in Salmonella isolates. These results indicate that other resistance mechanisms, including efflux pumps have an important role in aminoglycoside resistance of Salmonella.

Outer membrane protein OmpW is the receptor for typing phage VP5 in the Vibrio cholerae O1 El Tor biotype

Phage typing is used for the subtyping of clones of epidemic bacteria. In this study, we identified the outer membrane protein OmpW as the receptor for phage VP5, one of the typing phages for the Vibrio cholerae O1 El Tor biotype. A characteristic 11-bp deletion in ompW was observed in all epidemic strains resistant to VP5, suggesting that this mutation event can be used as a tracing marker in cholera surveillance.

Evaluation of monoclonal antibody based immunochromatographic strip test for direct detection of Vibrio cholerae O1 contamination in seafood samples

A strip test for the detection of Vibrio cholerae O1 was developed using two monoclonal antibodies (MAbs), VC-223 and VC-1226, specific to the lipopolysaccharides of Vibrio cholerae O1 Inaba and Ogawa serovars. The sensitivity of the test was 5 × 10(5)cfu/mL which was similar to that of dot blot test. The detection limit could be improved to 1cfu/mL of the original bacterial content after pre-incubation of the bacterium in alkaline peptone water (APW) for 12h. Detection of V. cholerae O1 in various fresh seafood samples such as shrimp, blood clam, mussel and oyster could be performed directly with sensitivities ranged from 5 × 10(5) to 10(6)cfu/mL. After pre-enrichment of the shrimp sample in APW, the detection sensitivities increased to 10(2) to 10CFU/mL of the original bacterial content after incubation for 12 and 24h. However, the detection sensitivities were also depending on the content of the other bacteria that might inhibit the growth of V. cholerae during pre-enrichment step. The V. cholerae O1 strip test has advantages in speed, and simplicity in not requiring sophisticated equipment or specialized skills and the sample could be directly examined without requirement for sample processing.

Differentiation among the Vibrio cholerae serotypes O1, O139, O141 and non-O1, non-O139, non-O141 using specific monoclonal antibodies with dot blotting

Seven different monoclonal antibodies (MAbs) specific to only Vibrio cholerae were produced using a combination of five representative serotypes of V. cholerae for immunization. The first three MAbs (VC-93, VC-82 and VC-223) were specific to the V. cholerae serogroup O1 with different avidity for the serotypes O1 Inaba and O1 Ogawa. The fourth and the fifth MAbs were specific to V. cholerae O139 (VC-812) or O141 (VC-191) serogroups, respectively. The sixth MAb (VC-26) bound to all three serogroups of V. cholerae. The seventh MAb (VC-63) bound to all twenty five isolates of V. cholerae used in this study. None of the seven MAbs showed cross-reactivity with other Vibrio spp. or closely-related V. cholerae species, V. mimicus or other gram-negative bacteria. The eighth MAbs (VC-201) specific to almost all Vibrio spp. was also obtained. In dot blotting, these MAbs can be used to detect a diluted pure culture of V. cholerae in solution with a sensitivity range of from 10(5) to 10(7) CFU ml(-1). However, the detection capability could be improved equivalent to that of PCR technique after preincubation of samples in alkaline peptone water (APW). Thus, these MAbs constitute convenient immunological tools that can be used for simple, rapid and simultaneous direct detection and differentiation of the individual serotypes of V. cholerae in complex samples, such as food and infected animals, without the requirement for bacterial isolation or biochemical characterization.

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.

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.