Evaluation of DNA Extraction Methods for Use in Combination with SYBR Green I Real-Time PCR To Detect Salmonella enterica Serotype Enteritidis in Poultry

The objective of this study was to develop a rapid, reproducible, and robust method for detecting Salmonella enterica serotype Enteritidis in poultry samples. First, for the extraction and purification of DNA from the preenrichment culture, four methods (boiling, alkaline lysis, Nucleospin, and Dynabeads DNA Direct System I) were compared. The most effective method was then combined with a real-time PCR method based on the double-stranded DNA binding dye SYBR Green I used with the ABI Prism 7700 system. The specificity of the reaction was determined by the melting temperature (T_m) of the amplicon obtained. The experiments were conducted both on samples of chicken experimentally contaminated with serotype Enteritidis and on commercially available poultry samples, which were also used for comparisons with the standard cultural method (i.e., ISO 6579/2001). The results of comparisons among the four DNA extraction methods showed significant differences except for the results from the boiling and Nucleospin methods (the two methods that produced the lowest threshold cycles). Boiling was selected as the preferred extraction method because it is the simplest and most rapid. This method was then combined with SYBR Green I real-time PCR, using primers SEFA-1 and SEFA-2. The specificity of the reaction was confirmed by the T_m, which was consistently specific for the amplicon obtained; the mean peak T_m obtained with curves specific for serotype Enteritidis was 82.56 ± 0.22°C. The standard curve constructed using the mean threshold cycle and various concentrations of serotype Enteritidis (ranging from 10³ to 10⁸ CFU/ml) showed good linearity (R² = 0.9767) and a sensitivity limit of less than 10³ CFU/ml. The results of this study demonstrate that the SYBR Green I real-time PCR constitutes an effective and easy-to-perform method for detecting serotype Enteritidis in poultry samples.     

Quantitative detection of Listeria monocytogenes in biofilms by real-time PCR

A quantitative method based on a real-time PCR assay to enumerate Listeria monocytogenes in biofilms was developed. The specificity for L. monocytogenes of primers targeting the listeriolysin gene was demonstrated using a SYBR Green I real-time PCR assay. The number of L. monocytogenes detected growing in biofilms was 6 x 10(2) CFU/cm2.     

Quantitative PCR for glucose transporter and tristetraprolin family gene expression in cultured mouse adipocytes and macrophages

Quantitative real-time PCR (qPCR) such as TaqMan and SYBR Green qPCR are widely used for gene expression analysis. The drawbacks of SYBR Green assay are that the dye binds to any double-stranded DNA which can generate false-positive signals and that the length of the amplicon affects the intensity of the amplification. Previous results demonstrate that TaqMan assay is more sensitive but generates lower calculated expression levels than SYBR Green assay in quantifying seven mRNAs in tung tree tissues. The objective of this study is to expand the analysis using animal cells. We compared both qPCR assays for quantifying 24 mRNAs including those coding for glucose transporter (Glut) and mRNA-binding protein tristetraprolin (TTP) in mouse 3T3-L1 adipocytes and RAW264.7 macrophages. The results showed that SYBR Green and TaqMan qPCR were reliable for quantitative gene expression in animal cells. This result was supported by validation analysis of Glut and TTP family gene expression. However, SYBR Green qPCR overestimated the expression levels in most of the genes tested. Finally, both qPCR instruments (Bio-Rad’s CFX96 real-time system and Applied Biosystems’ Prism 7700 real-time PCR instrument) generated similar gene expression profiles in the mouse cells. These results support the conclusion that both qPCR assays (TaqMan and SYBR Green qPCR) and both qPCR instruments (Bio-Rad’s CFX96 real-time system and Applied Biosystems’ Prism 7700 real-time PCR instrument) are reliable for quantitative gene expression analyses in animal cells but SYBR Green qPCR generally overestimates gene expression levels than TaqMan qPCR.     

粪便中肠球菌SYBR GreenI荧光定量PCR检测方法的建立

目的利用SYBR GreenI荧光定量PCR方法,建立肠球菌实时荧光PCR检测方法,并初步应用于粪便中肠球菌的检测。方法根据GenBank发表的肠球菌23S rRNA基因序列的保守区域设计合成特异性的引物;利用构建的质粒标准品绘制两种标准曲线,构建基因拷贝数、细菌数为分析指标的定量分析模型并初步应用于粪便标本的检测分析。结果所建立的SYBR GreenI荧光定量PCR方法检测灵敏度可达7个拷贝数/reaction。粪便样本根据实时荧光定量PCR方法所得的理论数值与培养菌值之间差异无显著性(P>0.05)。非炎性腹泻标本中菌数与健康成人标本中菌数差异无显著性(P>0.05)。灵敏度曲线所得的数值大于菌数标准曲线,可能由于DNA提取过程中存在部分的损失。检测粪便标本结果显示SYBR GreenI荧光定量PCR方法较平板计数法敏感、快捷、简便。结论本研究建立了一种灵敏、特异、简便易行的肠球菌定量检测方法。     

Comparison of the SYBR Green and the hybridization probe format for real-time PCR detection of HHV-6

A comparative study was conducted of a novel real-time quantitative PCR test (LightCycler System) with FastStart DNA Master(PLUS) SYBR Green I dye to detect DNA of human herpes virus 6 (HHV-6). Results were compared with those of a real-time quantitative PCR with hybridization probe (HP) formats using the fluorescence resonance energy transfer method, and with those of a single qualitative PCR test. The detection limit of the test with SYBR Green I dye was 20 copies of the virus, similar to that of the other two tests. The reproducibility was satisfactory. The new test has the same advantages as real-time PCR with HP formats and offers a greater versatility at lower cost.     

Direct detection of bacterial pathogens in representative dairy products using a combined bacterial concentration-PCR approach

AIMS: To develop a simple, rapid method to concentrate and purify bacteria and their nucleic acids from complex dairy food matrices in preparation for direct pathogen detection using polymerase chain reaction (PCR). METHODS AND RESULTS: Plain non-fat yogurt and cheddar cheese were each seeded with Listeria monocytogenes or Salmonella enterica serovar. Enteritidis in the range of 10(1)-10(6) CFU per 11-g sample. Samples were then processed for bacterial concentration using high-speed centrifugation (9700 g) followed by DNA extraction, PCR amplification, and amplicon confirmation by hybridization. Bacterial recoveries after centrifugation ranged from 53 to >100% and 71 to >100% for serovar. Enteritidis and L. monocytogenes, respectively, in the non-fat yogurt samples; and from 77 to >100% and 69 to >100% for serovar. Enteritidis and L. monocytogenes, respectively, in the cheddar cheese samples. There were no significant differences in recovery efficiency at different inocula levels, and losses to discarded supernatants were always <5%, regardless of dairy product or pathogen. CONCLUSIONS: When followed by pathogen detection using PCR and confirmation by amplicon hybridization, detection limits of 10(3) and 10(1) CFU per 11-g sample were achieved for L. monocytogenes and serovar. Enteritidis, respectively, in both product types and without prior cultural enrichment. SIGNIFICANCE AND IMPACT OF THE STUDY: This study represents progress toward the rapid and efficient direct detection of pathogens from complex food matrices at detection limits approaching those that might be anticipated in naturally contaminated products.     

Amplification efficiency of thermostable DNA polymerases

The amplification efficiencies of several polymerase chain reaction (PCR) enzymes were compared using real-time quantitative PCR with SYBR Green I detection. Amplification data collected during the exponential phase of PCR are highly reproducible, and PCR enzyme performance comparisons based upon efficiency measurements are considerably more accurate than those based on endpoint analysis. DNA polymerase efficiencies were determined under identical conditions using five different amplicon templates that varied in length or percentage GC content. Pfu- and Taq-based formulations showed similar efficiencies when amplifying shorter targets (<900 bp) with 45 to 56% GC content. However, when amplicon length or GC content was increased, Pfu formulations with dUTPase exhibited significantly higher efficiencies than Taq, Pfu, and other archaeal DNA polymerases. We discuss the implications of these results.     

Detection of periodontal pathogen Porphyromonas gingivalis by loop-mediated isothermal amplification method

A method for nucleic acid amplification, loop-mediated isothermal amplification (LAMP) was employed to develop a rapid and simple detection system for periodontal pathogen, Porphyromonas gingivalis. A set of six primers was designed by targeting the 16S ribosomal RNA gene. By the detection system, target DNA was amplified and visualized on agarose gel within 30 min under isothermal condition at 64 degrees C with a detection limit of 20 cells of P. gingivalis. Without gel electrophoresis, the LAMP amplicon was directly visualized in the reaction tube by addition of SYBR Green I for a naked-eye inspection. The LAMP reaction was also assessed by white turbidity of magnesium pyrophosphate (a by-product of LAMP) in the tube. Detection limits of these naked-eye inspections were 20 cells and 200 cells, respectively. Although false-positive DNA amplification was observed from more than 10(7) cells of Porphyromonas endodontalis, no amplification was observed in other five related oral pathogens. Further, quantitative detection of P. gingivalis was accomplished by a real-time monitoring of the LAMP reaction using SYBR Green I with linearity over a range of 10(2)-10(6) cells. The real-time LAMP was then applied to clinical samples of dental plaque and demonstrated almost identical results to the conventional real-time PCR with an advantage of rapidity. These findings indicate the potential usefulness of LAMP for detecting and quantifying P. gingivalis, especially in its rapidity and simplicity.     

PCR-BASED FLUORESCENT METHOD FOR RAPID DETECTION OF SALMONELLA TYPHIMURIUM IN POULTRY SAMPLES

A DNA binding fluorescence method based on polymerase chain reaction (PCR) products was evaluated for rapid detection of Salmonella Typhimurium in poultry products. Wash water samples of chicken carcasses and ground turkey were inoculated with S. Typhimurium to obtain final concentrations of 10° - 10⁵ CFU/mL. One mL of each sample was used to get the DNA template and 5 μL of the sample template was added into 25 μL of SYBR Green PCR Master Mix and two specific Salmonella ompC gene primers. The negative control was the same except 5 μL of each wash solution was added instead of 5 μL sample template. The reaction was carried out in a thermocycler. Finally, the fluorescence signal of each PCR product was measured using a fluorometer. The PCR products were also confirmed by ethidium bromide agarose gel, and the DNA concentrations of the PCR products were measured by a filter fluorescence photometer. The results showed that when bacterial cells increased from 0 to 2 CFU/mL, the fluorescence signal increased significantly. The PCR-based fluorescence method could detect the target bacteria in minutes after PCR amplification compared to hours by gel electrophoresis and also could be done at an earlier time during PCR amplification. The detection limit of this method for S. Typhimurium in the poultry samples was 2 CFU/mL without any enrichment.     

Quantitative detection of Proteus species by real-time polymerase chain reaction using SYBR Green I

A SYBR Green real-time polymerase chain reaction (PCR) method for rapid detection of Proteus species was developed and evaluated. Of 322 clinical and food samples tested, 75 samples were positive for Proteus species by using conventional PCR and real-time PCR assays. The results were consistent with standard culture methods and the Vitek auto-microbe system, indicating a 100 % specificity obtained by both PCR assays. For the real-time PCR method, the minimum detectable level was 10 colony forming units (CFU) /ml, which was a 10³ multiple higher than the conventional PCR method. Correlation coefficients of standard curves which were constructed using the threshold cycle (Ct) versus copy numbers of Proteus showed good linearity (R ²?=?0.997). In conclusion, several significant advantages such as higher sensitivity and rapidness were observed by using the SYBR Green real-time PCR method for identifying Proteus species.     

Comparison of SYTO9 and SYBR Green I for real-time polymerase chain reaction and investigation of the effect of dye concentration on amplification and DNA melting curve analysis

Following the initial report of the use of SYBR Green I for real-time polymerase chain reaction (PCR) in 1997, little attention has been given to the development of alternative intercalating dyes for this application. This is surprising considering the reported limitations of SYBR Green I, which include limited dye stability, dye-dependent PCR inhibition, and selective detection of amplicons during DNA melting curve analysis of multiplex PCRs. We have tested an alternative to SYBR Green I and report the first detailed evaluation of the intercalating dye SYTO9. Our findings demonstrate that SYTO9 produces highly reproducible DNA melting curves over a broader range of dye concentrations than does SYBR Green I, is far less inhibitory to PCR than SYBR Green I, and does not appear to selectively detect particular amplicons. The low inhibition and high melting curve reproducibility of SYTO9 means that it can be readily incorporated into a conventional PCR at a broad range of concentrations, allowing closed tube analysis by DNA melting curve analysis. These features simplify the use of intercalating dyes in real-time PCR and the improved reproducibility of DNA melting curve analysis will make SYTO9 useful in a diagnostic context.     

Comparison of methods of extracting Salmonella enterica serovar Enteritidis DNA from environmental substrates and quantification of organisms by using a general internal procedural control

This paper compares five commercially available DNA extraction methods with respect to DNA extraction efficiency of Salmonella enterica serovar Enteritidis from soil, manure, and compost and uses an Escherichia coli strain harboring a plasmid expressing green fluorescent protein as a general internal procedural control. Inclusion of this general internal procedural control permitted more accurate quantification of extraction and amplification of S. enterica serovar Enteritidis in these samples and reduced the possibility of false negatives. With this protocol it was found that the optimal extraction method differed for soil (Mobio soil DNA extraction kit), manure (Bio101 soil DNA extraction kit), and compost (Mobio fecal DNA extraction kit). With each method, as little as 1.2 x 10(3) to 1.8 x 10(3) CFU of added serovar Enteritidis per 100 mg of substrate could be detected by direct DNA extraction and subsequent S. enterica-specific TaqMan PCR. After bacterial enrichment, as little as 1 CFU/100 mg of original substrate was detected. Finally, the study presents a more accurate molecular analysis for quantification of serovar Enteritidis initially present in soil or manure using DNA extraction and TaqMan PCR.     

不同DNA抽提方法对普通PCR和实时荧光定量PCR方法检测家蚕微孢子虫的影响

为了优选快速、 灵敏、 特异的家蚕微孢子虫Nosema bombycis分子检测方法和DNA抽提方法, 本文通过对家蚕微孢子虫TaqMan探针荧光定量PCR检测方法和SYBR Green荧光定量PCR检测方法的建立以及反应体系优化, 并与普通PCR方法进行比较; 再采用4种不同DNA抽提方法分别对PCR和实时荧光定量PCR方法检测家蚕微孢子虫悬浮液的效果评价。结果显示: 不经过DNA抽提, 直接将家蚕微孢子虫发芽液进行PCR反应的效果优于其他方法, 检测灵敏度由高到低依次为直接法、 酚/氯仿抽提法、 动物组织DNA试剂盒抽提法和植物组织DNA试剂盒抽提法; TaqMan探针法检测家蚕微孢子虫发芽液的灵敏度和SYBR Green法相近, 达到微孢子10²个/mL, 两者均优于普通PCR方法。实验表明, 直接采用发芽液结合荧光定量PCR方法检测家蚕微孢子虫最为简便、 快速、 灵敏。该研究结果将有助于提高家蚕微粒子病监控技术和检疫能力, 对家蚕微粒子病的检疫和防治具有积极意义。     

Direct quantification of fungal DNA from soil substrate using real-time PCR

Detection and quantification of genomic DNA from two ecologically different fungi, the plant pathogen Fusarium solani f. sp. phaseoli and the arbuscular mycorrhizal fungus Glomus intraradices, was achieved from soil substrate. Specific primers targeting a 362-bp fragment from the SSU rRNA gene region of G. intraradices and a 562-bp fragment from the F. solani f. sp. phaseoli translation elongation factor 1 alpha gene were used in real-time polymerase chain reaction (PCR) assays conjugated with the fluorescent SYBR(R) Green I dye. Standard curves showed a linear relation (r(2)=0.999) between log values of fungal genomic DNA of each species and real-time PCR threshold cycles and were quantitative over 4-5 orders of magnitude. Real-time PCR assays were applied to in vitro-produced fungal structures and sterile and non-sterile soil substrate seeded with known propagule numbers of either fungi. Detection and genomic DNA quantification was obtained from the different treatments, while no amplicon was detected from non-seeded non-sterile soil samples, confirming the absence of cross-reactivity with the soil microflora DNA. A significant correlation (P<0.0001) was obtained between the amount of genomic DNA of F. solani f. sp. phaseoli or G. intraradices detected and the number of fungal propagules present in seeded soil substrate. The DNA extraction protocol and real-time PCR quantification assay can be performed in less than 2 h and is adaptable to detect and quantify genomic DNA from other soilborne fungi.     

Quantitative detection for low levels of Helicobacter pylori infection in experimentally infected mice by real-time PCR

Accurate diagnosis of Helicobacter pylori infection is important in both clinical practice and clinical research. Molecular methods are highly specific and sensitive, and various PCR-based tests have been developed to detect H. pylori in gastric biopsy specimens. We optimized a sensitive and specific quantitative SYBR Green I real-time PCR assay for detection of H. pylori based on amplification of the fragment of a 26-kDa Helicobacter species-specific antigen gene that allows for detection of 5 bacterial cells per PCR sample. Under the assay conditions, SYBR Green I real-time PCR is highly reproducible with a precise log-linear relation in the range of six orders of magnitude of bacterial DNA concentrations. For accurate comparison of H. pylori infection in different tissue samples, the amount of total host DNA in each sample is normalized by TaqMan real-time PCR of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) pseudogenes. The developed method was validated in prophilactically immunized and experimentally infected mice and revealed a level of H. pylori gastric colonisation that was below the limit of detection for a rapid urease test. This new method established for a quantitative analysis of H. pylori in the host's stomach may be useful in experimental studies evaluating new anti-H. pylori drugs and vaccines.     

基于SYBR Green I的双链DNA定量方法

摘要 基于SYBR Green I荧光染料与双链DNA（dsDNA）结合产生荧光的原理,建立一种高精度、高通量的双链DNA 定量方法。将梯度稀释后的基因组DNA及已知浓度的?DNA与等体积的SYBR Green I（4×）充分混合后,利用荧光定量PCR仪采集荧光信号,以ROX（1×）作为校正染料进行定量分析;同时利用紫外分光光度计对样品进行平行测定,比较该方法与紫外分光光度法的检测限与准确度。紫外分光光度法的检测限为2 ng/?l,而SYBR Green I荧光定量法的检测限可达到0.015 ng/?l,并且在0.015~2 ng/?l范围内,SYBR Green I荧光强度与?DNA浓度呈线性关系（R2=0.9999）,比紫外分光光度法灵敏100倍以上,并可准确定量低纯度的DNA样品。此方法具有重复性好、高通量的特点,仅需少量的生物样本即可满足定量要求,为分子生物学研究及临床检验等多个领域提供了一种可靠的dsDNA定量方法。     

DNA melting analysis: application of the "open tube" format for detection of mutant KRAS

High-resolution melting (HRM) analysis is a very effective method for genotyping and mutation scanning that is usually performed just after PCR amplification (the “closed tube” format). Though simple and convenient, the closed tube format makes the HRM dependent on the PCR mix, not generally optimal for DNA melting analysis. Here, the “open tube” format, namely the post-PCR optimization procedure (amplicon shortening and solution chemistry modification), is proposed. As a result, mutation scanning of short amplicons becomes feasible on a standard real-time PCR instrument (not primarily designed for HRM) using SYBR Green I. This approach has allowed us to considerably enhance the sensitivity of detecting mutant KRAS using both low- and high-resolution systems (the Bio-Rad iQ5–SYBR Green I and Bio-Rad CFX96–EvaGreen, respectively). The open tube format, though more laborious than the closed tube one, can be used in situations when maximal sensitivity of the method is needed. It also permits standardization of DNA melting experiments and the introduction of instruments of a “lower level” into the range of those suitable for mutation scanning.     

Detection of the Escherichia coli pathogenic gene eae with three real-time polymerase chain reaction methods

Several real-time polymerase chain reaction (PCR) methods are currently available to rapidly detect the presence of a specific DNA sequence. When used for detection of pathogenic organisms, the turnaround time for PCR-based methods is much lower than for traditional culture techniques. This study compared the sensitivity of three real-time PCR methods when detecting the Escherichia coli pathogenic gene eae to determine which method is most effective in identifying very low levels of the organism. The three methods were used to detect the eae gene over a range of DNA concentrations. The differences in sensitivity were statistically significant (p<0.05), and SYBR Green I PCR was found to have the lowest detection limit of the three; LUX primers had the highest detection limit. Therefore, using a defined DNA concentration for detecting the eae gene, SYBR Green I is the best alternative.