Genetically modified (GM) cotton lines have been approved for commercialization and widely cultivated in many countries, especially in China. As a step towards the development of reliable qualitative and quantitative PCR methods for detecting GM cottons, we report here the validation of the cotton (Gossypium hirsutum) endogenous reference control gene, Sad1, using conventional and real-time (RT)-PCR methods. Both methods were tested on 15 different G. hirsutum cultivars, and identical amplicons were obtained with all of them. No amplicons were observed when DNA samples from three species of genus Gossypium, Arabidopsis thaliana, maize, and soybean and others were used as amplified templates, demonstrating that these two systems are specific for the identification and quantification of G. hirsutum. The results of Southern blot analysis also showed that the Sad1 gene was two copies in these 15 different G. hirsutum cultivars. Furthermore, one multiplex RT-quantitative PCR employing this gene as an endogenous reference gene was designed to quantify the Cry1A(c) gene modified from Bacillus thuringiensis (Bt) in the insect-resistant cottons, such as Mon531 and GK19. The quantification detection limit of the Cry1A(c) and Sad1 genes was as low as 10 pg of genomic DNA. These results indicate that the Sad1 gene can be used as an endogenous reference gene for both qualitative and quantitative PCR detection of GM cottons.

We describe the development of a novel combined approach for high-throughput analysis of multiple DNA targets based on multiplex Microdroplet PCR Implemented Capillary gel electrophoresis (MPIC), a two-step PCR amplification strategy. In the first step, the multiple target DNAs are preamplified using bipartite primers attached with universal tail sequences on their 5'-ends. Then, the preamplified templates are compartmentalized individually in the microdroplet of the PCR system, and multiple targets can be amplified in parallel, employing primers targeting their universal sequences. Subsequently, the resulting multiple products are analyzed by capillary gel electrophoresis (CGE). Using genetically modified organism (GMO) analysis as a model, 24 DNA targets can be simultaneously detected with a relative limit of detection of 0.1% (w/w) and absolute limit of detection of 39 target DNA copies. The described system provides a promising alternative for high-throughput analysis of multiple DNA targets.

In this project, a highly precise quantitative method based on the digital polymerase chain reaction (dPCR) technique was developed to determine the weight of pork and chicken in meat products. Real-time quantitative polymerase chain reaction (qPCR) is currently used for quantitative molecular analysis of the presence of species-specific DNAs in meat products. However, it is limited in amplification efficiency and relies on standard curves based Ct values, detecting and quantifying low copy number target DNA, as in some complex mixture meat products. By using the dPCR method, we find the relationships between the raw meat weight and DNA weight and between the DNA weight and DNA copy number were both close to linear. This enabled us to establish formulae to calculate the raw meat weight based on the DNA copy number. The accuracy and applicability of this method were tested and verified using samples of pork and chicken powder mixed in known proportions. Quantitative analysis indicated that dPCR is highly precise in quantifying pork and chicken in meat products and therefore has the potential to be used in routine analysis by government regulators and quality control departments of commercial food and feed enterprises.

Meat products often consist of meat from multiple animal species, and inaccurate food product adulteration and mislabeling can negatively affect consumers. Therefore, a cost-effective and reliable method for identification and quantification of animal species in meat products is required. In this study, we developed a duplex droplet digital PCR (dddPCR) detection and quantification system to simultaneously identify and quantify the source of meat in samples containing a mixture of beef (Bos taurus) and pork (Sus scrofa) in a single digital PCR reaction tube. Mixed meat samples of known composition were used to test the accuracy and applicability of this method. The limit of detection (LOD) and the limit of quantification (LOQ) of this detection and quantification system were also identified. We conclude that our dddPCR detection and quantification system is suitable for quality control and routine analyses of meat products.

No abstract

One tomato ( Lycopersicon esculentum) gene, LAT52, has been proved to be a suitable endogenous reference gene for genetically modified (GM) tomato detection in a previous study. Herein are reported the results of a collaborative ring trial for international validation of the LAT52 gene as endogenous reference gene and its analytical systems; 14 GMO detection laboratories from 8 countries were invited, and results were finally received from 13. These data confirmed the species specificity by testing 10 plant genomic DNAs, less allelic variation and stable single copy number of the LAT52 gene, among 12 different tomato cultivars. Furthermore, the limit of detection of LAT52 qualitative PCR was proved to be 0.1%, which corresponded to 11 copies of haploid tomato genomic DNA, and the limit of quantification for the quantitative PCR system was about 10 copies of haploid tomato genomic DNA with acceptable PCR efficiency and linearity. Additionally, the bias between the test and true values of 8 blind samples ranged from 1.94 to 10.64%. All of these validated results indicated that the LAT52 gene is suitable for use as an endogenous reference gene for the identification and quantification of GM tomato and its derivates.

Reference materials are important in accurate analysis of genetically modified organism (GMO) contents in food/feeds, and development of novel reference plasmid is a new trend in the research of GMO reference materials. Herein, we constructed a novel multi-targeting plasmid, pSOY, which contained seven event-specific sequences of five GM soybeans (MON89788-5', A2704-12-3', A5547-127-3', DP356043-5', DP305423-3', A2704-12-5', and A5547-127-5') and sequence of soybean endogenous reference gene Lectin. We evaluated the specificity, limit of detection and quantification, and applicability of pSOY in both qualitative and quantitative PCR analyses. The limit of detection (LOD) was as low as 20 copies in qualitative PCR, and the limit of quantification (LOQ) in quantitative PCR was 10 copies. In quantitative real-time PCR analysis, the PCR efficiencies of all event-specific and Lectin assays were higher than 90%, and the squared regression coefficients (R(2)) were more than 0.999. The quantification bias varied from 0.21% to 19.29%, and the relative standard deviations were from 1.08% to 9.84% in simulated samples analysis. All the results demonstrated that the developed multi-targeting plasmid, pSOY, was a credible substitute of matrix reference materials, and could be used as a reliable reference calibrator in the identification and quantification of multiple GM soybean events.

Salmonella is the most common cause of bacterial food poisoning in humans worldwide. Thus, rapid and reliable methods for the detection of this pathogen are required. Real-time reverse-transcriptase polymerase chain reaction (rt-RT-PCR), which detects the presence of mRNA (shorter half-life than DNA) has shown great potential for detecting viable pathogens. We recently identified a few new potential specific DNA sequences for Salmonella enterica using a comparative genomics method (Chen et al., 2010). In the present study, we examined the expression of the Salmonella-specific sigDE operon (encoding invasion proteins within the pathogenicity island 5) under typical growth conditions to determine whether sigDE could be a useful viability marker for the bacterium. We then assayed sigDE mRNA from cells heat-treated at 60°C, 100°C, and 121°C (autoclaved), and found that mRNA was degraded in autoclaved bacterial samples. These results showed that the sigDE transcript is a suitable mRNA target for rt-RT-PCR with samples pretreated at 121°C. Thus, an rt-RT-PCR using sigDE primers was developed for the detection of viable Salmonella. An RNA internal amplification control was constructed by overlap extension PCR, synthesized using in vitro transcription with a T7 RNA polymerase promoter, and incorporated into the rt-RT-PCR system to eliminate false-negative results. The rt-RT-PCR system has the capability of specifically detecting all the tested S. enterica serovars, and the detection limit of this assay with cultures of Salmonella Typhimurium ATCC 13311 was 10(1) colony-forming units (CFU)/mL. After 18-h enrichment, sigDE-based rt-RT-PCR could detect as low as 10(0) CFU/mL of Salmonella from egg broth and milk.