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A highly precise, quantitative method based on the droplet digital polymerase chain reaction (ddPCR) technique was developed to identify and quantify the goat and sheep content in meat products. A formula for calculating raw meat weight based on DNA copy number was established. Exclusive specificity was verified using samples from 24 different animal species, and inclusive specificity between goat and sheep was tested using five different breeds for each species. The limit of detection and the limit of quantitation for both goat and sheep were 1 and 5 copies/μL, respectively, using a cloned plasmid containing goat- and sheep-specific target DNA fragments as calibrators. The accuracy and applicability of the method were verified using mixed powder samples with known proportions of goat and sheep meat, simulate meatball samples, and commercially available products, respectively. The results confirmed that the developed ddPCR methods are highly precise for identifying and quantifying the goat and sheep meat, indicating their potential applicability in future routine analyses.

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.