Multiplex Real-Time Polymerase Chain Reaction Assay for Simultaneous Detection and Quantification ofSalmonellaSpecies,Listeria monocytogenes, andEscherichia coliO157:H7 in Ground Pork Samples

A New Peptide Nucleic Acid Fluorescence In Situ Hybridization Probe for the Specific Detection of Salmonella Species in Food Matrices

Salmonella spp. is among the most central etiological agents in foodborne bacterial disorders. To identify Salmonella spp., numerous new molecular techniques have been developed conversely to the traditional culture-based methods. In this work, a new peptide nucleic acid fluorescence in situ hybridization (PNA-FISH) method was developed for the specific detection of Salmonella species, allowing a faster analysis compared with the traditional methods (ISO 6579-1: 2017). The method was optimized based on a novel PNA probe (SalPNA1692) combined with a blocker probe to detect Salmonella in food samples through an assessment of diverse-rich and selective enrichment broths. Our findings indicated that the best outcome was obtained using a 24-h pre-enrichment step in buffered peptone water, followed by RambaQuick broth selective enrichment for 16 h. For the enrichment step performance validation, fresh ground beef was artificially contaminated with two ranges of concentration of inoculum: a low level (0.2-2 colony-forming units [CFUs]/25 g) and a high level (2-10 CFUs/25 g). The new PNA-FISH method presented a specificity of 100% and a detection limit of 0.5 CFU/25 g of food sample, which confirms the great potential of applying PNA probes in food analysis.

Detection of Salmonella enterica and Listeria monocytogenes in alternative irrigation water by culture and qPCR-based methods in the Mid-Atlantic U.S

Alternative irrigation waters (rivers, ponds, and reclaimed water) can harbor bacterial foodborne pathogens like Salmonella enterica and Listeria monocytogenes, potentially contaminating fruit and vegetable commodities. Detecting foodborne pathogens using qPCR-based methods may accelerate testing methods and procedures compared to culture-based methods. This study compared detection of S. enterica and L. monocytogenes by qPCR (real-time PCR) and culture methods in irrigation waters to determine the influence of water type (river, pond, and reclaimed water), season (winter, spring, summer, and fall), or volume (0.1, 1, and 10 L) on sensitivity, accuracy, specificity, and positive (PPV), and negative (NPV) predictive values of these methods. Water samples were collected by filtration through modified Moore swabs (MMS) over a 2-year period at 11 sites in the Mid-Atlantic U.S. on a bi-weekly or monthly schedule. For qPCR, bacterial DNA from culture-enriched samples (n = 1,990) was analyzed by multiplex qPCR specific for S. enterica and L. monocytogenes. For culture detection, enriched samples were selectively enriched, isolated, and PCR confirmed. PPVs for qPCR detection of S. enterica and L. monocytogenes were 68% and 67%, respectively. The NPV were 87% (S. enterica) and 85% (L. monocytogenes). Higher levels of qPCR/culture agreement were observed in spring and summer compared to fall and winter for S. enterica; for L. monocytogenes, lower levels of agreement were observed in winter compared to spring, summer, and fall. Reclaimed and pond water supported higher levels of qPCR/culture agreement compared to river water for both S. enterica and L. monocytogenes, indicating that water type may influence the agreement of these results.

IMPORTANCE

Detecting foodborne pathogens in irrigation water can inform interventions and management strategies to reduce risk of contamination and illness associated with fresh and fresh-cut fruits and vegetables. The use of non-culture methods like qPCR has the potential to accelerate the testing process. Results indicated that pond and reclaimed water showed higher levels of agreement between culture and qPCR methods than river water, perhaps due to specific physiochemical characteristics of the water. These findings also show that season and sample volume affect the agreement of qPCR and culture results. Overall, qPCR methods could be more confidently utilized to determine the absence of Salmonella enterica and Listeria monocytogenes in irrigation water samples examined in this study.

Immunofluorescent-aggregation assay based on anti-Salmonella typhimurium IgG-AuNCs, for rapid detection of Salmonella typhimurium

Sensitive and rapid detection of pathogenic bacteria plays an important role in avoiding food poisoning. However, the practical application value of conventional assays for detection of foodborne bacteria, are limited by major drawbacks; these include the laboriousness of pure culture preparation, complexity of DNA extraction for polymerase chain reaction, and low sensitivity of enzyme-linked immunosorbent assay. Herein, we designed a non-complex strategy for the sensitive, quantitative, and rapid detection of Salmonella typhimurium with high specificity, using an anti-Salmonella typhimurium IgG-AuNC-based immunofluorescent-aggregation assay. Salmonella typhimurium was agglutinated with fluorescent anti-Salmonella typhimurium IgG-AuNC on a glass slide, and observed using a fluorescence microscope with photoexcitation and photoemission at 560 nm and 620 nm, respectively. Under optimized reaction conditions, the AuNC-based immunofluorescent-aggregation assay had a determination range between 7.0 × 10³ and 3.0 × 10⁸ CFU/mL, a limit of detection of 1.0 × 10³ CFU/mL and an assay response time of 3 min. The technique delivered good results in assessing real samples.

Rapid Detection of Single Viable Escherichia coli O157 Cells in Fresh Lettuce and Strawberry by Immunomagnetic Flow Cytometry in Combination with Pre-Enrichment

Enterohemorrhagic Escherichia coli are an important pathogen causing food poisoning. The rapid detection of viable E. coli O157 in vegetables and fruits at single-cell level is critical because of the low infective dose of this pathogen. In this study, an immunomagnetic flow cytometry (IMFC)-based method was developed to detect E. coli O157 in lettuce and strawberries inoculated with 1 CFU/25 g. This method developed immunomagnetic (IM)-beads to capture E. coli O157 cells. The pre-enrichment of E. coli O157 and IM-bead separation rapidly increased the concentration of cells to a detectable range for flow cytometry. Compared with the plate-based method, the diagnostic sensitivity and specificity of the IMFC-based method were 100% in 166 samples, including 100 artificially contaminated samples, 60 retail samples, and six O157-positive samples for proficiency testing. The developed IMFC-based method was found to be effective in detecting E. coli O157 at single-cell level in 25 g of lettuce or strawberry with relatively shorter associated time to results of 5.7 h. Therefore, the IMFC-based method could improve detection efficiency and also make early warnings in a short time.

Levels of Salmonella enterica and Listeria monocytogenes in Alternative Irrigation Water Vary Based on Water Source on the Eastern Shore of Maryland

Irrigation water sources have been shown to harbor foodborne pathogens and could contribute to the outbreak of foodborne illness related to consumption of contaminated produce. Determining the probability of and the degree to which these irrigation water sources contain these pathogens is paramount. The purpose of this study was to determine the prevalence of Salmonella enterica and Listeria monocytogenes in alternative irrigation water sources. Water samples (n = 188) were collected over 2 years (2016 to 2018) from 2 reclaimed water plants, 3 nontidal freshwater rivers, and 1 tidal brackish river on Maryland's Eastern Shore (ESM). Samples were collected by filtration using modified Moore swabs (MMS) and analyzed by culture methods. Pathogen levels were quantified using a modified most probable number (MPN) procedure with three different volumes (10 liters, 1 liter, and 0.1 liter). Overall, 65% (122/188) and 40% (76/188) of water samples were positive for S. enterica and L. monocytogenes, respectively. For both pathogens, MPN values ranged from 0.015 to 11 MPN/liter. Pathogen levels (MPN/liter) were significantly (P < 0.05) greater for the nontidal freshwater river sites and the tidal brackish river site than the reclaimed water sites. L. monocytogenes levels in water varied based on season. Detection of S. enterica was more likely with 10-liter filtration compared to 0.1-liter filtration. The physicochemical factors measured attributed only 6.4% of the constrained variance to the levels of both pathogens. This study shows clear variations in S. enterica and L. monocytogenes levels in irrigation water sources on ESM. IMPORTANCE In the last several decades, Maryland's Eastern Shore has seen significant declines in groundwater levels. While this area is not currently experiencing drought conditions or water scarcity, this research represents a proactive approach. Efforts, to investigate the levels of pathogenic bacteria and the microbial quality of alternative irrigation water are important for sustainable irrigation practices into the future. This research will be used to determine the suitability of alternative irrigation water sources for use in fresh produce irrigation to conserve groundwater.

Rapid Detection and Differentiating of the Predominant Salmonella Serovars in Chicken Farm by TaqMan Multiplex Real-Time PCR Assay

Salmonella has been known as an important zoonotic pathogen that can cause a variety of diseases in both animals and humans. Poultry are the main reservoir for the Salmonella serovars Salmonella Pullorum (S. Pullorum), Salmonella Gallinarum (S. Gallinarum), Salmonella Enteritidis (S. Enteritidis), and Salmonella Typhimurium (S. Typhimurium). The conventional serotyping methods for differentiating Salmonella serovars are complicated, time-consuming, laborious, and expensive; therefore, rapid and accurate molecular diagnostic methods are needed for effective detection and prevention of contamination. This study developed and evaluated a TaqMan multiplex real-time PCR assay for simultaneous detection and differentiation of the S. Pullorum, S. Gallinarum, S. Enteritidis, and S. Typhimurium. In results, the optimized multiplex real-time PCR assay was highly specific and reliable for all four target genes. The analytical sensitivity corresponded to three colony-forming units (CFUs) for these four Salmonella serovars, respectively. The detection limit for the multiplex real-time PCR assay in artificially contaminated samples was 500 CFU/g without enrichment, while 10 CFU/g after pre-enrichment. Moreover, the multiplex real-time PCR was applied to the poultry clinical samples, which achieved comparable results to the traditional bacteriological examination. Taken together, these results indicated that the optimized TaqMan multiplex real-time PCR assay will be a promising tool for clinical diagnostics and epidemiologic study of Salmonella in chicken farm and poultry products.

Graphene Oxide-Based Simple and Rapid Detection of Antibiotic Resistance Gene via Quantum Dot-Labeled Zinc Finger Proteins

With the increasing rise of antibiotic-resistant pathogens, a simple and rapid detection of antibiotic resistance gene (ARG) is crucial to mitigate the spreading of antibiotic resistance. DNA-binding zinc finger proteins (ZFPs) can be engineered to recognize specific double-stranded (ds) DNA sequences in ARG. Here, we designed a simple and rapid method to detect ARG in bacteria utilizing engineered ZFPs and 2D nanosheet graphene oxide (GO) as a sensing platform. Our approach relies on the on and off effect of fluorescence signal in the presence and absence of target ARG, respectively. By taking advantage of the unique quenching capability of GO due to its electronic property, quantum dot (QD)-labeled ZFPs are adsorbed onto the GO sheets, and their fluorescence signal is quenched by proximal GO sheets through fluorescence resonance energy transfer (FRET). In the presence of target DNA, ZFP binding to the target DNA induces dissociation from GO, thereby restoring the fluorescence signal. Our system detects target DNA through restoration of QD emission as the restored signal increases directly with target DNA concentrations. Engineered ZFPs were able to detect specific dsDNA of the tetracycline resistance gene tetM with high specificity after only 10 min incubation on our GO-based sensing system. Our sensing system employed one-step FRET-based ZFP and GO combined technology to enable rapid and quantitative detection of ARG, providing a limit of detection as low as 1 nM. This study demonstrated the application of GO in conjunction with engineered DNA-binding domains for the direct detection of dsDNA with great potential as a rapid and reliable screening and detecton method against the growing threat of antibiotic resistant bacteria.

Direct Metatranscriptome RNA-seq and Multiplex RT-PCR Amplicon Sequencing on Nanopore MinION - Promising Strategies for Multiplex Identification of Viable Pathogens in Food

Viable pathogenic bacteria are major biohazards that pose a significant threat to food safety. Despite the recent developments in detection platforms, multiplex identification of viable pathogens in food remains a major challenge. A novel strategy is developed through direct metatranscriptome RNA-seq and multiplex RT-PCR amplicon sequencing on Nanopore MinION to achieve real-time multiplex identification of viable pathogens in food. Specifically, this study reports an optimized universal Nanopore sample extraction and library preparation protocol applicable to both Gram-positive and Gram-negative pathogenic bacteria, demonstrated using a cocktail culture of E. coli O157:H7, Salmonella enteritidis, and Listeria monocytogenes, which were selected based on their impact on economic loss or prevalence in recent outbreaks. Further evaluation and validation confirmed the accuracy of direct metatranscriptome RNA-seq and multiplex RT-PCR amplicon sequencing using Sanger sequencing and selective media. The study also included a comparison of different bioinformatic pipelines for metatranscriptomic and amplicon genomic analysis. MEGAN without rRNA mapping showed the highest accuracy of multiplex identification using the metatranscriptomic data. EPI2ME also demonstrated high accuracy using multiplex RT-PCR amplicon sequencing. In addition, a systemic comparison was drawn between Nanopore sequencing of the direct metatranscriptome RNA-seq and RT-PCR amplicons. Both methods are comparable in accuracy and time. Nanopore sequencing of RT-PCR amplicons has higher sensitivity, but Nanopore metatranscriptome sequencing excels in read length and dealing with complex microbiome and non-bacterial transcriptome backgrounds.

Prevalence of Salmonella and Listeria monocytogenes in non-traditional irrigation waters in the Mid-Atlantic United States is affected by water type, season, and recovery method

Irrigation water contaminated with Salmonella enterica and Listeria monocytogenes may provide a route of contamination of raw or minimally processed fruits and vegetables. While previous work has surveyed specific and singular types of agricultural irrigation water for bacterial pathogens, few studies have simultaneously surveyed different water sources repeatedly over an extended period of time. This study quantified S. enterica and L. monocytogenes levels (MPN/L) at 6 sites, including river waters: tidal freshwater river (MA04, n = 34), non-tidal freshwater river, (MA05, n = 32), one reclaimed water holding pond (MA06, n = 25), two pond water sites (MA10, n = 35; MA11, n = 34), and one produce wash water site (MA12, n = 10) from September 2016—October 2018. Overall, 50% (84/168) and 31% (53/170) of sampling events recovered S. enterica and L. monocytogenes, respectively. Results showed that river waters supported significantly (p < 0.05) greater levels of S. enterica than pond or reclaimed waters. The non-tidal river water sites (MA05) with the lowest water temperature supported significantly greater level of L. monocytogenes compared to all other sites; L. monocytogenes levels were also lower in winter and spring compared to summer seasons. Filtering 10 L of water through a modified Moore swab (MMS) was 43.5 (Odds ratio, p < 0.001) and 25.5 (p < 0.001) times more likely to recover S. enterica than filtering 1 L and 0.1 L, respectively; filtering 10 L was 4.8 (p < 0.05) and 3.9 (p < 0.05) times more likely to recover L. monocytogenes than 1L and 0.1 L, respectively. Work presented here shows that S. enterica and L. monocytogenes levels are higher in river waters compared to pond or reclaimed waters in the Mid-Atlantic region of the U.S., and quantitatively shows that analyzing 10 L water is more likely recover pathogens than smaller samples of environmental waters.

Power of Scanning Electron Microscopy and Energy Dispersive X-Ray Analysis in Rapid Microbial Detection and Identification at the Single Cell Level

The demand for rapid, consistent and easy-to-use techniques for detecting and identifying pathogens in various areas, such as clinical diagnosis, the pharmaceutical industry, environmental science and food inspection, is very important. In this study, the reference strains of six food-borne pathogens, namely, Escherichia coli 0157: H7 ATCC 43890, Cronobacter sakazakii ATCC 29004, Salmonella Typhimurium ATCC 43971, Staphylococcus aureus KCCM 40050, Bacillus subtilis ATCC 14579, and Listeria monocytogenes ATCC 19115, were chosen for scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. In our study, the time-consuming sample preparation step for the microbial analysis under SEM was avoided, which makes this detection process notably rapid. Samples were loaded onto a 0.01-µm-thick silver (Ag) foil surface to avoid any charging effect. Two different excitation voltages, 10 kV and 5 kV, were used to determine the elemental information. Information obtained from SEM-EDX can distinguish individual single cells and detect viable and nonviable microorganisms. This work demonstrates that the combination of morphological and elemental information obtained from SEM-EDX analysis with the help of principal component analysis (PCA) enables the rapid identification of single microbial cells without following time-consuming microbiological cultivation methods.

Developing a multiplex real-time PCR with a new pre-enrichment to simultaneously detect four foodborne bacteria in milk

Aim: The aim of this study is to formulate a new single nonselective pre-enrichment medium (ELSS) that can support the concurrent growth of four major foodborne pathogens containing E. coli O157: H7, L. monocytogenes, S. aureus and S. enterica serovar Entertidis to develop a multiplex TaqMan Real-time PCR (mRT-PCR). Methods: The mRT-PCR with a new pre-enrichment was carried out for simultaneous detection and quantification of these foodborne bacteria. Results: By using mRT-PCR after 16 h pre-enrichment in ELSS, the detection limit of each pathogen was 1 CFU/25 ml contaminated milk, as well as inclusivity and exclusivity reached 100%. Conclusion: The mRT-PCR assay with pre-enrichment step is a fast and reliable technique for detecting single or multiple pathogens in food products.

Detection of Salmonella Typhimurium in Romaine Lettuce Using a Surface Plasmon Resonance Biosensor

Leafy vegetables have been associated with high-profile outbreaks causing severe illnesses. Timely and accurate identification of potential contamination is essential to ensure food safety. A surface plasmon resonance (SPR) assay has been developed for the detection of Salmonella Typhimurium in leafy vegetables. The assay utilizes a pair of well characterized monoclonal antibodies specific to the flagellin of S. Typhimurium. Samples of romaine lettuce contaminated with S. Typhimurium at different levels (between 0.9 and 5.9 log cfu/g) were pre-enriched in buffered peptone water. Three SPR assay formats, direct assay, sequential two-step sandwich assay, and pre-incubation one-step sandwich assay were evaluated. All three assay formats detect well even at a low level of contamination (0.9 log cfu/g). The SPR assay showed a high specificity for the detection of S. Typhimurium in the presence of other commensal bacteria in the romaine lettuce samples. The results also suggested that further purification of flagellin from the sample preparation using immunomagnetic separation did not improve the detection sensitivity of the SPR assay. The functional protocol developed in this study can be readily used for the detection of S. Typhimurium in leafy vegetables with high sensitivity and specificity.

Creek to Table: Tracking fecal indicator bacteria, bacterial pathogens, and total bacterial communities from irrigation water to kale and radish crops

The impact of microbially contaminated irrigation water on risks to produce safety and public health is a complex issue that is not well understood. This study tracked fecal indicators, pathogenic bacteria, and total bacterial communities from a creek water irrigation source to irrigated produce to assess the impact of irrigation events on soil and produce-associated microbiota. Kale and radishes were drip-irrigated using Mid-Atlantic creek water in October 2017. Plant and soil samples were collected immediately before and after irrigation, and for 3 consecutive days thereafter. All samples (n = 134), including irrigation water, were tested for generic Escherichia coli and total coliforms (TC) using standard membrane filtration or direct plating, and for Salmonella enterica and Listeria monocytogenes by selective enrichment. DNA extracted from all samples was PCR-amplified for the V3-V4 region of the 16S rRNA gene for bacterial community profiling. In soil, TC levels were significantly higher immediately and 3 days post-irrigation compared to pre-irrigation (p < 0.01). E. coli levels in soil increased after irrigation, but the difference was not significant (p = 0.31), and die-off was not observed. No E. coli were detected on kale leaves. TC increased over the study period on radish roots (p < 0.01) but not kale leaves (p = 0.43). Although target pathogens were detected in irrigation water, S. enterica was detected from only one post-irrigation kale sample and L. monocytogenes was not detected in the field. The 16S rRNA gene sequencing data revealed differences in bacterial community structure and composition across sample types and showed that radish soil and root surface bacterial communities were more strongly influenced by irrigation compared to kale samples. This study provides insights into the impact of irrigation water on fresh produce microbiota, revealing that, although irrigation did influence crop-associated microbiota (especially below ground) in the field, bacterial pathogens were not likely transferred to the crop.

Salmonella enterica in Soils Amended with Heat-Treated Poultry Pellets Survived Longer than Bacteria in Unamended Soils and More Readily Transferred to and Persisted on Spinach

Untreated biological soil amendments of animal origin (BSAAO) are commonly used as biological fertilizers but can harbor foodborne pathogens like Salmonella enterica, leading to potential transfer from soils to fruits and vegetables intended for human consumption. Heat-treated poultry pellets (HTPP) can provide produce growers with a slow-release fertilizer with a minimized risk of pathogen contamination. Little is known about the impact of HTPP-amended soil on the survival of Salmonella enterica The contributions of RpoS and formation of viable but nonculturable cells to Salmonella survival in soils are also inadequately understood. We quantified the survival of Salmonella enterica subsp. enterica serovar Newport wild-type (WT) and rpoS-deficient (ΔrpoS mutant) strains in HTPP-amended and unamended soil with or without spinach plants over 91 days using culture and quantitative PCR methods with propidium monoazide (PMA-qPCR). Simulated "splash" transfer of S. Newport from soil to spinach was evaluated at 35 and 63 days postinoculation (dpi). The S. Newport WT and ΔrpoS mutant reached the limit of detection, 1.0 log CFU/g (dry weight), in unamended soil after 35 days, whereas 2 to 4 log CFU/g (dry weight) was observed for both WT and ΔrpoS mutant strains at 91 dpi in HTPP-amended soil. S. Newport levels in soils determined by PMA-qPCR and plate count methods were similar (P > 0.05). HTPP-amended soils supported higher levels of S. Newport transfer to and survival on spinach leaves for longer periods of time than did unamended soils (P < 0.05). Salmonella Newport introduced to HTPP-amended soils survived for longer periods and was more likely to transfer to and persist on spinach plants than was S. Newport introduced to unamended soils.IMPORTANCE Heat-treated poultry pellets (HTPP) often are used by fruit and vegetable growers as a slow-release fertilizer. However, contamination of soil on farms may occur through contaminated irrigation water or scat from wild animals. Here, we show that the presence of HTPP in soil led to increased S. Newport survival in soil and to greater likelihood of its transfer to and survival on spinach plants. There were no significant differences in survival durations of WT and ΔrpoS mutant isolates of S. Newport. The statistically similar populations recovered by plate count and estimated by PMA-qPCR for both strains in the amended and unamended soils in this study indicate that all viable populations of S. Newport in soils were culturable.

Pathogen-specific DNA sensing with engineered zinc finger proteins immobilized on a polymer chip

A specific double-stranded DNA sensing system is of great interest for diagnostic and other biomedical applications. Zinc finger domains, which recognize double-stranded DNA, can be engineered to form custom DNA-binding proteins for the recognition of specific DNA sequences. As a proof of concept, a sequence-enabled reassembly of a TEM-1 β-lactamase system (SEER-LAC) was previously demonstrated to develop zinc finger protein (ZFP) arrays for the detection of a double-stranded bacterial DNA sequence. Here, we implemented the SEER-LAC system to demonstrate the direct detection of pathogen-specific DNA sequences present in E. coli O157:H7 on a lab-on-a-chip. ZFPs custom-designed to detect Shiga toxin in E. coli O157:H7 were immobilized on a cyclic olefin copolymer (COC) chip, which can function as a non-PCR based molecular diagnostic device. Pathogen-specific double-stranded DNA was directly detected by using engineered ZFPs immobilized on the COC chip with high specificity, providing a detection limit of 10 fmol of target DNA in a colorimetric assay. Therefore, in this study, we demonstrated the great potential of ZFP arrays on the COC chip for further development of a simple and novel lab-on-a-chip technology for the detection of pathogens.

Simultaneous detection of Salmonella enterica, Escherichia coli and Listeria monocytogenes in food using a light scattering sensor

AIM

To investigate the use of a light scattering sensor, BActerial Rapid Detection using Optical scattering Technology (BARDOT) coupled with a multipathogen selective medium, Salmonella, Escherichia and Listeria (SEL), for concurrent detection of the three major foodborne pathogens in a single assay.

METHODS AND RESULTS

BARDOT was used to detect and distinguish the three major pathogens, Salmonella enterica, Shiga toxin-producing Escherichia coli (STEC) and Listeria monocytogenes from food based on colony scatter signature patterns on SEL agar (SELA). Multiple strains of three test pathogens were grown on SELA, and BARDOT was used to generate colony scatter image libraries for inclusive (SEL Library) and exclusive (non-SEL Library) bacterial group. These pathogens were further differentiated using the SEL scatter image library. Raw chicken and hotdog samples were artificially inoculated with pathogens (100 CFU per 25 g each), and enriched in SEL broth at 37°C for 18 h and colonies were grown on SELA for 11-22 h before screening with BARDOT. The BARDOT sensor successfully detected and differentiated Salmonella, STEC and Listeria on SELA with high classification accuracy 92-98%, 91-98% and 83-98% positive predictive values (PPV) respectively; whereas the nontarget strains showed only 0-13% PPV. BARDOT-identified colonies were further confirmed by multiplex PCR targeting inlB gene of L. monocytogenes, stx2 of STEC and sefA of S. enterica serovar Enteritidis.

CONCLUSIONS

The results show that BARDOT coupled with SELA can efficiently screen for the presence of three major pathogens simultaneously in a test sample within 29-40 h.

SIGNIFICANCE AND IMPACT OF THE STUDY

This innovative SELA-BARDOT detection platform can reduce turnaround time and economic burden on food industries by offering a label-free, noninvasive on-plate multipathogen screening technology for reducing microbial food safety and public health concerns.

Incidence of fecal indicator and pathogenic bacteria in reclaimed and return flow waters in Arizona, USA

The quality of irrigation water used to cultivate produce that is consumed raw is an important issue with regard to food safety. In this study, the microbiological quality of potential irrigation water sources in Arizona was evaluated by testing for the presence of indicator and pathogenic bacteria. Reclaimed water samples were collected from two wastewater treatment plants and return flow samples were collected from two drainage canals and one return flow pond. Standard membrane filtration methods were used for detection of indicator bacteria. Water samples (n = 28) were filtered through cellulose ester membrane filters and bacterial populations were enumerated by placing the filters on selective agar. For detection of pathogens (Salmonella enterica, Listeria monocytogenes and Shiga toxin-producing E. coli (STEC)), water samples were filtered through Modified Moore swabs and enriched in Universal Pre-enrichment Broth, followed by selective enrichment broth for each pathogen. The enriched broth was streaked onto agar media selective for each pathogen. Presumptive colonies were confirmed by PCR/real-time PCR. Among the 14 reclaimed water samples from two sites, the ranges of recovered populations of E. coli, total coliforms, and enterococci were 0-1.3, 0.5-8.3 × 10³, and 0-5.5 CFU/100 mL, respectively. No L. monocytogenes, Salmonella or STEC were found. In the 13 return flow water samples from 3 sites, the ranges of recovered populations of E. coli, total coliforms and enterococci were 1.9-5.3 × 10², 6.5 × 10²-9.1 × 10⁴, and 2.9-3.7× 10³ CFU/100 mL, respectively. All samples were negative for L. monocytogenes. One (7.1%) of the return flow samples was positive for E. coli O145. Nine (64.3%) of the samples were positive for Salmonella. Both real-time PCR and culture-based methods were used for the detection of Salmonella and L. monocytogenes, and the results from the two methods were comparable. The findings of this study provide evidence that irrigation waters in Arizona, including reclaimed water and return flows, could be potential sources of bacterial contamination of produce. Additional work is needed to evaluate whether bacteria present in irrigation water sources transfer to the edible portion of irrigated plants and are capable of persisting through post-harvest activities.

Electrochemical sandwich immunoassay for Escherichia coli O157:H7 based on the use of magnetic nanoparticles and graphene functionalized with electrocatalytically active Au@Pt core/shell nanoparticles

A highly sensitive electrochemical sandwich immunoassay is described for determination of Escherichia coli O157:H7 (E. coli O157:H7). Silica coated magnetite nanoparticles (Fe₃O₄) were modified with primary antibody to capture E. coli O157:H7. Gold-platinum core/shell nanoparticles (Au@Pt NPs) with different Pt shell thicknesses were prepared via changing the molar ratio of H₂PtCl₆ to HAuCl₄ in the precursor solution. The optimized Au@Pt NPs exhibit enhanced activity in the electrocatalytic reduction of hydrogen peroxide (H₂O₂). The Au@Pt NPs were modified with graphene that was functionalized with Neutral Red, and then used as an electrochemical label for secondary antibodies and horseradish peroxidase (HRP). The sandwich immunocomplexes were magnetically absorbed on a 4-channel screen printed carbon electrode. Due to the catalysis of the Au@Pt NPs and HRP, the signal is strongly amplified in the presence of H₂O₂ when using thionine as the electron mediator. Under optimal conditions, the immunoassay has a linear response in the 4.0 × 10² to 4.0 × 10⁸ CFU·mL^-1 concentration range, with a limit of detection of 91 CFU·mL^-1 (at an S/N ratio of 3). Graphical abstract Preparation of Au@Pt core/shell nanoparticles with different Pt shell thickness (A), rGO-NR (B), rGO-NR-Au@Pt-Ab₂-HRP (C) and the preparation and the detection process of the immunoassay (D). rGO: reduced graphene oxide, GO: graphene oxide, NR: Neutral Red, HRP: horseradish peroxidase, AuNPs: gold nanoparticles, Fe₃O₄@SiO₂: Silica coated magnetite nanoparticles, 4-SPCE: 4-channel screen printed carbon electrode.

Microdevice-based solid-phase polymerase chain reaction for rapid detection of pathogenic microorganisms

We demonstrate the integration of DNA amplification and detection functionalities developed on a lab-on-a-chip microdevice utilizing solid-phase polymerase chain reaction (SP-PCR) for point-of-need (PON) DNA analyses. First, the polycarbonate microdevice was fabricated by thermal bonding  to contain microchambers as reservoirs for performing SP-PCR. Next, the microchambers were subsequently modified with polyethyleneimine and glutaraldehyde for immobilizing amine-modified forward primers. During SP-PCR, the immobilized forward primers and freely diffusing fluorescence-labeled reverse primers cooperated to generate target amplicons, which remained covalently attached to the microchambers for the fluorescence detection. The SP-PCR microdevice was used for the direct identifications of two widely detected foodborne pathogens, namely Salmonella spp. and Staphylococcus aureus, and an alga causing harmful algal blooms annually in South Korea, Cochlodinium polykrikoides. The SP-PCR microdevice would be versatilely applied in PON testing as a universal platform for the fast identification of foodborne pathogens and environmentally threatening biogenic targets.

Nanotechnology: Review of concepts and potential application of sensing platforms in food safety

In recent years a number of new nanotechnology based platforms have been developed for detection of wide variety of targets including infectious agents, protein biomarkers, nucleic acids, drugs, and cancer cells. Nanomaterials such as magnetic nanoparticles, quantum dots, carbon nanotubes, nanowires, and nanosensors like giant magnetoresistance (GMR) sensors are used to quantitatively detect biomolecules with, experimentally, relatively good accuracy. There has been a growing interest in the use of magnetic fields in biosensing applications. Because biological samples have no ferromagnetic property and therefore there is no interference with complex sample matrix, detection of infectious agents from minimally processed samples is possible. Here, we provide a brief overview of the recent emergence of nanotechnology-based techniques for the detection and monitoring of foodborne diseases. In addition, the potential applications and future perspectives of nanotechnology on food safety are discussed. Ultimately, the review is expected to stimulate and provide directions to the development and application of nanotechnology-based tests for the early detection, and eventual control of foodborne diseases.

Simultaneous detection of Escherichia coli O157:H7, Staphylococcus aureus and Salmonella by multiplex PCR in milk

Escherichia coli O157:H7, Staphylococcus aureus, and Salmonella are food-borne pathogens that cause serious gastrointestinal illness and frequent food safety accidents. This study aimed to develop a practical multiplex polymerase chain reaction (mPCR) technique for the simultaneous detection of these food-borne pathogens in culture broth and artificial food matrix. Pathogen-specific DNA sequences in the rfbE, nuc, and invA genes were used as targets to design primers for the identification of E. coli O157:H7, S. aureus, and Salmonella, respectively. As expected, the method produced species-specific bands of amplified products without any contaminating non-specific bands. The highest species specificity was established with primer concentrations of 0.1, 0.2, and 0.4 μM for E. coli O157:H7, S. aureus, and Salmonella, correspondingly. The detection sensitivity of this assay was 103 CFU/mL in culture broth, and the limit of detection was consistent with singleplex PCR in the food sample. The mPCR assay proposed here is an easy and convenient detection method, which will be valuable for microbial epidemiology and food safety investigations.

Screening and Detecting Salmonella in Different Food Matrices in Southern Tunisia Using a Combined Enrichment/Real-Time PCR Method: Correlation with Conventional Culture Method

A combined enrichment/ newly developed invA TaqMan^® real-time PCR (qPCR) method as a screening assay to detect Salmonella spp. in 500 naturally food matrices is evaluated. DNA template for qPCR was extracted from an overnight pre-enriched sample in buffered peptone water using lysis–guanidine isothiocyanate method. Heterologous internal amplification control (IAC) was incorporated during qPCR assays and co-amplified with the invA gene of the target pathogen. InvA qPCR exhibited 100% specificity when testing 94 Salmonella strains (inclusivity) and 32 non-Salmonella strains (exclusivity). The qPCR showed a consistent detection of two copies of the invA gene/PCR reaction, a good intra- and inter-run reproducibility with a good PCR efficiency (89.6%). QPCR was sensitive and showed Salmonella detection at 8.5 × 10⁰ CFU mL^-1 of artificially spiked poultry meat -BWP solution in less than 40 cycles. When analyzing 500 different food matrices and comparing the results with the ISO 6579:2002 conventional culture method, the sensitivity and specificity were 100 and 76.6%, respectively. QPCR showed Salmonella spp. DNA in raw poultry meat 27/45 (60%), milk 31/93 (33.3%), raw red meat 5/13 (38.5%), and fish 11/46 (23.9%) samples. The prevalence of Salmonella spp. in cakes, dairy, cooked meals, charcuterie products using qPCR was 11/14 (26.8%), 5/22 (22.7%), 32/150 (21.3%), and 5/20 (25%), respectively, compared to 0% as demonstrated by culture. S. Anatum was the most common serovar found associated with red meat compared to S. kentucky isolated from fish and poultry meat. In conclusion, our study is the first to use a combined enrichment/invA qPCR method as a screening assay to detect Salmonella DNA in different types of commercialized food in Southern Tunisia. QPCR results indicate that Salmonella contamination is common in milk and in other types of food samples.

Advanced molecular diagnostic techniques for detection of food-borne pathogens: Current applications and future challenges

The elimination of disease-causing microbes from the food supply is a primary goal and this review deals with the overall techniques available for detection of food-borne pathogens. Now-a-days conventional methods are replaced by advanced methods like Biosensors, Nucleic Acid-based Tests (NAT), and different PCR-based techniques used in molecular biology to identify specific pathogens. Bacillus cereus, Staphylococcus aureus, Proteus vulgaris, Escherichia coli, Campylobacter, Listeria monocytogenes, Salmonella spp., Aspergillus spp., Fusarium spp., Penicillium spp., and pathogens are detected in contaminated food items that cause always diseases in human in any one or the other way. Identification of food-borne pathogens in a short period of time is still a challenge to the scientific field in general and food technology in particular. The low level of food contamination by major pathogens requires specific sensitive detection platforms and the present area of hot research looking forward to new nanomolecular techniques for nanomaterials, make them suitable for the development of assays with high sensitivity, response time, and portability. With the sound of these, we attempt to highlight a comprehensive overview about food-borne pathogen detection by rapid, sensitive, accurate, and cost affordable in situ analytical methods from conventional methods to recent molecular approaches for advanced food and microbiology research.

A Multiplex RT-PCR Assay for S. aureus, L. monocytogenes, and Salmonella spp. Detection in Raw Milk with Pre-enrichment

This study firstly developed a multiplex real-time PCR (RT-PCR) technique combined with a pre-enrichment step to simultaneously detect Staphylococcus aureus (S. aureus), Listeria monocytogenes (L. monocytogenes) and Salmonella spp. in raw milk and the dairy farm environment (feces, soil, feed, water) in one reaction. Brain heart infusion (BHI) broth was selected for the enrichment step to increase the density of the target bacteria by using an incubation of 4 h before multiplex RT-PCR. The results showed that the detection limit of the multiplex real-time assay was approximately 102 CFU/mL for pure cultures and artificially contaminated milk without enrichment, while 12, 14, and 10 CFU/25 mL, respectively, for S. aureus, L. monocytogenes, and Salmonella spp. after pre-enrichment. The newly developed multiplex RT-PCR assay was applied to 46 dairy farm environmental samples and raw milk samples covering a wide variety of sample types. The results demonstrated that the multiplex RT-PCR assay coupled with the BHI enrichment broth was suitable for the simultaneous screening of S. aureus, L. monocytogenes, and Salmonella spp. in the pasture environment and in raw milk. The multiplex RT-PCR assay clearly and successfully shortened the total detection time and reduced labor compared to conventional culture-based methods for testing natural samples.

Next generation sequencing-based multigene panel for high throughput detection of food-borne pathogens

Contamination of food by chemicals or pathogenic bacteria may cause particular illnesses that are linked to food consumption, commonly referred to as foodborne diseases. Bacteria are present in/on various foods products, such as fruits, vegetables and ready-to-eat products. Bacteria that cause foodborne diseases are known as foodborne pathogens (FBPs). Accurate detection methods that are able to reveal the presence of FBPs in food matrices are in constant demand, in order to ensure safe foods with a minimal risk of causing foodborne diseases. Here, a multiplex PCR-based Illumina sequencing method for FBP detection in food matrices was developed. Starting from 25 bacterial targets and 49 selected PCR primer pairs, a primer collection called foodborne pathogen - panel (FPP) consisting of 12 oligonucleotide pairs was developed. The FPP allows a more rapid and reliable identification of FBPs compared to classical cultivation methods. Furthermore, FPP permits sensitive and specific FBP detection in about two days from food sample acquisition to bioinformatics-based identification. The FPP is able to simultaneously identify eight different bacterial pathogens, i.e. Listeria monocytogenes, Campylobacter jejuni, Campylobacter coli, Salmonella enterica subsp. enterica serovar enteritidis, Escherichia coli, Shigella sonnei, Staphylococcus aureus and Yersinia enterocolitica, in a given food matrix at a threshold contamination level of 10¹cell/g. Moreover, this novel detection method may represent an alternative and/or a complementary approach to PCR-based techniques, which are routinely used for FBP detection, and could be implemented in (parts of) the food chain as a quality check.

Development of Simple Multiplex Real-Time PCR Assays for Foodborne Pathogens Detection and Identification On Lightcycler

Most acute intestinal diseases are caused by food-borne pathogens. A fast and simple real-time PCR-based procedure for simultaneous detection of food contamination by any of the five food-borne pathogens: Campylobacter jejuni, Mycobacterium bovis, Enterobacter sakazaki, Shigella boydii, Clostridium perfrigens using multiplex EvaGreen real-time PCR for LightCycler was developed and evaluated. Real-time qPCR showed excellent sensitivity. Tm calling and Melting Curve Genotyping (MCG) were used for analysis of PCR product melting curves. The Melting Curve Genotyping option showed good performance for discrimination of positive samples containing DNA of single pathogen or pathogen mixtures from negative samples.

Multiplex real-time PCRs for detection of Salmonella, Listeria monocytogenes, and verotoxigenic Escherichia coli in carcasses of slaughtered animals

Introduction: The study objective was to develop and evaluate a new TaqMan multiplex real-time PCR method for Salmonella, L. monocytogenes, and verotoxigenic Escherichia coli (VTEC) detection in slaughtered animal carcasses.

Material and Methods: The procedure included an enrichment step, DNA extraction, and two multiplex real-time PCRs. The first PCR detected the invA and hly genes of Salmonella and L. monocytogenes respectively, the second the vtx1, vtx2, and eae genes of VTEC.

Results: The validation of this method resulted in 100% relative sensitivity, specificity, and accuracy as compared to the reference ISO methods. The limit of detection per swab sample was established at 1 cfu for Salmonella and L. monocytogenes and 2 cfu for VTEC. The authors analysed 265 slaughterhouse-collected swabs from cattle, pig, and poultry carcasses. Among 125 from cattle, 51 were positive for VTEC, 29 for Salmonella, and 1 for L. monocytogenes. Among swabs from pig carcasses (n = 95), three, two, and one sample were positive for these pathogens respectively. None of the microorganisms tested for was identified in 45 samples of poultry origin.

Conclusion: The obtained results showed that the method developed can rapidly identify the main bacterial pathogens that may contaminate carcasses of food-producing animals.

Rapid multiplex detection of 10 foodborne pathogens with an up-converting phosphor technology-based 10-channel lateral flow assay

The rapid high-throughput detection of foodborne pathogens is essential in controlling food safety. In this study, a 10-channel up-converting phosphor technology-based lateral flow (TC-UPT-LF) assay was established for the rapid and simultaneous detection of 10 epidemic foodborne pathogens. Ten different single-target UPT-LF strips were developed and integrated into one TC-UPT-LF disc with optimization. Without enrichment the TC-UPT-LF assay had a detection sensitivity of 10(4) CFU mL(-1) or 10(5) CFU mL(-1) for each pathogen, and after sample enrichment it was 10 CFU/0.6 mg. The assay also showed good linearity, allowing quantitative detection, with a linear fitting coefficient of determination (R(2)) of 0.916-0.998. The 10 detection channels did not cross-react, so multiple targets could be specifically detected. When 279 real food samples were tested, the assay was highly consistent (100%) with culture-based methods. The results for 110 food samples artificially contaminated with single or multiple targets showed a high detection rate (≥ 80%) for most target bacteria. Overall, the TC-UPT-LF assay allows the rapid, quantitative, and simultaneous detection of 10 kinds of foodborne pathogens within 20 min, and is especially suitable for the rapid detection and surveillance of foodborne pathogens in food and water.

Rapid methods for the detection of foodborne bacterial pathogens: principles, applications, advantages and limitations

The incidence of foodborne diseases has increased over the years and resulted in major public health problem globally. Foodborne pathogens can be found in various foods and it is important to detect foodborne pathogens to provide safe food supply and to prevent foodborne diseases. The conventional methods used to detect foodborne pathogen are time consuming and laborious. Hence, a variety of methods have been developed for rapid detection of foodborne pathogens as it is required in many food analyses. Rapid detection methods can be categorized into nucleic acid-based, biosensor-based and immunological-based methods. This review emphasizes on the principles and application of recent rapid methods for the detection of foodborne bacterial pathogens. Detection methods included are simple polymerase chain reaction (PCR), multiplex PCR, real-time PCR, nucleic acid sequence-based amplification (NASBA), loop-mediated isothermal amplification (LAMP) and oligonucleotide DNA microarray which classified as nucleic acid-based methods; optical, electrochemical and mass-based biosensors which classified as biosensor-based methods; enzyme-linked immunosorbent assay (ELISA) and lateral flow immunoassay which classified as immunological-based methods. In general, rapid detection methods are generally time-efficient, sensitive, specific and labor-saving. The developments of rapid detection methods are vital in prevention and treatment of foodborne diseases.

Rapid methods for the detection of foodborne bacterial pathogens: principles, applications, advantages and limitations

The incidence of foodborne diseases has increased over the years and resulted in major public health problem globally. Foodborne pathogens can be found in various foods and it is important to detect foodborne pathogens to provide safe food supply and to prevent foodborne diseases. The conventional methods used to detect foodborne pathogen are time consuming and laborious. Hence, a variety of methods have been developed for rapid detection of foodborne pathogens as it is required in many food analyses. Rapid detection methods can be categorized into nucleic acid-based, biosensor-based and immunological-based methods. This review emphasizes on the principles and application of recent rapid methods for the detection of foodborne bacterial pathogens. Detection methods included are simple polymerase chain reaction (PCR), multiplex PCR, real-time PCR, nucleic acid sequence-based amplification (NASBA), loop-mediated isothermal amplification (LAMP) and oligonucleotide DNA microarray which classified as nucleic acid-based methods; optical, electrochemical and mass-based biosensors which classified as biosensor-based methods; enzyme-linked immunosorbent assay (ELISA) and lateral flow immunoassay which classified as immunological-based methods. In general, rapid detection methods are generally time-efficient, sensitive, specific and labor-saving. The developments of rapid detection methods are vital in prevention and treatment of foodborne diseases.

Real-time PCR method combined with immunomagnetic separation for detecting healthy and heat-injured Salmonella Typhimurium on raw duck wings

Conventional culture detection methods are time consuming and labor-intensive. For this reason, an alternative rapid method combining real-time PCR and immunomagnetic separation (IMS) was investigated in this study to detect both healthy and heat-injured Salmonella Typhimurium on raw duck wings. Firstly, the IMS method was optimized by determining the capture efficiency of Dynabeads(®) on Salmonella cells on raw duck wings with different bead incubation (10, 30 and 60 min) and magnetic separation (3, 10 and 30 min) times. Secondly, three Taqman primer sets, Sal, invA and ttr, were evaluated to optimize the real-time PCR protocol by comparing five parameters: inclusivity, exclusivity, PCR efficiency, detection probability and limit of detection (LOD). Thirdly, the optimized real-time PCR, in combination with IMS (PCR-IMS) assay, was compared with a standard ISO and a real-time PCR (PCR) method by analyzing artificially inoculated raw duck wings with healthy and heat-injured Salmonella cells at 10(1) and 10(0) CFU/25 g. Finally, the optimized PCR-IMS assay was validated for Salmonella detection in naturally contaminated raw duck wing samples. Under optimal IMS conditions (30 min bead incubation and 3 min magnetic separation times), approximately 85 and 64% of S. Typhimurium cells were captured by Dynabeads® from pure culture and inoculated raw duck wings, respectively. Although Sal and ttr primers exhibited 100% inclusivity and exclusivity for 16 Salmonella spp. and 36 non-Salmonella strains, the Sal primer showed lower LOD (10(3) CFU/ml) and higher PCR efficiency (94.1%) than the invA and ttr primers. Moreover, for Sal and invA primers, 100% detection probability on raw duck wings suspension was observed at 10(3) and 10(4) CFU/ml with and without IMS, respectively. Thus, the Sal primer was chosen for further experiments. The optimized PCR-IMS method was significantly (P=0.0011) better at detecting healthy Salmonella cells after 7-h enrichment than traditional PCR method. However there was no significant difference between the two methods with longer enrichment time (14 h). The diagnostic accuracy of PCR-IMS was shown to be 98.3% through the validation study. These results indicate that the optimized PCR-IMS method in this study could provide a sensitive, specific and rapid detection method for Salmonella on raw duck wings, enabling 10-h detection. However, a longer enrichment time could be needed for resuscitation and reliable detection of heat-injured cells.

Assessment criteria and approaches for rapid detection methods to be used in the food industry

The number of commercially available kits and methods for rapid detection of foodborne pathogens continues to increase at a considerable pace, and the diversity of methods and assay formats is reaching a point where it is very difficult even for experts to weigh the advantages and disadvantages of different methods and to decide which methods to choose for a certain testing need. Although a number of documents outline quantitative criteria that can be used to evaluate different detection methods (e.g., exclusivity and inclusivity), a diversity of criteria is typically used by industry to select specific methods that are used for pathogen detection. This article is intended to provide an overall outline of criteria that the food industry can use to evaluate new rapid detection methods, with a specific focus on nucleic acid-based detection methods.

Evaluation of a multiplex selective enrichment broth SEL for simultaneous detection of injured Salmonella, Escherichia coli O157:H7 and Listeria monocytogenes

Although many rapid and high throughput molecular methods have been developed in the recent years for the multiplex detection of foodborne pathogens, the simultaneous recovery and enrichment of sublethally injured cells is still a problem that needs to be considered. Combined with previous established multiplex real-time PCR assay, the capability of simultaneous recovery and enrichment of sublethally injured Salmonella, E. coli O157:H7 and L. monocytogenes cells was evaluated in a multiplex selective enrichment broth SEL. The injured cells were obtained by heat shock. After evaluation of different procedures, 1 h of recovery period prior to 20 h of enrichment was proved to be necessary for the detection of less than 10 CFU/5 mL broth of injured L. monocytogenes. When the detection method was applied to artificially contaminated ground beef, all the three injured pathogens could be simultaneously detected without discrimination by real-time PCR combined with SEL broth, the detection limit was < 5 CFU/10 g ground beef. Comparatively, when BPW was employed as the enrichment broth in the same detection procedure, injured L. monocytogenes could not be detected if the initially spiked level was below 10² CFU/10 g ground beef. Considering the capability of co-enrichment and high detection effectiveness, the real-time PCR assay combined with SEL broth herein appears to be a promising tool for high-throughput screening of a large number of processed food samples, which require either single or multiple pathogen detection. More important, the sublethally injured foodborne pathogen cells were also detectable.

Application of multiplex PCR assay based on uidR and fliCH7 genes for detection of Escherichia coli O157:H7 in milk

A highly sensitive and specific multiplex PCR assay has been developed to detect the presence of Escherichia coli O157:H7 from naturally contaminated raw milk samples within 10 h. The primers explored in the assay were targeted against the uidR gene specific for all types of E. coli and the fliCH7 gene specific for the h7 flagellar antigen of E. coli O157:H7. The multiplex PCR assay developed was found to be highly specific as it produced PCR products of 152 bp (E. coli specific) and 625 bp (E. coli O157:H7 specific). The assay was tested for its specificity against different serotypes of E. coli as well as other pathogenic strains like Salmonella, Shigella, Klebsiella, Enterobacter, Staphylococcus aureus, Lactobacillus and Lactococcus etc. When this multiplex PCR assay was directly applied to 24 raw milk samples collected from different sources, E. coli O157:H7 could be detected in one of the milk samples without 4 h enrichment in CT-SMAC broth and three samples after 4 h enrichment in CT-SMAC broth. However, all the pasteurized milk samples gave a negative signal for this organism.