A rapid, sensitive and automated method for detection of Salmonella species in foods using AG-9600 AmpliSensor Analyzer

A capillary polymerase chain reaction for Salmonella detection from poultry meat

AIMS

In this study, a capillary polymerase chain reaction (cPCR) was applied for Salmonella detection from poultry meat.

METHODS AND RESULTS

Salmonella detection limits of the optimized cPCR were determined with DNA templates from the samples of tetrathionate broth (TTB), Rappaport Vassiliadis broth (RVB) and selenite cystine broth (SCB) artificially contaminated with 10-fold dilutions of 6 x 10(8) CFU ml(-1) of pure Salmonella enterica ssp. enterica serovar Enteritidis 64K stock culture. Detection limits of cPCR from TTB, RVB and SCB were found as 6, 6 x 10(1) and 6 x 10(4) CFU ml(-1), respectively. In addition, detection limits of bacteriology were also determined as 6 CFU ml(-1) with TTB and SCB, and 6 x 10(1) CFU ml(-1) with RVB. A total of 200 samples, consisting of 100 chicken and 100 turkey meat samples, were tested with optimized cPCR and bacteriology. Eight and six per cent of the chicken meat samples were found to harbour Salmonella by cPCR and standard bacteriology, respectively. Of six Salmonella isolates, four belonged to serogroup D, two to serogroup B.

CONCLUSIONS

The TTB cultures of both artificially and naturally contaminated samples were found to be superior to those of RVB and SCB cultures in their cPCR results. This cPCR, utilizing template from 18-h TTB primary enrichment broth culture, takes approximately 40 min in the successful detection of Salmonella from poultry meat.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that cPCR from TTB enrichment culture of poultry meat would enable rapid detection of Salmonella in laboratories with low sample throughput and limited budget.

Rapid detection of Salmonella from hydrodynamic pressure-treated poultry using molecular beacon real-time PCR

A real-time polymerase chain reaction (PCR) assay was evaluated to detect Salmonella in hydrodynamic pressure (HDP)-treated chicken using molecular beacon probes available as a commercial kit (iQ-Check, Bio-Rad Laboratories). The sensitivity and accuracy of the assay were compared with the conventional USDA microbiological procedure using artificially contaminated minced chicken. Chicken fillets were irradiated at 10 kGy to completely destroy any naturally occurring Salmonella. These fillets were minced and inoculated with as low as 2+/-1 cfu of S. typhimurium per 25 g chicken. The minced chicken samples were vacuum packed in multi-layer barrier bags, heat shrunk, and treated with HDP. Results showed that all inoculated samples (n=36) were detected by the PCR assay and conventional USDA procedure. Similarly, all uninoculated controls (n=11) were negative by both PCR assay and USDA procedure. As few as 2+/-1 cfu could be detected from 25 g HDP-treated chicken following 16-18 h enrichment in buffered peptone water. Real-time PCR proved to be an effective method for Salmonella detection in HDP-treated chicken with high sensitivity and more importantly, a rapid and high-throughput detection in 18 h, compared to 3-8 days for the conventional microbiological methods. HDP treatment, which has been reported to reduce spoilage bacteria in various meats, was unable to kill pathogenic Salmonella in minced chicken.

Rapid detection and counting of viable beer-spoilage lactic acid bacteria using a monoclonal chemiluminescence enzyme immunoassay and a CCD camera

A chemiluminescence enzyme immunoassay carried out with a monoclonal antibody (MAb) and a charge-coupled device (CCD) camera was developed for rapid enumeration of viable beer-spoilage lactic acid bacteria. LA-4 MAb, which recognizes a broad spectrum of lactic acid bacteria isolated from several breweries across Spain, was produced and characterized. Test samples were filtered through polycarbonate membranes, and the membranes with retained bacteria were incubated at 31 degrees C for 2 days. They were then subjected to a two-step chemiluminescence enzyme immunoassay with MAb LA-4, and light-emitting points were detected and counted with a CCD camera. Eighteen out of 19 beer-spoilage lactic acid bacteria analysed produced luminous spots that could be enumerated. Results provided by the immunochemiluminescence assay correlated very well with those obtained by visual plate counting within a range of 3-100 CFU/100 ml. Correlation coefficients were 0.994 for four strains in sterile saline solution and 0.984 for 14 strains in artificially contaminated beer. The excellent agreement suggests that luminous spots detected within 2 days of culture are produced only by viable cells.

Rapid detection of Salmonella from poultry by real-time polymerase chain reaction with fluorescent hybridization probes

Detection of Salmonella by bacteriologic methods is known to be time consuming. Therefore, we have developed a real-time probe-specific polymerase chain reaction (PCR) to rapidly detect Salmonella invA gene-based PCR products from chicken feces and carcasses by a fluorescence resonance energy transfer assay. The sensitivity and the specificity of this system were determined as 3 colony-forming units ml(-1) and 100%, respectively. Overnight tetrathionate broth enrichment cultures of chicken feces and carcass samples were used in template preparation for PCR. Also, a standard bacteriology was performed (National Poultry Improvement Plan-U.S. Department of Agriculture, Bacteriological Analytical Manual-Food and Drug Administration Center for Food Safety and Applied Nutrition) for confirmation. Seventy-two cloacal swab, 147 intestine, and 50 carcass (neck) samples were examined. Thirteen (8.8%) and 25 (17%) of the intestinal samples were found to harbor Salmonella by bacteriology and PCR, respectively. Forty-five of 50 (90%) carcass samples were Salmonella positive by both methods. Salmonella was not detected from cloacal swab samples. Results indicate that this assay has the potential for use in routine monitoring and detection of Salmonella in infected flocks and carcasses.

A new fluorescent quantitative polymerase chain reaction technique

To perform real-time detection of specific genes, a new complex probe has been designed and synthesized. Based on fluorescence resonance energy transfer (FRET), this complex probe is composed of a long-fluorescent reporter probe and a short-quenching probe. The 5' end of the fluorescent probe is connected to a fluorescein molecule, and its 3' end is linked to an extending blocking molecule. The 3' end of the quenching probe is connected to a quenching molecule-p-methyl red (Dabcyl). The quenching probe is complementary to the 5' end of the fluorescent probe. When there is no template, the two probes combine to form a complex probe and therefore no fluorescence is produced; when there are templates, the fluorescent probe hybridizes with the templates first, and the fluorescence is not quenched. The fluorescence intensity produced is in direct proportion to the template quantity. In accordance with the principles of reaction of the complex probe, we have studied the probe's FRET nature and the factors that affect it, including the quenching probe and amplified fragment length, the proper proportion of the fluorescent probe to the quenching probe, and the magnesium ion concentration. Experimental results showed that the quenching probe and its amplified fragment length had an obvious impact on the function of the complex probe. The quenching probe used in the present experiment is up to 21 nucleotides long, with an amplified fragment of 127bp. The most preferable reaction system is obtained when the proportion of the fluorescent probe to the quenching probe is 1:1, and the concentration of magnesium ions is 3mmol/L. The complex probe is easy to synthesize. The quenching is thorough with good accuracy and specificity. The sensitivity reaches 10(2) copies with a very large dynamic quantitation range. Accurate quantitation can be achieved with samples detected within 10(2)-10(9) copies. The complex probe method can be used to detect virus infection levels, transgenic copy quantities, single nucleotide polymorphisms, etc.

Detection and identification of salmonellas from poultry-related samples by PCR

A polymerase chain reaction (PCR) assay was developed for the generic detection of Salmonella sp. and the identification of S. Enteritidis (SE), S. Gallinarum (SG), S. Pullorum (SP) and S. Typhimurium (ST) in material collected in the field from poultry. The specificity and sensitivity of the assay combined with Rappaport-Vassiliadis selective enrichment broth (PCR-RV) were determined, and field samples were analyzed to verify the validity of the method application. Specificity of the assay was tested using 29 SE, 11 SG, 10 ST and 10 SP strains, along with 75 strains of 28 other Salmonella serovars and 21 strains of other bacterial genera. The assay was 100% specific for Salmonella detection and ST identification. The primer pair for SE, SG and SP also detected S. Berta. PCR detection limits for Salmonella at the genus level were 2 ST, 8 SE, 1.1x10(3) SG and 1.8x10(5) SP cells. At the serovar level, detection limits were 7 ST, 1.2x10(3) SE, 4.4x10(7) SG and 1.8x10(6) SP cells. At the genus level, PCR-RV detected approximately 128% more positive field samples than the standard microbiological techniques and results were ready in 48h instead of 7 days. PCR-RV method is diagnostic of Salmonella at the genus level and ST at the serovar level, although other tests are needed to identify SE, SG and SP to serovar level.

Detection of salmonellae in chicken feces by a combination of tetrathionate broth enrichment, capillary PCR, and capillary gel electrophoresis

This report describes a rapid detection procedure for salmonellae from chicken feces by the combination of tetrathionate primary enrichment (preenrichment [PE])-bacterial lysis-capillary PCR and capillary gel electrophoresis. Pure Salmonella enterica serovar Enteritidis 64K was reisolated and detected by capillary PCR after buffered peptone water and nutrient broth, tetrathionate broth base Hajna (TTBH), and tetrathionate broth (TTB) preenrichments. When the same culture was mixed with intestinal homogenate, bacteriological reisolation and capillary PCR detection was achieved only by TTBH and TTB preenrichments. Capillary gel electrophoresis revealed that a Salmonella genus-specific 281-bp PCR product was detected when Salmonella strains but not non-Salmonella strains were tested. The detection limit of capillary PCR with whole-cell DNA extracted from pure Salmonella enterica serovars Enteritidis 64K, Typhimurium LT2-CIP60-62, and Gallinarum 64K was 3, 3, and 9 CFU ml(-1), respectively. The detection limit of capillary PCR from whole-cell DNA extracted from intestinal homogenate artificially contaminated with the same three strains was 3, 3, and 7 CFU ml(-1), respectively. We compared the results of the capillary PCR and bacteriological examination from the natural samples. Thirty-five of 53 naturally contaminated samples produced a specific PCR product. In 9 of the 35 PCR-positive samples, Salmonella could not be detected bacteriologically either by PE or a primary and delayed secondary enrichment (DSE) combination. In the 18 PCR-negative samples, 4 samples were found to harbor Salmonella by both PE and DSE and 14 samples were positive after DSE. Fifty-three additional intestinal homogenate samples, which were negative by their PE and DSE in bacteriological examination, were found to be also negative by their PCRs. The total time required to detect Salmonella with the capillary PCR method we used was approximately 20 h. If samples are from clinically diseased birds, the total time for PCR and detection is reduced to 2 h since the 18-h PE is not required. These results indicate that TTB enrichment, bacterial lysis, and genus-specific capillary PCR combined with capillary gel electrophoresis constitute a sensitive and selective procedure which has the potential to rapidly identify Salmonella-infected flocks.

Lectin and carbohydrate affinity capture surfaces for mass spectrometric analysis of microorganisms

In a preliminary report (Bundy, J. L.; Fenselau, C. Anal. Chem 1999, 71, 1460-1463), we demonstrated the use of lectin-derivatized surfaces to capture and concentrate complex carbohydrates as well as microorganisms from sample matrixes unamenable to direct MALDI mass spectrometry. Here, we extend the work to include samples representative of a wider variety of microorganisms of importance to human health and of enveloped viruses. In this study, lectins were immobilized directly to a membrane surface via primary amines. A complementary approach was also explored, using immobilized carbohydrates to capture bacteria via microbial lectins expressed on their surfaces. The carbohydrate-based surfaces were constructed by first immobilizing streptavidin to the membrane, followed by attachment of a commercially produced biotin/carbohydrate polymer. Acid treatment of the sample prior to mass spectrometric analysis permits the observation of protein biomarkers from the captured microbial samples in the 5-20 kDa mass range. Bacteria samples were detected from physiological buffers, urine, milk, and processed chicken samples using the biocapture probes. Viral samples were detected from culture based on glycoprotein moieties desorbed directly from the surface. The carbohydrate-based system provided greater sensitivity than the lectin system, possibly due to the larger number of accessible saccharide ligands on the polymer.

Rapid and specific detection of PCR products using light-up probes

Newly developed light-up probes offer an attractive tool for PCR product detection. The light-up probe, which consists of a thiazole orange derivative linked to a peptide nucleic acid oligomer, hybridizes specifically to complementary nucleic acids. Upon hybridization the thiazole orange moiety interacts with the nucleic acid bases and the probe becomes brightly fluorescent. This eliminates the need to separate bound from unbound probes and reduces the risk of cross contamination during sample handling. We demonstrate here the applicability of light-up probes in two different PCR assays, one directed towards the human beta-actin gene and the other towards the invA gene of Salmonella. The probes do not interfere with the PCR reaction and can either be included in the sample mixture or added after completed amplification. The specificity of the probe is found to be excellent: a single-base mismatch in the target sequence is sufficient to prevent probe binding as indicated by the lack of fluorescence increase. Furthermore, a clear correlation is found between the intensity of gel bands and the measured probe fluorescence in solution, which suggests that the amount of PCR products can be quantified using light-up probes.

Probeliatrade mark PCR system for rapid detection of Salmonella in milk powder and ricotta cheese

The Probeliatrade mark Salmonella sp. PCR amplification and detection kits (Sanofi Diagnostics Pasteur, Marnes La Coquette, France) were evaluated for the rapid and specific detection of Salmonella agona artificially inoculated into skim milk powder and ricotta cheese. The Probeliatrade mark results were compared with those obtained using the Australian Standard Method. Using a pure culture of Salm. agona, the detection limit of Probeliatrade mark was between 8 and 79 cfu ml-1, equivalent to 0.2-2 cfu per PCR reaction. Detection of Salm. agona inoculated in skim milk powder (at 5-10 cfu g-1, stored at 5, 15 or 25 degrees C) and ricotta cheese (at 1-2, 10-20 and 100-200 cfu per 25 g) was effected by using non-selective enrichment prior to the PCR determinations. For all of the 40 milk powder samples and 12 ricotta cheese samples, the Probeliatrade mark results were consistent with those using the Australian Standard Method. Using Probeliatrade mark, Salmonella was detected to genus level in the dairy products within 24-28 h, whereas the cultural technique required 3-4 d for presumptive positive isolates and further time for confirmation.

Rapid detection of stressed Salmonella spp. in dairy and egg products using immunomagnetic separation and PCR

The rapid detection of an average of 5.9 stressed Salmonella cells in 25 g of food product using immunomagnetic separation (IMS) and PCR is described. For pasteurised egg yolk, egg yolk powder, ice-cream, whole egg, egg white and cheeses made from pasteurised milk PCR was applied after 16 h of preenrichment in buffered peptone water (BPW) using IMS and alkaline lysis as sample preparation method. For whole egg and egg white the BPW was supplemented with iron. For milk powder, and raw milk cheeses, the 16-h preenrichment in BPW was followed by IMS and a 4-h enrichment in Rappaport-Vassiliadis broth. In the latter case, PCR was applied on the enrichment medium after centrifugation and alkaline lysis. For PCR the primers ST11 and ST15 (Aabo et al., 1993) producing a fragment of 429 bp were used. An internal PCR control, designed to be co-amplified with the target DNA using the same primers but producing a smaller fragment of 240 bp, was used.

An automated fluorescent PCR method for detection of shiga toxin-producing Escherichia coli in foods

An automated fluorescence-based PCR system (a model AG-9600 AmpliSensor analyzer) was investigated to determine whether it could detect Shiga toxin-producing Escherichia coli (STEC). The AmpliSensor PCR assay involves amplification-mediated disruption of a fluorogenic DNA signal duplex (AmpliSensor) that is homologous to conserved target sequences in a 323-bp amplified fragment of Shiga toxin genes stx1, stx2, and stxe. Using the Amplisensor assay, we detected 113 strains of STEC belonging to 50 different serotypes, while 18 strains of non-Shiga-toxin-producing E. coli and 68 strains of other bacteria were not detected. The detection limits of the assay were less than 1 to 5 CFU per PCR mixture when pure cultures of five reference strains were used and 3 CFU per 25 g of food when spiked ground beef samples that were preenriched overnight were used. The performance of the assay was also evaluated by using 53 naturally contaminated meat samples and 48 raw milk samples. Thirty-two STEC-positive samples that were confirmed to be positive by the culture assay were found to be positive when the AmpliSensor assay was used. Nine samples that were found to be positive when the PCR assay was used were culture negative. The system described here is an automated PCR-based system that can be used for detection of all serotypes of STEC in food or clinical samples.