A comparison of standard cultural methods for the detection of foodborne Salmonella

Joint concanavalin A-aptamer enabled dual recognition for anti-interference visual detection of Salmonella typhimurium in complex food matrices

Given that food poisoning and infectious diseases caused by Salmonella typhimurium (S. typhimurium) draw intensive public health concerns, developing rapid, accurate, and cost-effective approaches to detect the pathogen is of crucial importance. Herein, we proposed a concanavalin A (Con A)-aptamer joint strategy to realize dual recognition for the strongly specific, visual, and highly sensitive determination of S. typhimurium. Compared with currently used single identification strategies, Con A and aptamer could recognize different sites of S. typhimurium to enhance the utilization rate of these sites for better sensing. The developed assay offered specific detection of S. typhimurium against other bacteria in a remarkably wide concentration range of 7.0 × 10¹ ∼ 7.0 × 10⁹ CFU/mL, along with a detection limit as low as 23 CFU/mL. Real sample analyses of milk and pork demonstrated the excellent reliability and practicability of our assay, providing great potential for food safety analysis.

Prevalence, antimicrobial resistance profile and comparison of methods for the isolation of salmonella in chicken liver from Argentina

This study was conducted to estimate the apparent prevalence of Salmonella spp. in chicken livers obtained from markets in Entre Ríos, Argentina, using two culture methods (preenrichment and direct selective agar plating). We also determined the antimicrobial resistance of Salmonella isolated strains and evaluated the performance of the two culture methods and selective-differential plating media used for Salmonella isolation. Of 666 chicken livers studied, 32 organs (4.8%) related to 4 poultry slaughterhouse companies were positive for Salmonella sp. using one or two culture methods. Fifty Salmonella strains were isolated from the positive liver samples and were typed into 3 serovars: S. ser. Schwarzengrund (78%), S. ser. Enteritidis (18%), and S. ser. Typhimurium 4(%). More than one Salmonella serovar was found in livers belonging to two chicken slaughterhouse companies. All strains were susceptible to all antibiotics tested, with the exception of erythromycin (100% resistant) and streptomycin (22% intermediate sensitivity). Overall, 32 (4.80%) and 3 (0.45%) of the chicken liver samples were positive for Salmonella sp. in preenrichment method and direct selective agar plating method, respectively; these percentages were significantly different (P=0.0001; kappa=0.16). There was also a statistical difference in relative accuracy, sensitivity and negative predictive value between the preenrichment method and the direct selective agar plating method; the first had greater values for these parameters than the direct selective agar plating method. These parameters were statistically different between MacConkey agar (MCA) and modified lysine iron (MLIA) in the two culture methods; the second had greater values than MCA for both culture methods. This study shows that even though serovars that are important for public health were isolated, the prevalence of Salmonella sp. is low in chicken livers from Entre Rios, Argentina. The isolated strains do not have multi-resistance patterns. Furthermore, the preenrichment method and MLIA are superior to the direct selective agar plating method and MCA for Salmonella sp. isolation from chicken liver samples, respectively.

Relative effectiveness of selected preenrichment media for the detection of Salmonella from leafy green produce and herbs

Four buffered preenrichment media (BAX® System MP Media (BAX)), Universal Preenrichment Broth (UPB), modified Buffered Peptone Water (mBPW), and Buffered Peptone Water (BPW)) were compared with lactose broth (LB) in the Bacteriological Analytical Manual's (BAM) Salmonella culture method for the analysis of 9 leafy green produce and herb types. Artificially contaminated test portions were pre-enriched in each medium and the results were analyzed statistically using Fisher's Exact 2-tailed F test (p < 0.05) with pairwise comparisons. There was no difference in recovery of Salmonella from curly parsley and basil among the five media (p > 0.05). UPB was consistently among the most effective media for recovery of Salmonella from the nine produce types; however, S. Typhimurium and S. Newport were isolated from cabbage more frequently with mBPW than with UPB (p < 0.05). Comparisons of the results among the preenrichment media from all experimental trials, with leafy green produce and herbs, demonstrate that Salmonella is more effectively detected and isolated using buffered enrichments than with the currently recommended LB (p < 0.05). There were no significant differences among the buffered preenrichments for the detection of Salmonella-positive test portions of the produce tested (BAX (160 Salmonella-positive test portions/480 test portions), UPB (176/480), mBPW (184/480), BPW (169/480), LB (128/480))(p > 0.05).

Magnetic-optical nanohybrids for targeted detection, separation, and photothermal ablation of drug-resistant pathogens

A rapid, sensitive and quantitative immunoassay for the targeted detection and decontamination of E. coli based on Fe3O4 magnetic nanoparticles (MNPs) and plasmonic popcorn-shaped gold nanostructure attached single-walled carbon nanotubes (AuNP@SWCNT) is presented. The MNPs were synthesized as the support for a monoclonal antibody (mAb@MNP). E. coli (49979) was captured and rapidly preconcentrated from the sample with the mAb@MNP, followed by binding with Raman-tagged concanavalin A-AuNP@SWCNTs (Con A-AuNP@SWCNTs) as detector nanoprobes. A Raman tag 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) generated a Raman signal upon 670 nm laser excitation enabling the detection and quantification of E. coli concentration with a limit of detection of 10(2) CFU mL(-1) and a linear logarithmic response range of 1.0 × 10(2) to 1.0 × 10(7) CFU mL(-1). The mAb@MNP could remove more than 98% of E. coli (initial concentration of 1.3 × 10(4) CFU mL(-1)) from water. The potential of the immunoassay to detect E. coli bacteria in real water samples was investigated and the results were compared with the experimental results from the classical count method. There was no statistically significant difference between the two methods (p > 0.05). Furthermore, the MNP/AuNP@SWCNT hybrid system exhibits an enhanced photothermal killing effect. The sandwich-like immunoassay possesses potential for rapid bioanalysis and the simultaneous biosensing of multiple pathogenic agents.

Culture versus PCR for Salmonella Species Identification in Some Dairy Products and Dairy Handlers with Special Concern to Its Zoonotic Importance

A total of 200 samples of milk and dairy products as well as 120 samples of dairy handlers were randomly collected from different dairy farms and supermarkets in Dakahlia Governorate, Egypt. The conventional cultural and serotyping methods for detection of Salmonella in dairy products were applied and the results were compared with those obtained by molecular screening assay using (ttr sequence). The obtained results revealed that 21% of milk and dairy products (42/200) were positive for Salmonella species using enrichment culture-based PCR method, while 12% of different dairy samples (24/200) were found to be positive for Salmonella species by using the conventional culture methods. Two stool specimens out of 40 apparently healthy dairy handlers were positive by the PCR method. Serotyping of Salmonella isolates revealed that 58.3% (14/24) from different dairy products were contaminated with Salmonella Typhimurium. We conclude that the enrichment culture-based PCR assay has high sensitivity and specificity for detection of Salmonella species in dairy products and handlers. High incidence of Salmonella Typhimurium in the examined dairy samples highlights the important role played by milk and dairy products as a vehicle in disease prevalence. Great effort should be applied for reducing foodborne risk for consumers.

Comparison of a real-time PCR method with a culture method for the detection of Salmonella enterica serotype enteritidis in naturally contaminated environmental samples from integrated poultry houses

Conventional culture methods have traditionally been considered the "gold standard" for the isolation and identification of foodborne bacterial pathogens. However, culture methods are labor-intensive and time-consuming. A Salmonella enterica serotype Enteritidis-specific real-time PCR assay that recently received interim approval by the National Poultry Improvement Plan for the detection of Salmonella Enteritidis was evaluated against a culture method that had also received interim National Poultry Improvement Plan approval for the analysis of environmental samples from integrated poultry houses. The method was validated with 422 field samples collected by either the boot sock or drag swab method. The samples were cultured by selective enrichment in tetrathionate broth followed by transfer onto a modified semisolid Rappaport-Vassiliadis medium and then plating onto brilliant green with novobiocin and xylose lysine brilliant Tergitol 4 plates. One-milliliter aliquots of the selective enrichment broths from each sample were collected for DNA extraction by the commercial PrepSEQ nucleic acid extraction assay and analysis by the Salmonella Enteritidis-specific real-time PCR assay. The real-time PCR assay detected no significant differences between the boot sock and drag swab samples. In contrast, the culture method detected a significantly higher number of positive samples from boot socks. The diagnostic sensitivity of the real-time PCR assay for the field samples was significantly higher than that of the culture method. The kappa value obtained was 0.46, indicating moderate agreement between the real-time PCR assay and the culture method. In addition, the real-time PCR method had a turnaround time of 2 days compared with 4 to 8 days for the culture method. The higher sensitivity as well as the reduction in time and labor makes this real-time PCR assay an excellent alternative to conventional culture methods for diagnostic purposes, surveillance, and research studies to improve food safety.

Development and evaluation of DNA and RNA real-time assays for food analysis using the hilA gene of Salmonella enterica subspecies enterica

The objective of this study was the development of DNA and RNA real-time PCR methods for detection of food-borne Salmonella sp. as rapid alternatives to the traditional cultural method (ISO 6579, 2004) in fresh meat carcasses and processed meat samples. These PCR methods were based on the hilA sequence, with primers and hybridisation probes designed against this gene target. The primers and probes were evaluated for their efficiency and dynamic range and subsequently the specificity of the assay was tested using 106 Salmonella enterica subspecies enterica strains and 30 non-salmonellae strains. An internal amplification control (IAC) was also developed for incorporation. The optimum copy number of IAC was determined to be 500 copies per reaction. A complementary enrichment protocol was adapted from the existing standard ISO 6579:2004 and consisted of enrichment in Buffered Peptone Water (BPW) 22 ± 2 h and a second selective enrichment for 6 h in Rappaport Vassiliadis with Soya (RVS). The DNA and RNA-based real-time PCR protocols, were applied to meat samples inoculated with Salmonella enterica subspecies enterica strains, including swabs from meat carcasses and minced beef samples which were heat treated or frozen. The developed methods have the potential as useful alternatives to the standard ISO 6579:2004 method for the detection of Salmonella enterica subspecies enterica on carcass swabs and raw meat using hilA as a target. The DNA assay is a useful tool for the screening of meat samples in the abattoir within 3 days of slaughter or in a food production process and the RNA-based assay has the potential to detect viable Salmonella enterica subspecies enterica in ready-to-eat products.

Interference of raw milk autochthonous microbiota on the performance of conventional methodologies for Listeria monocytogenes and Salmonella spp. detection

Pathogen detection in foods by reliable methodologies is very important to guarantee microbiological safety. However, peculiar characteristics of certain foods, such as autochthonous microbiota, can directly influence pathogen development and detection. With the objective of verifying the performance of the official analytical methodologies for the isolation of Listeria monocytogenes and Salmonella in milk, different concentrations of these pathogens were inoculated in raw milk treatments with different levels of mesophilic aerobes, and then submitted to the traditional isolation procedures for the inoculated pathogens. Listeria monocytogenes was inoculated at the range of 0.2-5.2 log CFU/mL in treatments with 1.8-8.2 log CFU/mL. Salmonella Enteritidis was inoculated at 0.9-3.9 log CFU/mL in treatments with 3.0-8.2logCFU/mL. The results indicated that recovery was not possible or was more difficult in the treatments with high counts of mesophilic aerobes and low levels of the pathogens, indicating interference of raw milk autochthonous microbiota. This interference was more evident for L. monocytogenes, once the pathogen recovery was not possible in treatments with mesophilic aerobes up to 4.0 log CFU/mL and inoculum under 2.0 log CFU/mL. For S. Enteritidis the interference appeared to be more non-specific.

The effect of pre-enrichment protocol on the sensitivity and specificity of PCR for detection of naturally contaminated Salmonella in raw poultry compared to conventional culture

Salmonella spp. are the leading cause of foodborne illness worldwide. Conventional culture techniques for the detection of Salmonella spp. are labor intensive and time consuming. Several rapid detection methods have been developed over the past few years. However, standard methods for sample handling and preparation have not been established and limited data are available on the sensitivity and specificity of these methods for detection of Salmonella in naturally contaminated retail meat. Using culture as the gold standard for Salmonella detection in naturally contaminated raw poultry products, the sensitivity and specificity of a polymerase chain reaction (PCR) detection method was determined under varying enrichment protocols. Chicken meat samples (ground, boneless/skinless breast meat, and bone-in breast meat with skin) from retail grocery stores were pre-enriched in buffered peptone water (BPW) and Salmonella specific primers ST 11 and ST 15 were used to amplify a 429 bp region of random fragment target specific to all Salmonella spp. There was a significant decrease (P-value<0.001) in the sensitivity of the PCR test when BPW pre-enrichment alone (85%) was used compared to the sensitivity achieved after both BPW enrichment and selective enrichment with RV and TT-H (100%). PCR failed to detect any positive samples when no pre-enrichment was conducted. A minimum of 12h pre-enrichment was required for detection of Salmonella by PCR at a limit of 100 colony forming unit (cfu)/1 ml of sample. No detectable amplification product was seen in those naturally contaminated meat samples testing negative by culture methods.

Preenrichment versus direct selective agar plating for the detection of Salmonella Enteritidis in shell eggs

The relative effectiveness of two methods for the recovery of Salmonella Enteritidis (SE) from jumbo and medium shell eggs was compared. The first method used in the comparison consisted of a preenrichment of the sample, and the second method was developed by the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS). Three bulk lots of blended, pooled eggs, each containing 220 liquid whole eggs that were thoroughly mixed manually were artificially inoculated with different levels of SE cells between approximately 10(0) and 10(3) CFU/ml. Twenty samples containing the contents of approximately 10 eggs each (by weight) were withdrawn from each of the inoculated bulk lots and incubated for 4 days at room temperature (ca. 23 degrees C). For the APHIS method, each sample was cultured by direct plating onto brilliant green (BG), brilliant green with novobiocin (BGN), xylose lysine desoxycholate (XLD), and xylose lysine agar Tergitol 4 (XLT4) agars. For the preenrichment method, 25-g portions from each pool were enriched in modified tryptic soy broth with 30 mg/liter of FeSO4. After 24 h of incubation, the preenrichments were subcultured to tetrathionate and Rappaport-Vassiliadis broths, and streaked to BG, BGN, bismuth sulfite, XLD, and XLT4 agar plates. SE isolates were confirmed biochemically and serologically. In all of the experiments, the preenrichment method recovered significantly more SE isolates (P < 0.05) of all the phage types and inoculum levels than did the APHIS method. From a total of 539 jumbo egg test portions analyzed, 381 (71%) were SE-positive by the preenrichment method and 232 (43%) were positive by the APHIS method. From a total of 360 medium egg test portions analyzed, 223 (62%) were SE-positive by the preenrichment method and 174 (48%) were positive by the APHIS method. The preenrichment method provided greater sensitivity for the isolation of SE in contaminated egg slurries than did the APHIS method.

Application of nested polymerase chain reaction to detection of Salmonella in poultry environment

Isolation of Salmonella from environmental and processing-plant poultry samples requires the sampling of large numbers of areas within the poultry house or plant. Subsequently, the required number of samples necessitates a large volume of work for a microbiology laboratory, especially when the protocol requires the inclusion of a delayed secondary enrichment for the isolation of Salmonella. This study examined the use of the polymerase chain reaction (PCR) to identify those secondary enrichments containing Salmonella. The unique Salmonella virulence gene invA was chosen as the target for the development of a nested PCR because of its uniform distribution among Salmonella serotypes. The use of nested PCR primers increased the sensitivity of detection 100-fold, resulting in the detection of as few as four cells. There was a strong, statistically significant positive correlation between PCR and culture results as determined by chi-square (P < 0.001) and kappa (kappa = 0.915; excellent agreement) tests. Using PCR to screen primary enrichments for presumptive Salmonella contamination, we improved our efficiency at isolating Salmonella upon secondary enrichment by 20%, and no false negatives were observed. This method will not only validate the use of secondary enrichment procedures but also reduce costs and manpower required for the surveillance of Salmonella.

Detection of Salmonella enteritidis in shell and liquid eggs using enrichment and plating

Detection methods using various enrichment and plating media and immunoconcentration for Salmonella enteritidis in shell and liquid eggs were evaluated. For liquid egg samples naturally contaminated with S. enteritidis, pre-enrichment in 225 ml of buffered peptone water with cysteine followed by selective enrichment in 10 ml of tetrathionate broth was the superior, resulting in the detection of S. enteritidis in all samples on six of the seven types of selective agar substrate investigated. This enrichment procedure also enabled detection of S. enteritidis in most of artificially inoculated shell egg and pasteurized liquid egg samples.

Evaluation of motility enrichment on modified semi-solid Rappaport-Vassiladis medium (MSRV) for the detection of Salmonella in foods

The detection and identification of Salmonella spp. is still troublesome and time consuming to the food industry. Employing the modified semi-solid Rappaport-Vassiliadis medium (MSRV), presumptive results for Salmonella can be obtained in 48 h, representing an interesting alternative to the standard methods. The specificity and sensitivity of the MSRV method were evaluated in this research. The efficiency of this method was also compared with the methodology recommended by the US Food and Drug Administration (FDA) using bismuth sulfite agar, XLT4 agar and Rambach agar. A total of 146 food samples comprised of 41 chicken thighs, 35 Brazilian fresh pork sausages, 35 samples of cocoa powder and/or granulated cocoa and 35 samples of grated fresh coconut, were examined. Overall, the rapid method (direct + indirect) and the standard culture detected 96.1% and 84.6% of the positive samples, respectively. No Salmonella was detected in the coconut or cocoa samples by any of the methods. Eighteen (43.9%) chicken thigh samples were contaminated with the microorganism. The rapid method (direct + indirect) and the standard culture detected 94.4% and 88.9% of these, respectively. Salmonella was detected in eight (22.8%) fresh pork sausage samples. The MSRV method detected Salmonella in all eight samples, while the standard gave positive results in six (75%). When compared with the standard method, the indirect method showed 86.4% sensitivity and 96.8% specificity, while the direct MSRV showed a sensitivity of 71.4% and specificity of 99.2%. Combined, both MSRV methods showed 95.5% sensitivity and 96.8% specificity. The MSRV medium also reduces the time necessary for the isolation of Salmonella from foods.

Update on media for isolation of Enterobacteriaceae from foods

Testing for 'total' Enterobacteriaceae, coliforms and Escherichia coli as marker organisms in foods and detection of specific pathogens of the family Enterobacteriaceae, including pathogenic E. coli, Salmonella, Shigella and Yersinia spp. is widely applied in many food control laboratories. This review describes some recent developments in culture media for these organisms. Methods for enumeration of E. coli include the standard MPN technique, a membrane-filter method and the use of media containing chromogenic and fluorogenic indicators for beta-D-glucuronidase (GUD) activity. Shiga toxin-producing E. coli O157 strains usually do not ferment sorbitol and are GUD-negative. These characteristics are used in selective media for these organisms, such as cefixime tellurite sorbitol MacConkey agar. For the detection of salmonellae, motility enrichment in Modified Semisolid Rappaport-Vassiliadis (MSRV) medium shows equal or better results than the use of standard Rappaport-Vassiliadis broth. Addition of nitrofurantoin to diagnostic semisolid salmonella agar and to xylose lysine desoxycholate agar favours the isolation of S. enteritidis. Recently developed salmonella media use different selective and diagnostic properties, such as acid formation from propylene glycol, glucuronate fermentation, fermentation of glycerol and addition of Tergitol 4 as selective agent. The isolation of Shigella spp. from foods is rather difficult and further evaluation of suggested isolation systems and the development of more effective methods for the isolation of this pathogen are needed. Yersinia enterocolitica includes both pathogenic and nonpathogenic biotypes and serogroups. As no single procedure will recover all pathogenic strains of Y. enterocolitica, the use of two isolation procedures in parallel is recommended.

Evaluation of a new enrichment broth for the isolation of Salmonella spp. from poultry products

Four selective enrichment broths were compared for the detection of Salmonella spp. in naturally contaminated poultry products and the recovery of atypical Salmonella strains in suspensions of pure cultures. In analysis of 100 poultry samples, the sensitivities observed were 94.0% for Müller-Kauffmann Tetrathionate-Brilliant Green (MKTBG), 97.6% for Rappaport Vassiliadis (RV), 42.2% for Selenite Cystine (SC) and 97.6% for the new broth KIMAN (Whitley Impedance Broth supplemented with 20 mg/l of novobiocin sodium salt, 10 mg/l of malachite green oxalate and 40 g/l of potassium iodide). The two most efficient broths--RV and KIMAN for recovery of atypical Salmonella strains (gallinarum biotypes gallinarum and pullorum, typhi, paratyphi A) were less toxic than MKTBG but more toxic than s.c. broth. According to these results, the use of RV and KIMAN could be a good combination to assure maximal recovery of Salmonella strains.

Salmonellae and food safety

Salmonella is one of the most important foodborne pathogens around the world. The knowledge that very low numbers of Salmonella cells can be infectious emphasizes the need for stringent food safety measures Traditional methods for isolating and identifying Salmonella in food rely on preenrichment, selective enrichment in selective and differential media, biochemical tests, and serological confirmation. Recent advances in diagnostic technology have considerably altered testing methods for foodborne Salmonella. Many commercial assay systems and kits that use newer technologies are available to facilitate the identification of Salmonella in foods. These systems include miniaturized biochemical tests, new media formulations, automated instrumentation, DNA/RNA probes, antibody-dependent assays, and polymerase chain reaction. The technologies used for these systems are described, and the various kit formats are compared. Among the limitations of detection methods in terms of food safety are timeliness, limits of detection, and differentiation of virulent and nonvirulent isolates. Current efforts of prevention measures and strategies at different links of the food chain such as consumer education and hazard analysis and critical control point (HACCP) programs are reviewed, Global approaches to food safety are needed..

Detection of Salmonella typhimurium in dairy products with flow cytometry and monoclonal antibodies

Flow cytometry, combined with fluorescently labelled monoclonal antibodies, offers advantages of speed and sensitivity for the detection of specific pathogenic bacteria in foods. We investigated the detection of Salmonella typhimurium in eggs and milk. Using a sample clearing procedure, we determined that the detection limit was on the order of 10(3) cells per ml after a total analysis time of 40 min. After 6 h of nonselective enrichment, the detection limits were 10 cells per ml for milk and 1 cell per ml for eggs, even in the presence of a 10,000-fold excess of Escherichia coli cells.

A comparison of six different plating media used in the isolation of Salmonella

Seventeen Canadian Federal, Provincial and Public Health Laboratories took part in different phases of a comparative/collaborative study that evaluated rapid methods to the standard Health Protection Branch (HPB) method for the detection of Salmonella. A variety of commercial media were tested, including Brilliant Green Sulpha Agar, Bismuth Sulphite Agar, Hektoen Enteric Agar, Xylose Lysine Deoxycholate Agar, EF-18 Agar and Rambach Agar. Each laboratory compared up to six of these different plating media. Plating of 123 salmonellae cultures and 28 artificially-inoculated foods showed the recovery of Salmonella spp. on the six plating media to be within one log. Therefore, quantitative testing of the media showed them to be comparable in the recovery of salmonellae. Qualitative testing of the six media during the comparative/collaborative study of various methods showed that EF-18 Agar recovered the greatest number of isolates. Hektoen Enteric Agar ranked second, with the other agars being comparable in their recovery of Salmonella spp. Problems with the various media are summarized. Based on our results and those of other researchers, it is recommended that Bismuth Sulphite Agar be compulsory and that at least one other agar be used for newly developed cultural procedures.