Evaluation and comparison of rapid methods for the detection of Salmonella in naturally contaminated pine nuts using different pre enrichment media

Evaluation of universal preenrichment broth and comparison of rapid molecular methods for the detection of Salmonella from spent sprout irrigation water (SSIW)

Sprouts and spent sprout irrigation water (SSIW) present unique challenges for the development of a Salmonella detection method in food matrices. This study aimed to compare universal preenrichment broth (UPB) and lactose broth (LB) as preenrichment media for cultural and rapid screening methods and to compare their abilities to recover Salmonella in SSIW samples from different sprout varieties (i.e., alfalfa, broccoli, and mung bean sprouts). The associated co-enriched microbiota with different sprout varieties using different preenrichment media were also examined using a quasi-metagenomic approach. The performance of media and detection methods was compared using the relative level of detection (RLOD) value, as recommended by ISO 16140-2:2016. The level of detection (LOD) for Salmonella culture method with UPB was similar to that with LB in low aerobic plate count (APC) background samples (the relative LOD, i.e., RLOD, was nearly 1 after adjusting for the effects of SSIW variety and serovar), but significantly lower than that with LB in high APC background samples (RLOD = 0.32). The LOD for Salmonella with selected rapid methods was comparable to each other (RLOD from 0.97 to 1.50) and to the culture method (RLOD from 0.69 to 1.03), and no significant difference was detected between preenrichment broths in low APC background samples with RLOD values between 0.76 and 1.04. In samples with a high APC background, however, a drastic difference in LOD was observed between methods and between preenrichment broths for each method. The RLOD ranged from 0.03 to 0.32 when UPB was compared to LB as preenrichment broth. The composition and relative abundance (RA) of co-enriched microbiota was affected by multiple factors including food matrices, preenrichment media and Salmonella contamination. Altogether, this study validated UPB as a better preenrichment broth than LB for the detection of Salmonella enterica from SSIW. This study also suggested UPB may also be an optimal preenrichment medium for rapid screening methods when APC level is high. The observation of potential exclusion of Salmonella in preenrichment through the overgrowth of competitive microflora from the quasi-metagenomic study provided novel information that may be used to further optimize preenrichment formulations.

Multi-laboratory validation study of a real-time PCR method for detection of Salmonella in baby spinach

The FDA Bacteriological Analytical Manual (BAM) Salmonella culture method takes at least 3 days for a presumptive positive result. The FDA developed a quantitative PCR (qPCR) method to detect Salmonella from 24-h preenriched cultures, using ABI 7500 PCR system. The qPCR method has been evaluated as a rapid screening method for a broad range of foods by single laboratory validation (SLV) studies. The present multi-laboratory validation (MLV) study was aimed to measure the reproducibility of this qPCR method and compare its performance with the culture method. Sixteen laboratories participated in two rounds of MLV study to analyze twenty-four blind-coded baby spinach test portions each. The first round yielded ∼84% and ∼82% positive rates across laboratories for the qPCR and culture methods, respectively, which were both outside the fractional range (25%-75%) required for fractionally inoculated test portions by the FDA's Microbiological Method Validation Guidelines. The second round yielded ∼68% and ∼67% positive rates. The relative level of detection (RLOD) for the second-round study was 0.969, suggesting that qPCR and culture methods had similar sensitivity (p > 0.05). The study demonstrated that the qPCR yields reproducible results and is sufficiently sensitive and specific for the detection of Salmonella in food.

Combination of filtration and immunomagnetic separation based on real-time PCR to detect foodborne pathogens in fresh-cut apple

Rapid detection methods require pre-enrichment culture in order to detect low levels of foodborne pathogens. To rapidly detect foodborne pathogens, enrichment culture processes could be replaced. Filtration and immunomagnetic separation methods have been identified to effectively concentrate and separate target pathogens from foods. In this study, a combination of filtration and immunomagnetic separation (IMS) has enabled the rapid and sensitive detection of foodborne pathogens. The pretreatment method, including separation and concentration procedures, increased sensitivity 10-100-fold. The sensitivity of a combination method using filtration and IMS to detect Escherichia coli O157:H7 and Salmonella enterica subsp. enterica serovar Typhimurium was 10⁰-10¹ CFU/10 mL. In fresh-cut apples, IMS combined with filtration effectively improved the detection limit of real-time PCR to 2.70 × 10¹ CFU/g in E. coli O157:H7 and 1.80 × 10² CFU/g in Salmonella. The filtration simplified processing of large-volumes (250 mL) and effectively concentrated pathogens while decreasing immunomagnetic beads used in IMS. Bacterial concentration by IMS combined with filtration increased sensitivity 10-100-fold compared with control. In addition, the application of IMS effectively removed concentrated residual food material (10-15 mg/mL) after filtration, improving relative sensitivity. In conclusion, this method may detect foodborne pathogen in foods such as fresh-cut fruits in a more rapid and sensitive fashion than traditional culture-based methods.

Direct detection of Salmonella from poultry samples by DNA isothermal amplification

1. Salmonellosis is one of the most important diseases in public health and it is usually associated with poultry product consumption. This study aimed to validate rapid methods to detect Salmonella spp. from poultry samples. 2. A DNA isothermal amplification method, previously developed for other matrices, was applied for the specific detection of Salmonella spp. from various samples, including poultry tissues, drag and boot swabs, faeces and feed. A new procedure was validated with Salmonella spp. serotypes and isolates from other enteric bacterial species, as well as naturally contaminated poultry samples. 3. The study demonstrated the successful development and implementation of a procedure, including a DNA isothermal amplification method, for the detection of Salmonella spp. directly from tissues, drag and boot swabs, faeces and feed. The whole procedure can be performed in less than 24 hours and it has been successfully used in a veterinary diagnostic laboratory.

Application of LpxC enzyme inhibitor to inhibit some fast-growing bacteria in human gut bacterial culturomics

BACKGROUND

Culturomics can ascertain traces of microorganisms to be cultivated using different strategies and identified by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry or 16S rDNA sequencing. However, to cater to all requirements of microorganisms and isolate as many species as possible, multiple culture conditions must be used, imposing a heavy workload. In addition, the fast-growing bacteria (e.g., Escherichia) surpass the slow-growing bacteria in culture by occupying space and using up nutrients. Besides, some bacteria (e.g., Pseudomonas) suppress others by secreting antibacterial metabolites, making it difficult to isolate bacteria with lower competence. Applying inhibitors to restrain fast-growing bacteria is one method to cultivate more bacterial species from human feces.

RESULTS

We applied CHIR-090, an LpxC enzyme inhibitor that has antibacterial activity against most Gram-negative bacteria, to culturomics of human fresh feces. The antibacterial activity of CHIR-090 was first assessed on five Gram-negative species of bacteria (Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Proteus vulgaris, and Bacteroides vulgatus), all of which are commonly isolated from the human gut. Then, we assessed suitable concentrations of the inhibitor. Finally, CHIR-090 was applied in blood culture bottles for bacterial cultivation. In total, 102 species from five samples were identified. Of these, we found one new species, two species not reported previously in the human gut, and 11 species not previously isolated from humans.

CONCLUSIONS

CHIR-090 can suppress E. coli, P. aeruginosa, K. pneumoniae, Pro. vulgaris, but not B. vulgatus. Compared with the non-inhibitor group, CHIR-090 increased bacteria isolation by 23.50%, including four species not reported in humans and one new species. Application of LpxC enzyme inhibitor in culturomics increased the number of species isolated from the human gut.

Evaluation of Incompatibility Group I1 (IncI1) Plasmid-Containing Salmonella enterica and Assessment of the Plasmids in Bacteriocin Production and Biofilm Development

Mobile genetic elements, such as plasmids, can potentially increase the ability of bacteria to infect and persist in vertebrate host cells. IncI1 plasmids are widely distributed in Salmonella from food animal sources and associated with clinically important strains. These plasmids often encode antimicrobial resistance; however, little is known about their impact on the virulence of Salmonella strains. To assess the potential impact of the plasmids on virulence, 43 IncI1-positive Salmonella isolates from human and animal sources were subjected to whole genome sequence (WGS) analyses and evaluated for their abilities to invade and persist for 48 h in Caco-2 human intestinal epithelial cells, form biofilms and encode bacteriocins. Draft WGS data were submitted to predict the presence of virulence and antimicrobial resistance genes, plasmid replicon types present, conduct plasmid multilocus sequence typing (pMLST), and core genome MLST (cgMLST) in the isolates. Caco-2 cells were infected with Salmonella strains and incubated for both one and 48 h for the invasion and persistence assays, respectively. Additionally, Salmonella isolates and IncI1 plasmid carrying transconjugants (n = 12) generated in Escherichia coli were assessed for their ability to produce biofilms and bacteriocin inhibition of growth of other bacteria. All Salmonella isolates infected Caco-2 cells and persisted in the cells at 48 hrs. Persistent cell counts were observed to be significantly higher than invasion assay cell counts in 26% of the isolates. Among the IncI1 plasmids, there were 18 pMLST types. Nearly 35% (n = 15) of Salmonella isolates produced biofilms; however, none of the IncI1-positive transconjugants produced increased biofilms compared to the recipient. Approximately 65% (n = 28) of isolates and 67% (n = 8) of IncI1-positive transconjugants were able to inhibit growth of at least one E. coli strain; however, none inhibited the growth of strains from species other than E. coli. The study characterized IncI1 positive Salmonella isolates and provided evidence about the potential contributions of IncI1 plasmids virulence phenotypes and areas where they do not. These findings should allow for more focused efforts to assess the impact of plasmids on bacterial pathophysiology and human health.

Challenges in Recovering Foodborne Pathogens from Low-Water-Activity Foods

There are numerous obstacles to the detection of foodborne pathogens in foods that exhibit a low water activity (a_w). These obstacles include the presence of antimicrobial compounds, particulates, PCR inhibitors, and fatty matrices. New approaches should be sought to increase the sensitivity of pathogen testing in low-a_w foods and to overcome the effects of various inhibitors and antimicrobials. The U.S. Food and Drug Administration and other laboratories are working toward this goal. This review will address these issues while delineating specific inhibitors and antimicrobials that impede testing of low-a_w foods. A review of relevant rapid and conventional testing methodologies for Salmonella in low-a_w foods will also be discussed.

Simultaneous and Ultrasensitive Detection of Foodborne Bacteria by Gold Nanoparticles-Amplified Microcantilever Array Biosensor

Foodborne pathogens, especially bacteria, are explicitly threatening public health worldwide. Biosensors represent advances in rapid diagnosis with high sensitivity and selectivity. However, multiplexed analysis and minimal pretreatment are still challenging. We fabricate a gold nanoparticle (Au NP)-amplified microcantilever array biosensor that is capable of determining ultralow concentrations of foodborne bacteria including Escherichia coli O157:H7, Vibrio parahaemolyticus, Salmonella, Staphylococcus aureus, Listeria monocytogenes, Shigella, etc. The method is much faster than using conventional tools without germiculturing and PCR amplification. The six pairs of ssDNA probes (ssDNA₁ + ssDNA₂ partially complementary to the target gene) that originated from the sequence analysis of the specific gene of the bacteria were developed and validated. The ssDNA₁ probes were modified with -S-(CH₂)₆ at the 5′-end and ready to immobilize on the self-assembled monolayers (SAMs) of the sensing cantilevers in the array and couple with Au NPs, while 6-mercapto-1-hexanol SAM modification was carried out on the reference cantilevers to eliminate the interferences by detecting the deflection from the environment induced by non-specific interactions. For multianalyte sensing, the target gene sequence was captured by the ssDNA₂-Au NPs in the solution, and then the Au NPs-ssDNA₂-target complex was hybridized with ssNDA₁ fixed on the beam of the cantilever sensor, which results in a secondary cascade amplification effect. Integrated with the enrichment of the Au NP platform and the microcantilever array sensor detection, multiple bacteria could be rapidly and accurately determined as low as 1–9 cells/mL, and the working ranges were three to four orders of magnitude. There was virtually no cross-reaction among the various probes with different species. As described herein, it holds great potential for rapid, multiplexed, and ultrasensitive detection in food, environment, clinical, and communal samples.

Evaluation of the Genetics and Functionality of Plasmids in Incompatibility Group I1-Positive Salmonella enterica

Salmonella is a predominant foodborne pathogen in the United States and other countries. Mobile genetic elements such as plasmids allow Salmonella to adapt to external stress factors such as nutrient deprivation and host factors. Incompatibility group I1 (IncI1) plasmid-carrying Salmonella enterica strains were examined to determine the presence of plasmid-associated genes and their influence on phenotypic characteristics. The objective of this study was to understand the genetic determinants on IncI1 plasmids and their impact on antimicrobial susceptibility, competitive growth inhibition of Escherichia coli, and plasmid transfer. Primers were designed for genes that play a role in virulence, antimicrobial resistance, and plasmid transfer based on previously sequenced IncI1 plasmids. Polymerase chain reaction assays were conducted on 92 incompatibility group I1 (IncI1)-positive S. enterica strains. Phenotypic characterization included conjugation assays, antimicrobial susceptibility testing, and bacteriocin production based on the inhibition of growth of colicin-negative E. coli J53. The antimicrobial resistance genes aadA1, tetA, sul1, and bla_CMY were detected in 88%, 87%, 80%, and 48% of the strains, respectively. Over half of the strains were resistant or intermediately resistant to streptomycin (85%), sulfonamides (76%), tetracycline (74%), and ampicillin (68%) and 57% of the strains inhibited growth of E. coli J53 strain. Among putative virulence genes, colicin-associated colI and cib were detected in 23% and 35% of strains and imm and ccdA were present in 58% and 54% of strains, respectively. Approximately 61% of strains contained plasmids that conjugally transferred antimicrobial resistance, including 83% where the recipient received IncI1 plasmids. Most of the strains carried an assortment of transfer associated (pil and tra) genes with between 63% and 99% of strains being positive for individual genes. Taken together the study affirms that IncI1 plasmids likely play roles in the dissemination of antimicrobial resistance and virulence-associated factors among enteric organisms.

Detection of Salmonella enterica subsp. enterica Serovar Cubana from Naturally Contaminated Chick Feed

Because some significant outbreaks of human salmonellosis have been traced to contaminated animal feed, the rapid and efficient detection of Salmonella in feed is essential. However, the current U.S. Food and Drug Administration Bacteriological Analytical Manual (BAM) culture method that uses lactose broth as a preenrichment medium has not reliably supported the results of real-time PCR assays for certain foods. We evaluated the BAM culture method and a quantitative real-time PCR (qPCR) assay using two preenrichment media, modified buffered peptone water and lactose broth, to detect Salmonella enterica subsp. enterica serovar Cubana in naturally contaminated chick feed. After 24 h of incubation, the qPCR method was as sensitive as the culture method when modified buffered peptone water was used as the preenrichment medium but less sensitive than culture when lactose broth was used. After 48 h of incubation, detection of Salmonella Cubana by qPCR and by culture in either preenrichment medium was equivalent. We also compared the performance of the traditional serotyping method, which uses pure cultures of Salmonella grown on blood agar, to two molecular serotyping methods. The serotyping method based on whole genome sequencing also requires pure cultures, but the PCR-based molecular serotyping method can be done directly with the enriched culture medium. The PCR-based molecular serotyping method provided simple and rapid detection and identification of Salmonella Cubana. However, whole genome sequencing allows accurate identification of many Salmonella serotypes and highlights variations in the genomes, even in tight genomic clusters. We also compared the genome of the chick feed isolate with 58 Salmonella Cubana strains in GenBank and found that the chick feed isolate was very closely related to an isolate from a foodborne outbreak involving alfalfa sprouts.

Evaluation of Three Real-Time PCR Methods for Detection of Salmonella from Cloves

The purpose of the study was to evaluate three real-time PCR platforms for rapid detection of Salmonella from cloves and to compare three different DNA extraction methods. Six trials were conducted with two clove cultivars, Ceylon and Madagascar, and three Salmonella serotypes, Montevideo, Typhimurium, and Weltevreden. Each trial consisted of 20 test portions. The preenrichment cultures were used to perform PCR for comparison of the effectiveness of U.S. Food and Drug Administration, Pacific Regional Laboratory Southwest (FDA-PRLSW), Applied Biosystems Inc. (ABI) MicroSEQ, and GeneDisc platforms for detection of Salmonella. Three DNA extraction methods were used: standard extraction method for each PCR platform, boil preparation, and LyseNow food pathogen DNA extraction cards. The results from real-time PCR correlated well with FDA Bacteriological Analytical Manual culture assay results, with a wide range of cycle threshold (C_T) values among the three PCR platforms for intended positive samples. The mean C_T values for MicroSEQ (16.36 ± 2.78) were significantly lower than for PRLSW (20.37 ± 3.45) and GeneDisc (23.88 ± 2.90) (P < 0.0001). Pairwise comparisons between PCR platforms using different DNA extraction methods indicate that the C_T values are inversely proportional to the relative DNA quantity (RDQ) yields by different platform-extraction combinations. The pairing of MicroSEQ and boil preparation generated the highest RDQ of 120 and the lowest average C_T value of 14.48, whereas the pairing of GeneDisc and LyseNow generated the lowest RDQ of 0.18 and the highest average C_T of 25.97. Boil preparation yielded higher RDQ than the other extraction methods for all three PCR platforms. Although the MicroSEQ platform generated the lowest C_T values, its sensitivity was compromised by narrow separations between the positive and negative samples. The PRLSW platform generated the best segregation between positive and negative groups and is less likely to produce false results. In conclusion, FDA-PRLSW was the most efficient PCR assay for Salmonella detection, and boil preparation was the best method for DNA extraction.

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.

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).

Development and Validation of a Cultural Method for the Detection and Isolation of Salmonella in Cloves

Detection of Salmonella in some spices, such as cloves, remains a challenge due to their inherent antimicrobial properties. The purpose of this study was to develop an effective detection method for Salmonella from spices using cloves as a model. Two clove varieties, Ceylon and Madagascar, were used in the study. Cloves were inoculated with Salmonella enterica subsp. enterica serotypes Montevideo, Typhimurium, or Weltevreden at about 1, 3, or 6 log CFU/25 g. Two test portion sizes, 10 and 25 g, were compared. After adding Trypticase soy broth (TSB) to the weighed cloves for preenrichment, three preenrichment methods were compared: cloves were left in the TSB for 24 h during preenrichment (PreE1), or the cloves-TSB mixture was shaken vigorously for 30 s (PreE2) or 60 s (PreE3), and the decanted material was transferred to a new bag for 24 h of preenrichment. The rest of the procedures were carried out according to the U.S. Food and Drug Administration Bacteriological Analytical Manual (BAM). At the low inoculation level (<1 log CFU/25 g), the detection rate was low across the three preenrichment methods, with the highest for PreE3 and lowest for PreE1. At the medium and high inoculation levels (3 and 6 log CFU/25 g), all samples from PreE2 and PreE3 were positive for Salmonella , whereas PreE1 produced only 12 positive samples from the 48 samples at the medium inoculation level and 38 positive samples from the 48 samples at the high inoculation level. Therefore, PreE3 with 25 g of cloves per sample was more effective than the other two tested methods. This newly designed method was then validated by comparing with the BAM method in six trials, with each trial consisting of 40 test samples. The results showed that PreE3 detected Salmonella from 88 of 120 inoculated test samples compared with only 31 positive from 120 test samples with the BAM method. Thus, our newly designed method PreE3 was more sensitive and easier to operate than the current BAM method for detection of Salmonella in cloves.

Early Recovery of Salmonella from Food Using a 6-Hour Non-selective Pre-enrichment and Reformulation of Tetrathionate Broth

Culture based methods are commonly employed to detect pathogens in food and environmental samples. These methods are time consuming and complex, requiring multiple non-selective and selective enrichment broths, and usually take at least 1 week to recover and identify pathogens. Improving pathogen detection in foods is a primary goal for regulatory agencies and industry. Salmonella detection in food relies on a series of culture steps in broth formulations optimized to resuscitate Salmonella and reduce the abundance of competitive bacteria. Examples of non-selective pre-enrichment broths used to isolate Salmonella from food include Lactose, Universal Pre-enrichment, BPW, and Trypticase Soy broths. Tetrathionate (TT) and Rappaport-Vassiliadis (RV) broths are employed after a 24-h non-selective enrichment to select for Salmonella and hamper the growth of competitive bacteria. In this study, we tested a new formulation of TT broth that lacks brilliant green dye and has lower levels of TT . We employed this TT broth formulation in conjunction with a 6-h non-selective pre-enrichment period and determined that Salmonella recovery was possible one day earlier than standard food culture methods. We tested the shortened culture method in different non-selective enrichment broths, enumerated Salmonella in the non-selective enrichments, and used 16S rRNA gene sequencing to determine the proportional abundances of Salmonella in the TT and RV selective enrichments. Together these data revealed that a 6-h non-selective pre-enrichment reduces the levels of competitive bacteria inoculated into the selective TT and RV broths, enabling the recovery of Salmonella 1 day earlier than standard culture enrichment methods.

Prevalence and Characteristics of Salmonella Serotypes Isolated from Fresh Produce Marketed in the United States

Salmonella continues to rank as one of the most costly foodborne pathogens, and more illnesses are now associated with the consumption of fresh produce. The U.S. Department of Agriculture Microbiological Data Program (MDP) sampled select commodities of fresh fruit and vegetables and tested them for Salmonella, pathogenic Escherichia coli, and Listeria. The Salmonella strains isolated were further characterized by serotype, antimicrobial resistance, and pulsed-field gel electrophoresis profile. This article summarizes the Salmonella data collected by the MDP between 2002 and 2012. The results show that the rates of Salmonella prevalence ranged from absent to 0.34% in cilantro. A total of 152 isolates consisting of over 50 different serotypes were isolated from the various produce types, and the top five were Salmonella enterica serotype Cubana, S. enterica subspecies arizonae (subsp. IIIa) and diarizonae (subsp. IIIb), and S. enterica serotypes Newport, Javiana, and Infantis. Among these, Salmonella serotypes Newport and Javiana are also listed among the top five Salmonella serotypes that caused most foodborne outbreaks. Other serotypes that are frequent causes of infection, such as S. enterica serotypes Typhimurium and Enteritidis, were also found in fresh produce but were not prevalent. About 25% of the MDP samples were imported produce, including 65% of green onions, 44% of tomatoes, 42% of hot peppers, and 41% of cantaloupes. However, imported produce did not show higher numbers of Salmonella-positive samples, and in some products, like cilantro, all of the Salmonella isolates were from domestic samples. About 6.5% of the Salmonella isolates were resistant to the antimicrobial compounds tested, but no single commodity or serotype was found to be the most common carrier of resistant strains or of resistance. The pulsed-field gel electrophoresis profiles of the produce isolates showed similarities with Salmonella isolates from meat samples and from outbreaks, but there were also profile diversities among the strains within some serotypes, like Salmonella Newport.

Development and evaluation of an ELIME assay to reveal the presence of Salmonella in irrigation water: Comparison with Real-Time PCR and the Standard Culture Method

A reliable, low-cost and easy-to-use ELIME (Enzyme-Linked-Immuno-Magnetic-Electrochemical) assay for detection of Salmonella enterica in irrigation water is presented. Magnetic beads (MBs), coupled to a strip of eight-magnetized screen-printed electrodes localized at the bottom of eight wells (8-well/SPE strip), effectively supported a sandwich immunological chain. Enzymatic by-product is quickly measured by chronoamperometry, using a portable instrument. With the goal of developing a method able to detect a wide range of Salmonella serotypes, including S. Napoli and S. Thompson strains responsible for various community alerts, different kinds of MBs, antibodies and blocking agents were tested. The final system employs MBs coated with a broad reactivity monoclonal antibody anti-salmonella and blocked with dry milk. For a simple and rapid assay these two steps were performed in a preliminary phase, while the two sequential incubations for the immuno-recognition events were merged in a single step of 1h. In parallel a Real-Time PCR (RTi-PCR) method, based on a specific locked nucleic acid (LNA) fluorescent probe and an internal amplification control (IAC), was carried out. The selectivity of the ELIME and RTi-PCR assays was proved by inclusivity and exclusivity tests performed analyzing different Salmonella serotypes and non-target microorganisms, most commonly isolated from environmental sources. Furthermore, both methods were applied to experimentally and not experimentally contaminated irrigation water samples. Results confirmed by the ISO culture method, demonstrated the effectiveness of ELIME and RTi-PCR assays to detect a low number of salmonella cells (1-10 CFU/L) reducing drastically the long analysis time usually required to reveal this pathogen.