Epithelial entry rather than the ensuing systemic immune response determines the pathogenicity of two Salmonella enterica serovar Typhimurium strains in a mouse model

Functional amino acid supplementation attenuates the negative effects of plant-based nursery diets on the response of pigs to a subsequent Salmonella Typhimurium challenge

Functional amino acids (FAA) attenuate the effects of Salmonella challenge in pigs. However, this may be affected by protein source (PS). The objective of the present study was to determine the effects of nursery dietary PS and FAA supplementation on growth performance and immune status of pigs subsequently challenged with Salmonella Typhimurium (ST). Thirty-two weanling pigs (8.7 ± 0.23 kg) were assigned to a feeding program for 31 d in a 2 × 2 factorial arrangement. Factors were dietary PS (plant-based [PB] vs. animal-based [AB]) and FAA profile (basal [FAA-] or supplemented [FAA+; Thr, Met, and Trp at 120% of requirements]). Pigs were subsequently placed on a common grower diet and, after a 7-d adaptation, were inoculated with ST and monitored for 7 d postinoculation. Growth performance, rectal temperature, fecal score, gut health, ST shedding score, intestinal colonization and translocation, and blood parameters of acute-phase response and antioxidant balance were measured pre- and postinoculation. Data were analyzed with a 2 (AB vs. PB) × 2 (FAA- vs. FAA+) factorial arrangement of treatments and differences between means were considered significant at P ≤ 0.05. Postinoculation fecal score was worse, ST shedding, cecal myeloperoxidase, and cecal and colonic ST colonization were greater in PB compared to AB pigs (P < 0.05). Translocation of ST to spleen was decreased by FAA+ (P < 0.05), regardless of dietary PS. Postinoculation, AB pigs had greater average daily gain compared to PB-FAA- (P < 0.05). Pigs fed AB-FAA- showed increased average daily feed intake compared to PB-FAA- pigs (P < 0.05) and feed efficiency was increased in AB-FAA+ compared to PB-FAA- pigs (P < 0.05). Feeding PB ingredients in nursery diets seems to increase susceptibility of pigs to Salmonella. Moreover, FAA supplementation partially attenuated the negative effects of PB diets on the response of pigs to ST challenge.

An aptamer biosensor based dual signal amplification system for the detection of salmonella typhimurium

Salmonella, a typical foodborne pathogen, always seriously threatens the health and even life of both humans and animals. However, highly sensitive and fast quantitative methods for its detection are remaining to be challenged. Herein, we presented an efficient method with dual signal amplification strategy by combining immune hybridization chain reaction (HCR) with surface enhanced Raman scattering (SERS) to high sensitively detect Salmonella typhimurium in food. After sample preparation, S. typhimurium were specifically captured by immunomagnetic beads (IMBs), then aptamers and hairpin-probes were added to trigger HCR to form nicked dsDNA, finally 4',6-Diamidino-2-phenylindole dihydrochloride (DAPI) was incubated with HCR products and then the whole system was mixed with AgNP colloid to detect the SERS intensity at 1610 cm^-1. As a result, a good linear relationship was achieved between SERS intensities and corresponding concentrations of S. typhimurium ranging from 10 to 10⁵ CFU/mL, with a limit of detection (LOD) of 6 CFU/mL in 3.5 h. The proposed method has been successfully applied to capture and detect the S. typhimurium in spiked milk samples, and the results were consistent with those of the traditional plate counting method. The method, with combination of HCR and SERS, achieves double amplification of the detection signal and significantly improves the detection sensitivity of S. typhimurium. And it also shows good application potential for the highly sensitive detection of other contaminants in food.

Aptamer-based fluorometric determination of Salmonella Typhimurium using Fe3O4 magnetic separation and CdTe quantum dots

Based on the high sensitivity and stable fluorescence of CdTe quantum dots (QDs) in conjunction with a specific DNA aptamer, the authors describe an aptamer-based fluorescence assay for the determination of Salmonella Typhimurium. The fluorescence detection and quantification of S. Typhimurium is based on a magnetic separation system, a combination of aptamer-coated Fe₃O₄ magnetic particles (Apt-MNPs) and QD-labeled ssDNA2 (complementary strand of the aptamer). Apt-MNPs are employed for the specific capture of S. Typhimurium. CdTe QD-labeled ssDNA2 was used as a signaling probe. Simply, the as-prepared CdTe QD-labeled ssDNA2 was first incubated with the Apt-MNPs to form the aptamer-ssDNA2 duplex. After the addition of S. Typhimurium, they could specifically bind the DNA aptamer, leading to cleavage of the aptamer-ssDNA2 duplex, accompanied by the release of CdTe QD-labeled DNA. Thus, an increased fluorescence signal can be achieved after magnetic removal of the Apt-MNPs. The fluorescence of CdTe QDs (λexc/em = 327/612 nm) increases linearly in the concentration range of 10 to 10¹⁰ cfu•mL^-1, and the limit of detection is determined to be 1 cfu•mL^-1. The detection process can be performed within 2 h and is successfully applied to the analysis of spiked food samples with good recoveries from 90% to 105%.

Aptamer based SERS detection of Salmonella typhimurium using DNA-assembled gold nanodimers

The authors describe a surface-enhanced Raman scattering (SERS) based aptasensor for Salmonella typhimurium (S. typhimurium). Gold nanoparticles (AuNPs; 35 nm i.d.) were functionalized with the aptamer (ssDNA 1) and used as the capture probe, while smaller (15 nm) AuNPs were modified with a Cy3-labeled complementary sequence (ssDNA 2) and used as the signalling probe. The asymmetric gold nanodimers (AuNDs) were assemblied with the Raman signal probe and the capture probe via hybridization of the complementary ssDNAs. The gap between two nanoparticles is a "hot spot" in which the Raman reporter Cy3 is localized. It experiences a strong enhancement of the electromagnetic field around the particle. After addition of S. typhimurium, it will be bound by the aptamer which therefore is partially dehybridized from its complementary sequence. Hence, Raman intensity drops. Under the optimal experimental conditions, the SERS signal at 1203 cm^-1 increases linearly with the logarithm of the number of colonies in the 10² to 10⁷ cfu·mL^-1 concentration range, and the limit of detection is 35 cfu·mL^-1. The method can be performed within 1 h and was successfully applied to the analysis of spiked milk samples and performed very well and with high specificity. Graphical abstract DNA-assembled asymmetric gold nanodimers (AuNDs) were synthesized and appllied in a SERS-based aptasensor for S. typhimurium. Capture probe was preferentially combined with S. typhimurium and the structure of the AuNDs was destroyed. The "hot spot" vanished partly, this resulting in the decreased Raman intensity of Cy3.

Stress Response Protein BolA Influences Fitness and Promotes Salmonella enterica Serovar Typhimurium Virulence

The intracellular pathogen Salmonella enterica serovar Typhimurium has emerged as a major cause of foodborne illness, representing a severe clinical and economic concern worldwide. The capacity of this pathogen to efficiently infect and survive inside the host depends on its ability to synchronize a complex network of virulence mechanisms. Therefore, the identification of new virulence determinants has become of paramount importance in the search of new targets for drug development. BolA-like proteins are widely conserved in all kingdoms of life. In Escherichia coli, this transcription factor has a critical regulatory role in several mechanisms that are tightly related to bacterial virulence. Therefore, in the present work we used the well-established infection model Galleria mellonella to evaluate the role of BolA protein in S Typhimurium virulence. We have shown that BolA is an important player in S Typhimurium pathogenesis. Specifically, the absence of BolA leads to a defective virulence capacity that is most likely related to the remarkable effect of this protein on S Typhimurium evasion of the cellular response. Furthermore, it was demonstrated that BolA has a critical role in bacterial survival under harsh conditions since BolA conferred protection against acidic and oxidative stress. Hence, we provide evidence that BolA is a determining factor in the ability of Salmonella to survive and overcome host defense mechanisms, and this is an important step in progress to an understanding of the pathways underlying bacterial virulence.IMPORTANCE BolA has been described as an important protein for survival in the late stages of bacterial growth and under harsh environmental conditions. High levels of BolA in stationary phase and under stresses have been connected with a plethora of phenotypes, strongly suggesting its important role as a master regulator. Here, we show that BolA is a determining factor in the ability of Salmonella to survive and overcome host defense mechanisms, and this is an important step in progress to an understanding of the pathways underlying bacterial virulence. This work constitutes a relevant step toward an understanding of the role of BolA protein and may have an important impact on future studies in other organisms. Therefore, this study is of utmost importance for understanding the genetic and molecular bases involved in the regulation of Salmonella virulence and may contribute to future industrial and public health care applications.

Aptasensors for quantitative detection of Salmonella Typhimurium

Salmonella is one of the most frequent causes of food borne infectious disease. Among nearly 2500 documented serotypes are reported, Salmonella Typhimurium is the number one serotype associated with salmonellosis worldwide. Many different methods have been developed for the detection and quantification of S. typhimurium. Most of these assays are usually expensive, time consuming and require difficult sample preparation steps. Therefore, it is necessary to develop rapid, robust, cost-effective and sensitive alternative detection methods. In the last years, aptasensors, used for detection of S. typhimurium in different samples. In this review, recent advances and applications of aptasensors for the detection and quantification of S. typhimurium in details have been summarized.

Salmonella typhimurium detection using a surface-enhanced Raman scattering-based aptasensor

Surface-enhanced Raman spectroscopy (SERS) has been used in a variety of biological applications due to its high sensitivity and specificity. Here, we report a SERS-based aptasensor approach for quantitative detection of pathogenic bacteria. A SERS substrate bearing Au@Ag core/shell nanoparticles (NPs) is functionalized with aptamer 1 (apt 1) for the capture of target molecules. X-rhodamine (ROX)-modified aptamer 2 (apt 2) is used as recognition element and Raman reporter. Salmonella typhimurium specifically interacted with the aptamers to form Au@Ag-apt 1-target-apt 2-ROX sandwich-like complexes. As a result, the concentration of S. typhimurium was determined using this developed aptasensor structure, and a calibration curve is obtained in the range of 15 to 1.5 × 10(6) cfu/mL with a limit of detection of 15 cfu/mL. Our method was successfully applied to real food samples, and the results are consistent with the results obtained using plate counting methods. We believe that the developed method shows potential for the rapid and sensitive detection of pathogenic bacteria in food safety assurance.

Development of an engineered bioluminescent reporter phage for the sensitive detection of viable Salmonella typhimurium

Because foodborne illnesses continuously threaten public health, rapid and sensitive detection of pathogens in food has become an important issue. As an alternative to time-consuming and laborious conventional detection methods, a technique using recombinant reporter phages has been developed. Here, we developed an advanced bioluminescent reporter phage SPC32H-CDABE by inserting a bacterial luxCDABE operon into the Salmonella temperate phage SPC32H genome. Whole SPC32H genome sequencing enabled the selection of nonessential genes, which can be replaced with approximately 6-kb luxCDABE operon, which provides both luciferase (LuxAB) and its substrate, fatty aldehyde, as generated by fatty acid reductase (LuxCDE). Thus, the SPC32H-CDABE detection assay is simpler and more efficient compared to the luxAB-based assay because the substrate addition step is excluded. At least 20 CFU/mL of pure S. Typhimurium culture was detectable using SPC32H-CDABE within 2 h, and the signals increased proportionally to the number of cells contaminated in lettuce, sliced pork, and milk. These results thereby demonstrate that this phage successfully detects live Salmonella without appreciable interference from food components. Furthermore, the presented data suggest that SPC32H-CDABE represents a promising easy-to-use diagnostic tool for the detection of Salmonella contamination in food.