Improved bioluminescent enzyme immunoassay for the rapid detection of Salmonella in chicken meat samples

Luciola mingrelica firefly luciferase as a marker in bioluminescent immunoassays

Chemical modification of the enzymes with biospecific macromolecules is used in various fields of biotechnology to impart new functions or improve their properties and is a fast and convenient way to get the final products. The preparation of highly active, stable, and functionally active conjugates of the thermostable luciferase through the NH2-groups or free SH-groups of the enzyme with target molecules of different molecular weight (albumin, avidin from chicken eggs, antibodies, and progesterone) is described. The obtained conjugates were successfully tested as a reporter in bioluminescent immunoassay for the detection of the molecules and pathogens. Thus, the luc-albumin (Luc-Alb) and luc-insulin (Luc-Ins) conjugates were used in competitive ELISA for the detection of an analyte (albumin or insulin) in the samples. Luc-progesterone (Luc-Pg) was used in the rapid homogeneous immunoassay of progesterone by the BRET technique with the detection limit of 0.5 ng/ml. Luciferase conjugates with avidin (Luc-Avi) and secondary and primary antibodies (Luc-RAM and Luc-Sal) were used for enzyme immunoassay detection of Salmonella paratyphi A cells with the cell detection limit of 5 × 104 CFU/ml. To reduce the detection limit of Salmonella cells, we developed a pseudo-homogeneous bioluminescent enzyme immunoassay of cells using a new matrix for the analyte capture-polystyrene microparticles coated with Pluronic F108, covalently labeled with Sal antibodies. This allowed to achieve efficient trapping of cells from solution, significantly reduced nonspecific sorption and decreased the cell detection limit to 2.7 × 103 CFU/ml without prior concentration of the sample. The methodology that was developed in this study can be applied for the development of novel bioanalytical systems based on firefly luciferases.

A Novel Streptavidin-luciferase Fusion Protein: Preparation, Properties and Application in Hybridization Analysis of DNA

A streptavidin-luciferase fusion protein comprising the thermostable mutant form of firefly luciferase Luciola mingrelica and minimal core streptavidin was constructed. The streptavidin-luciferase fusion was mainly produced in a tetrameric form with high luciferase and biotin-binding activities. It was shown that fusion has the same K_m values for ATP and luciferin and the bioluminescence spectra as initial luciferase. The linear dependence of the bioluminescence signal on the content of the fusion was observed within the range of 10^-18 -10^-13 mol per well. Successful application of obtained fusion in a biospecific bioluminescence assay based on biotin-streptavidin interactions was demonstrated by the example of a specific DNA hybridization analysis. A DNA hybridization analysis for Escherichia coli cells identification was developed using unique for these cells gadB fragment encoding glutamate decarboxylase. The amplified biotinylated GadB fragments were hybridized with the immobilized oligonucleotide probes; then, the biotin in the DNA duplexes was detected using the streptavidin-luciferase fusion protein. To reach the high sensitivity of the assay, we optimized the conditions of the assay. It was shown that the use of Pluronic for plate modification resulted in a significant reduction in the DNA detection limit which finally was 0.4 ng per well.

Firefly Luciferase-based Fusion Proteins and their Applications in Bioanalysis

Firefly luciferase is widely used in molecular biology and bioanalytical systems as a reporter molecule due to the high quantum yield of the bioluminescence, availability of stable mutant forms of the enzyme with prescribed spectral characteristics and abundance of bacterial expression systems suitable for production of recombinant proteins in limitless quantities. In this review, we described fusion proteins of luciferase with biotin-binding domain and streptavidin, with proteins A and G, antibodies, with DNA- and RNA-binding proteins, as well as fusion proteins designed for BRET systems. The firefly luciferase-based fusion proteins are represented as an effective tool for the development of different bioanalytical systems such as (1) systems in which luciferase is attached to the surface of the target and the bioluminescence signal is detected from the specific complexes formed; (2) BRET-based systems, in which the specific interaction induces changes in the bioluminescence spectrum; and (3) systems that use modified or split luciferases, in which the luciferase activity changes under the action of the analyte. All these systems have wide application in biochemical analysis of physiologically important compounds, for the detection of pathogenic bacteria and viruses, for evaluation of protein-protein interactions, assaying of metabolites involved in cell communication and cell signaling.

Visual detection technique for efficient screening and isolation of Salmonella based on a novel enrichment assay using chromatography membrane

To detect Salmonella more efficiently and isolate strains more easily, a novel and simple detection method that uses an enrichment assay and two chromogenic reactions on a chromatography membrane was developed. Grade 3 chromatography paper is used as functionalized solid phase support (SPS), which contains specially optimized medium. One reaction for screening is based on the sulfate-reducing capacity of Salmonella. Hydrogen sulfide (H2S) generated by Salmonella reacts with ammonium ferric citrate to produce black colored ferrous sulfide. Another reaction is based on Salmonella C8 esterase that is unique for Enterobacteriaceae except Serratia and interacts with 4-methylumbelliferyl caprylate (MUCAP) to produce fluorescent umbelliferone, which is visible under ultraviolet light. A very low detection limit (10(1) CFU ml(-1)) for Salmonella was achieved on the background of 10(5) CFU ml(-1) Escherichia coli. More importantly, testing with more than 1,000 anal samples indicated that our method has a high positive detection rate and is relatively low cost, compared with the traditional culture-based method. It took only 1 day for the preliminary screening and 2 days to efficiently isolate the Salmonella cells, indicating that the new assay is specific, rapid, and simple for Salmonella detection. In contrast to the traditional culture-based method, this method can be easily used to screen and isolate targeted strains with the naked eye. The results of quantitative and comparative experiments showed that the visual detection technique is an efficient alternative method for the screening of Salmonella spp. in many applications of large-sized samples related to public health surveillance.

Antibody-integrated and functionalized graphite-encapsulated magnetic beads, produced using ammonia gas plasma technology, for capturing Salmonella

Salmonella spp. is the single and most important causative agent of foodborne infections, especially involving foods such as eggs, milk and meat. To prevent infection, a reliable surveillance system is required that can quickly and sensitively detect Salmonella. Here, we describe the development of antibody-integrated magnetic beads that are functionalized by a novel strategy using ammonia gas plasma. Ammonia plasma, produced by a radio frequency (RF) power supply, was allowed to react with the surface of graphite-encapsulated magnetic beads, resulting in the introduction of amino groups. An anti-Salmonella antibody was then anchored by sulfide groups present on the protein surface to the amino groups of the magnetic beads via N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP). The potential usefulness of these magnetic beads for capturing Salmonella was examined as follows. The beads were incubated with Salmonella in liquid medium and then separated from the supernatant by applying a magnetic field. After thorough washing, adsorption of Salmonella to the beads was confirmed by immunochromatography, polymerase chain reaction and a direct culture assay. Our findings indicate that the capture and concentration of Salmonella using the antibody-integrated magnetic beads was more efficient than commercial Dynabeads® anti-Salmonella, which are conventionally used for concentrating Salmonella from liquid cultures. We believe this novel bead technology will contribute to the enhanced detection of Salmonella.

Anodic stripping voltammetry of anti-Vi antibody functionalized CdTe quantum dots for the specific monitoring of Salmonella enterica serovar Typhi

CdTe QD based stripping voltammetry for Vi capsular polysaccharide detection. The technique has provided an insight into the competence of CdTe QD and GNP immuno-conjugates. This is a novel approach to characterize the efficiency of immuno-conjugates of QDs and GNPs.

Scano-magneto immunoassay based on carbon nanotubes/gold nanoparticles nanocomposite for Salmonella enterica serovar Typhimurium detection

To improve sensitivity of S. enterica serovar Typhimurium detection, multiwalled carbon nanotubes (MWCNTs) and gold nanoparticles (AuNPs) were combined and used as a label to amplify signal in a scanometric based assay. In this study, the MWCNTs/AuNPs nanocomposite was fabricated by directly assemble of Au(3+) to MWCNTs and allowed growing of AuNPs along the MWCNTs surface. This MWCNTs/AuNPs nanocomposite was then attached to anti-S. typhimurium antibody (MWCNTs/AuNPs/Ab(1)) and used as a detecting molecule. Upon binding to Salmonella, they were pre-concentrated by magenetic beads/antibody (MBs/Ab(2)) forming a sandwich immuno-complex which is later spotted on a nitrocellulose membrane coated slide. Silver reduction was applied to amplify signal. The detection limit of 42CFU/ml was achieved when 2% BSA was used as a blocking agent. Given different types of real samples testing, chicken broth was found to give lowest detection limit, followed by orange juice low fat and whole milk. Selectivity testing was performed by using Escherichia coli as interference and found slightly cross-reactivity which could be due to specificity of the Ab used. By virtue of using a slide for multi-samples spotting and a flatbed scanner for signal-read out acquisition, this scano-magneto immunoassay could enable low-cost detection as well as high throughput screening.

Selection, characterization, and application of DNA aptamers for the capture and detection of Salmonella enterica serovars

Sensitive and specific pre-analytical sample processing methods are needed to enhance our ability to detect and quantify food borne pathogens from complex food and environmental samples. In this study, DNA aptamers were selected and evaluated for the capture and detection of Salmonella enterica serovar. Typhimurium. A total of 66 candidate sequences were enriched against S. Typhimurium outer membrane proteins (OMPs) with counter-selection against Escherichia coli OMPs and lipopolysaccharides (LPS). Specificity of the selected aptamers was evaluated by gel-shift analysis against S. Typhimurium OMP. Five Salmonella-specific aptamer candidates were selected for further characterization. A dilution-to-extinction capture protocol using pure cultures of S. Typhimurium further narrowed the field to two candidates (aptamers 33 and 45) which showed low-end detection limits of 10-40CFU. DNase protection assays applied to these aptamers confirmed sequence-specific binding to S. Typhimurium OMP preparations, while South-Western blot analysis combined with mass spectrometry identified putative membrane proteins as targets for aptamer binding. Aptamer 33 was bound to magnetic beads and used for the capture of S. Typhimurium seeded into whole carcass chicken rinse samples, followed by detection using quantitative real-time RT-PCR. In a pull-down assay format, detection limits were 10(1)-10(2)CFU S. Typhimurium/9mL rinsate, while in a recirculation format, detection limits were 10(2)-10(3)CFU/25mL rinsate. Reproducible detection at <10(1)S. typhimurium CFU/g was also achieved in spike-and-recovery experiments using bovine feces. The pull-down analysis using aptamer 33 was validated on 3 naturally infected chicken litter samples confirming their applicability in the field. This study demonstrates the applicability of Salmonella specific aptamers for pre-analytical sample processing as applied to food and environmental sample matrices.

Adding a selective enrichment step to the iQ-CheckTM real-time PCR improves the detection of Salmonella in naturally contaminated retail turkey meat products

AIMS

The aim of this study was to compare the real-time iQ-Check Salmonella kit (Bio-Rad) with the immunocapture assay RapidCheck Salmonella method, and a conventional culture method (FSIS, USDA) in detecting Salmonella in naturally contaminated turkey meat products. This study was also designed to determine if a selective enrichment step might improve the real-time detection of Salmonella.

METHODS AND RESULTS

Using the culture method, Salmonella was recovered from 49 out of 99 retail turkey meat samples collected. RapidCheck failed to detect 11 Salmonella samples that were positive by the culture method. The iQ-Check real-time PCR also failed to detect three samples that were positive by the culture method. However, when carried out after a selective enrichment step, the iQ-Check real-time PCR detected all 49 Salmonella samples recovered by the culture method. The iQ-Check real-time PCR detected the presence of Salmonella in some samples that were not recovered by the culture method.

CONCLUSIONS

Adding a selective enrichment step to the iQ-Check real-time PCR improves the detection of Salmonella in naturally contaminated turkey meat samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

The iQ-Check Salmonella real-time PCR can be used as a rapid method to monitor Salmonella in turkey meat, together with conventional culture methodology.