A highly selective enrichment broth combined with real-time PCR for detection of Staphylococcus aureus in food samples

Isothermal Amplification and CRISPR/Cas12a-System-Based Assay for Rapid, Sensitive and Visual Detection of Staphylococcus aureus

Staphylococcus aureus exists widely in the natural environment and is one of the main food-borne pathogenic microorganisms causing human bacteremia. For safe food management, a rapid, high-specificity, sensitive method for the detection of S. aureus should be developed. In this study, a platform for detecting S. aureus (nuc gene) based on isothermal amplification (loop-mediated isothermal amplification-LAMP, recombinase polymerase amplification-RPA) and the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas12a) proteins system (LAMP, RPA-CRISPR/Cas12a) was proposed. In this study, the LAMP, RPA-CRISPR/Cas12a detection platform and immunochromatographic test strip (ICS) were combined to achieve a low-cost, simple and visualized detection of S. aureus. The limit of visual detection was 57.8 fg/µL of nuc DNA and 6.7 × 102 CFU/mL of bacteria. Moreover, the platform could be combined with fluorescence detection, namely LAMP, RPA-CRISPR/Cas12a-flu, to establish a rapid and highly sensitive method for the detection of S. aureus. The limit of fluorescence detection was 5.78 fg/µL of genomic DNA and 67 CFU/mL of S. aureus. In addition, this detection platform can detect S. aureus in dairy products, and the detection time was ~40 min. Consequently, the isothermal amplification CRISPR/Cas12a platform is a useful tool for the rapid and sensitive detection of S. aureus in food.

Phage-Based Biosensing for Rapid and Specific Detection of Staphylococcus aureus

Staphylococcus aureus (S. aureus) is a major foodborne pathogen. Rapid and specific detection is crucial for controlling staphylococcal food poisoning. This study reported a Staphylococcus phage named LSA2302 showing great potential for applications in the rapid detection of S. aureus. Its biological characteristics were identified, including growth properties and stability under different pH and temperature conditions. The genomic analysis revealed that the phage has no genes associated with pathogenicity or drug resistance. Then, the phage-functionalized magnetic beads (pMB), serving as a biological recognition element, were integrated with ATP bioluminescence assays to establish a biosensing method for S. aureus detection. The pMB enrichment brought high specificity and a tenfold increase in analytical sensitivity during detection. The whole detection process could be completed within 30 min, with a broad linear range of 1 × 104 to 1 × 108 CFU/mL and a limit of detection (LOD) of 2.43 × 103 CFU/mL. After a 2 h pre-cultivation, this method is capable of detecting bacteria as low as 1 CFU/mL. The recoveries of S. aureus in spiked skim milk and chicken samples were 81.07% to 99.17% and 86.98% to 104.62%, respectively. Our results indicated that phage-based biosensing can contribute to the detection of target pathogens in foods.

Quantitative Detection of Viable but Nonculturable Vibrio parahaemolyticus in Frozen Bivalve Molluscs

Vibrio parahaemolyticus is a foodborne pathogen diffusely distributed in the marine environment and often isolated from raw seafood belonging to different species, mostly shellfish. Ingestion of under- or uncooked seafood contaminated by V. parahaemolyticus can cause severe gastrointestinal symptoms in humans. Due to its ability to withstand low temperatures, Vibrio spp. could survive in frozen seafoods for long periods by entering the viable but nonculturable state (VBNC) and may constitute an unrecognized source of food contamination and infection. In the present study, seventy-seven frozen bivalve molluscs (35 mussels; 42 clams) were subjected to the detection and enumeration of viable V. parahaemolyticus using standard culture methods. VBNC forms were detected and quantified by applying an optimized protocol based on Propidium Monoazide (PMA) and Quantitative PCR (qPCR). All samples were negative for both the detection and enumeration of V. parahaemolyticus by the standard culture methods. VBNC forms were detected in 11.7% of the samples (9/77), with values ranging from 1.67 to 2.29 Log CFU/g. Only clam samples were positive for the detection of VBNC forms. The results of this study highlighted that VBNC V. parahaemolyticus may be present in frozen bivalve molluscs. Further data on the prevalence of VBNC V. parahaemolyticus in frozen seafood are needed in order to perform a robust risk assessment.

PMA-qPCR method for the selective quantitation of viable lactic acid bacteria in fermented milk

The number of viable lactic acid bacteria (LAB) is a key indicator of the quality of fermented milk. Currently, the combination of propidium monoazide (PMA) and qPCR has been applied in the quantification of viable bacteria in various matrices. In this research, the PMA-qPCR method was used to detect the number of viable bacteria of each LAB species in fermented milk. By analyzing pheS gene and 16S rRNA gene sequence similarities in five species of LAB, namely Lactobacillus delbrueckii subsp. bulgaricus, Lactiplantibacillus plantarum, Streptococcus thermophilus, Lactobacillus helveticus, and Lactococcus lactis subsp. lactis, the pheS gene resolved species identities better and was thus selected to design specific primers and probes. The pheS gene was cloned into the pUC19 vector and used to construct a standard curve for absolute quantification. Standard curves for quantification were constructed for each LAB species for serial dilutions between 1011 and 106 CFU/mL, with R2 > 0.99. The number of viable bacteria in the fermented milk detected by PMA-qPCR was significantly lower than that of qPCR (P < 0.05), indicating that PMA inhibited the amplification of DNA from dead cells. This was corroborated by the results from bacterial staining and plate count experiments. The proposed PMA-qPCR method provided rapid qualitative and quantitative determination of the number of viable bacteria for each LAB species in fermented milk within 3 h.

Simultaneous detection of viable Salmonella spp., Escherichia coli , and Staphylococcus aureus in bird's nest, donkey‐hide gelatin, and wolfberry using PMA with multiplex real‐time quantitative PCR

Salmonella spp., Escherichia coli, and Staphylococcus aureus are common microbial contaminants within the homology of medicine and food that can cause serious food poisoning. This study describes a highly efficient, sensitive, specific, and simple multiplex real‐time quantitative PCR (mRT‐qPCR) method for the simultaneous detection of viable Salmonella spp., E. coli, and S. aureus. Primers and probes were designed for the amplification of the target genes invA, uidA, and nuc. Dead bacterial genetic material was excluded by propidium monoazide (PMA) treatment, facilitating the detection of only viable bacteria. This method was capable of detecting Salmonella spp., E. coli, and S. aureus at 10², 10², and 10¹ CFU/ml, respectively, in pure culture. PMA combined with mRT‐qPCR can reliably distinguish between dead and viable bacteria with recovery rates from 95.7% to 105.6%. This PMA‐mRT‐qPCR technique is a highly sensitive and specific method for the simultaneous detection of three pathogens within the homology of medicine and food.

Architecting of an aptasensor for the staphylococcus aureus analysis by modification of the screen-printed carbon electrode with aptamer/Ag-Cs-Gr QDs/NTiO2

Given the many issues bacterial infections cause to humans and the necessity for their detection, in this work we developed a robust aptasensor for prompt, ultrasensitive, and selective analysis of staphylococcus aureus bacterium (S. aureus). A nanocomposite of Ag nanoparticles, chitosan, graphene quantum dots, and nitrogen-doped TiO₂ nanoparticles (Ag-Cs-Gr QDs/NTiO₂) was synthesized, and thoroughly characterized by XRD, FT-IR, and FE-SEM spectroscopic methods. The surface of screen-printed carbon electrodes modified with Ag-Cs-Gr QDs/NTiO₂ nanocomposite was utilized as a compatible platform for aptamer attachment. The aptasensor accurately determined S. aureus in the dynamic range of 10-5 × 10⁸ CFU/mL with detection limit of 3.3 CFU/mL. The monitoring of the practical performance of aptasensor in human serum samples revealed its superiority over the conventional methods (relative recovery of 96.25-103.33%). The Ag-Cs-Gr QDs/NTiO₂-based aptasensor offers facile, biocompatibility, good repeatability, reproducibility (RSD = 3.66%), label free and stabile strategy for sensitive S. aureus analysis free from biomolecules interferences in actual specimens.

Upconversion nanoparticles-based FRET system for sensitive detection of Staphylococcus aureus

Staphylococcus aureus (S. aureus) is a pathogenic bacterium that seriously endangers food safety. Herein, a rapid, sensitive and specific aptasensor based on upconversion fluorescence resonance energy transfer (FRET) was developed for S. aureus detection in food. Aptamer-functionalized gold nanoparticles (AuNPs-aptamers) were bonded to cDNA-modified upconversion nanoparticles (UCNPs-cDNA) by complementary pairing, resulting in fluorescence quenching. After adding S. aureus into the system, the aptamers preferentially combined with S. aureus, dissociated UCNPs-cDNA from AuNPs-aptamers, and the fluorescence was recovered. Under optimized conditions, there was a significant linear correlation between fluorescence intensity and S. aureus concentration over the range 47-4.7 × 10⁷ CFU/mL (R² = 0.9904) with a detection limit of 10.7 CFU/mL. Furthermore, the precision and accuracy of the developed biosensor were validated using standard plate count method, yielding no significant differences. The proposed method has potential application for rapid and sensitive quantification of S. aureus in food.

Electrochemical immunosensor for determination of Staphylococcus aureus bacteria by IgY immobilized on glassy carbon electrode with electrodeposited gold nanoparticles

A new ultrasensitive immunosensor is proposed based on the covalently attached anti-protein A antibody (IgY) on deposited gold nanoparticle (AuNP)-modified glassy carbon electrode (GCE) for the electrochemical measurement of Staphylococcus aureus (S. aureus). Chicken IgY as a capture antibody provides highly selective and specific binding to the target bacteria and selectively captures the S. aureus in its three-dimensional space. Due to that it can eliminate the interference from protein G-producing Streptococcus. In addition, the electron-transfer characteristic of [Fe(CN)₆]^4-/3- is hindered by this combination; as it is reflected on the electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) curves. The proposed immunosensor displays a wide linear dynamic range from 10 to 10⁷ CFU mL^-1 with a detection limit of 3.3 CFU mL^-1 with RSD 3.0%. It is capable to accurately determine S. aureus in milk and human blood serum as a complex matrix sample with satisfactory recovery of ∼ 97-103%. The immunosensor also displays high selectivity over other bacteria and acceptable stability. Presumably, our study can be regarded as the first one to report chicken IgY in order to detect S. aureus based on an electrochemical method.Graphical abstract.

Single-Layered Graphene/Au-Nanoparticles-Based Love Wave Biosensor for Highly Sensitive and Specific Detection of Staphylococcus aureus Gene Sequences

In this work, a single-layered graphene (SLG)/Au-nanoparticles (NPs)-based Love wave biosensor was prepared for detection of Staphylococcus aureus (S. aureus) gene sequences. The annealing process was proposed to obtain a larger-area interface with ssDNA. The sensitivity was verified by detecting S. aureus gene sequences with a linear detection ranging from 0 to 10 nmol/L, where the limit of detection (LOD) was only 12.4 pg/mL. The stable state of the biosensors based on SLG/Au NPs in the liquid phase could be kept for more than 0.5 h (±0.1°), and the specificity was verified by detecting noncomplementary ssDNA and one- and three-base mismatched ssDNA. The results of our study suggest that a Love wave biosensor based on SLG/Au NPs hold great potential in clinical testing and diagnosis.

Multiplex touchdown PCR assay to enhance specificity and sensitivity for concurrent detection of four foodborne pathogens in raw milk

AIMS

The aim of this study was to develop a multiplex touchdown PCR (multiplex TD-PCR) for rapid and simultaneous detection of four major foodborne pathogens to avoid mispriming and unwanted production during gene amplification. Touchdown PCR is the modified form of standard PCR, which enhances specificity, sensitivity.

METHODS AND RESULTS

For this reason, a multiplex TD-PCR assay with a pre-enrichment step was developed to detect four foodborne pathogens namely Escherichia coli O157:H7, Listeria monocytogenes, Staphylococcus aureus, and Salmonella enterica serovar Enteritidis in pure culture and raw milk samples. The results showed that this protocol can eliminate the unwanted band or reduce significantly. The detection sensitivity of the single and multiplex TD-PCR was one cell per ml in pure culture. Furthermore, the detection limit of multiplex TD-PCR was one cell per 25 ml for artificially contaminated raw milk. We obtained similar results for detection of aforementioned pathogens in raw milk, after comparing the multiplex TD-PCR method with the traditional culture, except in one or two samples.

CONCLUSIONS

Hence, the proposed multiplex TD-PCR method could be confirmed as an effective way for rapid optimization of PCR reactions to increase specificity, sensitivity during gene amplification.

SIGNIFICANCE AND IMPACT OF THE STUDY

Hence, due to its simplicity, cost-effectiveness and being time-saving, it seems that this method is reasonable and economical for rapid optimization of PCR reactions.