Development of a specific nanobody and its application in rapid and selective determination of Salmonella enteritidis in milk

Development of a nanobody-based immunoassay for the detection of Escherichia coli O157:H7 in food samples

Rapid and accurate detection of foodborne pathogens is crucial for food safety. E. coli O157:H7, a major foodborne pathogen, can cause severe illness even at low concentration, highlighting the necessity for effective detection methods. This study focuses on developing nanobodies specific to E. coli O157:H7, aiming to provide a more efficient, cost-effective, and stable analytical reagents which can be available internationally. An alpaca was immunized and a phage-displayed nanobody library was constructed. Following panning, nanobodies were expressed and characterized for their affinity and specificity. A nanobody-based immunoassay was developed, with a limit of detection of 8.7 × 103 cfu/mL. The matrix effect was evaluated, and the assay successfully detected E. coli O157:H7 in various food samples (with 5 cfu spiked), including ground beef, orange juice, and milk. The study developed a nanobody-based immunoassay for E. coli O157:H7 detection, underscoring the practical applicability of nanobodies for testing food safety monitoring.

Structural Insights Into the Mechanisms of Recognition of Recombinant Full-Length Antibodies for the Detection of Staphylococcus aureus Enterotoxins C1, C2, and C3

Food poisoning caused by Staphylococcus aureus (S. aureus) and its enterotoxins has become a global food safety issue. Herein, three types of Staphylococcal enterotoxins (SE), including SEC1, SEC2, and SEC3, were successfully expressed and immunized in mice to prepare monoclonal antibodies (mAbs). We screened a pair of mAbs 16E12-9B7 from 12 strains that could simultaneously recognize SEC1, SEC2, and SEC3. Furthermore, the genes from hybridoma cells 9B7 and 16E12 were extracted, amplified, and inserted into expression vectors to obtain recombinant antibodies (rAbs), whose affinities were consistent with those of ascites antibodies. The paired rAbs 16E12-9B7 were applied to a gold immunochromatographic strip (GICS) system to enable the rapid detection of SEC1, SEC2, and SEC3 with visual limits of detection (vLOD) of 2, 0.5, and 2 ng/mL in milk samples. Noticeably, we found that hydrogen bonds and salt bridges played a significant role in these antigen-antibody interactions. The key sites for 9B7 were ASN28 and SER31 in complementarity determining region (CDR) and the key sites for 16E12 were SER32 in the 16E12 variable light chain (VL) and ARG100, SER101, and TYR102 in the 16E12 variable heavy chain (VH). The analysis of key rAbs sites has potential in the screening of mutant antibodies.

Detection of Salmonella by competitive ELISA of lipopolysaccharide secreted into the culture medium

Detection of Salmonella in food is topical due to known cases of salmonellosis epidemics. Immunochemical methods including ELISA are widely used for Salmonella detection. Traditionally, commercial ELISA kits are based on sandwich technique and detect lipopolysaccharide (LPS), which is considered to be the component of the outer membrane of Gram-negative bacteria. Our aim was elaboration of competitive ELISA test for Salmonella detection in food with improved parameters. It was shown that in the Salmonella culture after the standard sample preparation procedure LPS is present mainly outside cells as a component of outer membrane vesicles. Improved sample preparation procedure includes separation of bacteria from the medium and analysis of the medium, which increases analytical sensitivity. Immobilization of the bovine serum albumin (BSA)-LPS conjugate in microplate wells allows to obtain a more homogeneous coating than immobilization of LPS itself. Thus, we have developed test system for Salmonella detection in food by competitive ELISA of LPS secreted into the culture medium with the immobilized BSA-LPS conjugate and monoclonal antibodies (mAb) to LPS core in the liquid phase. New competitive ELISA test is high sensitive, give reproducible results, allows the detection of any Salmonella serotype and is important for the protection of human health.

Preparation of divalent camelid single-domain antibody and its application in immunoassays for Salmonella detection in food

Salmonella-related foodborne infections are commonly caused by the serovars of S. Typhimurium, which can be detected using antibody-based immunoassays. The monovalent variable domain of the camelid heavy chain antibody (VHH) performs excellently in constructing multivalent VHH variants, which generally exhibit higher affinities with antigens and consequently enhance the assay sensitivity. In this study, the divalent variants of VHHs (diVHHs) targeting S. Typhimurium were generated by encoding the monovalent VHH genes assembled in tandem with a flexible linker peptide (G4S)2. Soluble diVHHs were produced in a prokaryotic expression system and purified with a yield of 4.22 mg/L. Benefiting from their stability and antigen-binding abilities towards tested Salmonella serovars, diVHH-based immunoassays were developed. The diVHH-based sandwich immunoassay, using diVHH as capture antibody, exhibited a detection limit of 1.04×102 CFU/mL and enabled as low as 10 CFU/mL S. Typhimurium to be detected after 6 h of enrichment in lettuce. Furthermore, this assay can be applied to spiked lettuce, chicken, and pork samples, showing acceptable recoveries ranging from 83 to 106%. This study presented feasible strategies for VHH multivalence and established a superior sensitivity VHH-based immunoassay for monitoring and analyzing Salmonella contamination in food.

Detection of AFB1 by Immunochromatographic Test Strips Based on Double-Probe Signal Amplification with Nanobody and Biotin-Streptavidin System

Aflatoxin B1 (AFB1) is highly toxic and difficult to prevent. It is mainly produced by fungi and exists in plants and animals and is classified by the World Health Organization as a class I carcinogen, posing a serious threat to human and animal health. Therefore, it is important to establish an efficient, sensitive, and on-site detection method for AFB1 to protect human health. The immunochromatographic test strip method is simple, sensitive, and can achieve real-time detection. However, traditional immunochromatographic test strips have low sensitivity due to their relatively weak optical properties. In this study, Nb-G8 was biotinylated using a chemical method. Two sizes of gold nanoflowers (AuNFs) were prepared and combined with biotinylated G8 and streptavidin to form two types of probes. These probes were sprayed on gold standard pads and expanded pads, respectively, to enhance the signals through the high affinity interaction between streptavidin and biotin. Under the optimal experimental conditions, the half maximal inhibitory concentration (IC50) of this method was 5.0 ng/mL and the limit of detection (IC10) was 0.03 ng/mL, which increased the sensitivity of the test strip by four-fold compared with that of the traditional biotinylated nanoantibody immunochromatography test strip and had a wider detection range. In conclusion, the use of a high-affinity amplification signal between biotin and streptavidin is a valuable method for the detection of aflatoxin.

Synthesis of dual models multivalent activatable aptamers based on HCR and RCA for ultrasensitive detection of Salmonella typhimurium

Aptamers have superior structural properties and have been widely used in bacterial detection methods. However, the problem of low affinity still exists in complex sample detection. In contrast, hybridization chain reaction (HCR)-based model I and rolling circle amplification (RCA)-based model II multivalent activatable aptamers (multi-Apts) can fulfill the need for low-cost, rapid, highly sensitive and high affinity detection of S. typhimurium. In our research, two models of multi-Apts were designed. First, a monovalent activatable aptamer (mono-Apt) was constructed by fluorescence resonance energy transfer (FRET) with an S. typhimurium aptamer and its complementary chain of BHQ1. Next, the DNA scaffold was obtained by HCR and RCA, and the multi-Apts were obtained by self-assembly of the mono-Apt with a DNA scaffold. In model I, when target was presented, the complementary chain BHQ1 was released due to the binding of multi-Apts to the target and was subsequently adsorbed by UIO66. Finally, a FRET-based fluorescence detection signal was obtained. In mode II, the multi-Apts bound to the target, and the complementary chain BHQ1 was released to become the trigger chain for the next round of amplification of HCR with a fluorescence detection signal. HCR and RCA based multi-Apts were able to detect S. typhimurium as low as 2 CFU mL-1 and 1 CFU mL-1 respectively. Multi-Apts amplification strategy provides a new method for early diagnosis of pathogenic microorganisms in foods.

NMR Immunosensor Based on a Targeted Gadolinium Nanoprobe for Detecting Salmonella in Milk

Detecting harmful pathogens in food is not only a crucial aspect of food quality management but also an effective way to ensure public health. In this paper, a complete nuclear magnetic resonance biosensor based on a novel gadolinium (Gd)-targeting molecular probe was developed for the detection of Salmonella in milk. First, streptavidin was conjugated to the activated macromolecular polyaspartic acid (PASP) via an amide reaction to generate SA-PASP. Subsequently, the strong chelating and adsorption properties of PASP toward the lanthanide metal gadolinium ions were exploited to generate the magnetic complex (SA-PASP-Gd). Finally, the magnetic complex was linked to biotinylated antibodies to obtain the bioprobe and achieve the capture of Salmonella. Under optimal experimental conditions, the sensor we have constructed can achieve a rapid detection of Salmonella within 1.5 h, with a detection limit of 7.1 × 103 cfu mL-1.

Facile construction of sandwich ELISA based on double-nanobody for specific detection of α-hemolysin in food samples

α-hemolysin (Hla), a toxin secreted by Staphylococcus aureus (S. aureus), has been proved to be involved in the occurrence and aggravation of food poisoning. Hence, it is quite essential to establish its rapid detection methods to guarantee food safety. Sandwich ELISA based on nanobody is well known to be viable for toxins, but there is absence of nanobody against Hla, let alone a pair for it. Therefore, in this paper, we screened specific nanobodies by bio-panning and obtained the optimal nanobody pair for sandwich ELISA firstly. Then, RANbody, a novel nanobody owning both recognition and catalytic capability, is generated in a single step and at low cost through molecular recombination technology. Subsequently, sandwich ELISA was developed to detect Hla based on the nanobody and RANbody, that not only eliminated the use of secondary antibodies and animal-derived antibody, but also reduced detection time and cost, compared with traditional sandwich ELISA. Lastly, the performance has been evaluated, especially for specificity which showed no response to other hemolysins and a low limit of detection of 10 ng/mL. Besides, the proposed sandwich ELISA exhibits favorable feasibility and was successfully employed for the detection of Hla in milk and pork samples.

Nanobodies-based colloidal gold immunochromatographic assay for specific detection of parathion

Parathion is one of organophosphorus pesticide, which has been prohibited in agricultural products due to its high toxicity to human beings. However, there are still abuse cases for profit in agricultural production. Hence, we established nanobodies-based colloidal gold immunochromatographic assay (GICA) in which nanobodies (Nbs) as an excellent recognition element, greatly improving the stability and sensitivity of ICA. Under the optimal conditions, the developed Nbs-based GICA showed a cut-off value of 50 ng/mL for visual judgment and a half-inhibitory concentration (IC50) of 2.39 ng/mL for quantitative detection. The limit of detection (LOD) was as low as 0.15 ng/mL which was significantly 50-fold higher sensitivity than the commercial mAb-ICA. Additionally, this method exhibited good recoveries for the detection of cabbage, cucumber, and orange samples and excellent correlation with the UPLC-MS/MS method. The results showed that this method developed in this work based on nanobody can be used in practical detection of parathion in foods and nanobody is novel prospective antibody resource for immunoassays of chemical contaminants.

Material-specific binding peptides empower sustainable innovations in plant health, biocatalysis, medicine and microplastic quantification

Material-binding peptides (MBPs) have emerged as a diverse and innovation-enabling class of peptides in applications such as plant-/human health, immobilization of catalysts, bioactive coatings, accelerated polymer degradation and analytics for micro-/nanoplastics quantification. Progress has been fuelled by recent advancements in protein engineering methodologies and advances in computational and analytical methodologies, which allow the design of, for instance, material-specific MBPs with fine-tuned binding strength for numerous demands in material science applications. A genetic or chemical conjugation of second (biological, chemical or physical property-changing) functionality to MBPs empowers the design of advanced (hybrid) materials, bioactive coatings and analytical tools. In this review, we provide a comprehensive overview comprising naturally occurring MBPs and their function in nature, binding properties of short man-made MBPs (<20 amino acids) mainly obtained from phage-display libraries, and medium-sized binding peptides (20-100 amino acids) that have been reported to bind to metals, polymers or other industrially produced materials. The goal of this review is to provide an in-depth understanding of molecular interactions between materials and material-specific binding peptides, and thereby empower the use of MBPs in material science applications. Protein engineering methodologies and selected examples to tailor MBPs toward applications in agriculture with a focus on plant health, biocatalysis, medicine and environmental monitoring serve as examples of the transformative power of MBPs for various industrial applications. An emphasis will be given to MBPs' role in detecting and quantifying microplastics in high throughput, distinguishing microplastics from other environmental particles, and thereby assisting to close an analytical gap in food safety and monitoring of environmental plastic pollution. In essence, this review aims to provide an overview among researchers from diverse disciplines in respect to material-(specific) binding of MBPs, protein engineering methodologies to tailor their properties to application demands, re-engineering for material science applications using MBPs, and thereby inspire researchers to employ MBPs in their research.

Affinity purification of mAb from serum-containing hybridoma culture supernatant through a novel nanobody that discriminates mouse IgG from bovine IgG by recognizing the mouse kappa constant region (mCK)

When purifying mAb from serum-containing hybridoma culture supernatant, it is essential that mouse IgG remains free from contaminations of bovine IgG. However, the broadly used Protein A resin cannot achieve this goal due to binding between both mouse and bovine IgG. Here, a novel nanobody-based affinity purification magnetic beads that discriminates mouse IgG from bovine IgG was developed. To bind all subtypes of mouse IgG (IgG1, IgG2a, IgG2b and IgG3) that contain the kappa light chain, mCK (mouse kappa constant region)-specific nanobody binders were selected from an immune phage display VHH library; this library was constructed with peripheral blood mononuclear cells (PBMCs), which were collected from Bactrian camels immunized with a mix of intact mouse IgGs (IgG1, IgG2a, IgG2b and IgG3). A novel clone that exhibited a higher expression level and a higher binding affinity was selected (4E6). Then, the 4E6 nanobody in the format of VHH-hFC (human Fc) was conjugated on magnetic beads with a maximal binding capacity of 15.41±0.69 mg mouse IgG/mL beads. Furthermore, no bovine IgG could be copurified from hybridoma culture supernatant with immunomagnetic beads. This approach is valuable for the large-scale in vitro production of highly pure antibodies by hybridoma cells.

A novel nanoparticle-based fluorescent sandwich immunoassay for specific detection of Salmonella Typhimurium

The diseases caused by foodborne pathogens have a serious impact on human health and social stability. Conventional detection methods can involve long assay times and complex pretreatment steps, making them unsuitable for rapid, large-scale analysis of food samples. We constructed a novel nano-fluorescence sandwich immunosorbent immunoassay (nano-FSIA) to rapidly detect Salmonella Typhimurium in food, based on strong covalent binding between streptavidin and biotin. We used antibodies coupled to large particle-size fluorescent microspheres as fluorescent probes for direct quantitative analysis of S. typhimurium in milk. The optimized parameters were determined, and specificity and sensitivity were validated in phosphate-buffered saline (PBS) and milk. The results demonstrated a wide dynamic detection range for S. typhimurium (103-108 colony forming units [CFU]/mL), with the limit of detection in PBS and milk at 234 and 346 CFU/mL, respectively. The results of nano-FSIA were consistent with those of plate counts and enzyme-linked immunosorbent assays, providing an effective and promising single-bacterium counting method for the rapid detection of Salmonella.

Enhanced sandwich immunoassay based on bivalent nanobody as an efficient immobilization approach for foodborne pathogens detection

BACKGROUND

Nanobodies (Nbs), which consist of only antigen-binding domains of heavy chain antibodies, have been used in a various range of applications due to their excellent properties. Nevertheless, the size of Nbs is so small that their antigen binding sites may be sterically hindered after random fixation as capture antibodies, thus leading to poor detection performance in immunoassays. To address this problem, we have focused on the multivalent modification of Nbs, wanted to retain the advantage of good stability through enlarging the size of Nbs to a certain extent, while improve its affinity and reduce its influence by spatial orientation.

RESULTS

Here, we designed homo- and heterodimeric Nbs based on Nb413 and Nb422 which recognize different epitopes of Salmonella. The affinity of engineered bivalent nanobodies for S. Enteritidis were 2 orders of magnitude higher compared to monovalent Nbs and low to sub-nM KD, as calculated by Scatchard analysis. To further explore the potential of bivalent Nbs for the detection of Salmonella, we established a sandwich ELISA based on bivalent and phage-displayed Nbs (BNb-ELISA) for multiplex Salmonella determination. Compared with monovalent Nb-based ELISA, the limit of detection (LOD) of the BNb-ELISA was shown to increase 7.5-fold to 2.364 × 103 CFU mL-1 for S. Enteritidis. In addition, the feasibility of this approach for S. Enteritidis detection in real samples was evaluated, with recoveries ranging from 73.0 % to 125.6 % and coefficients of variation (CV) below 7.68 %.

SIGNIFICANCE AND NOVELTY

In this study, we developed for the first time bivalent Nbs against Salmonella and examined their improved affinity and impact on the performance of ELISA assay. It confirmed the high binding affinity and good ability of dimeric Nbs to reduce the occupation of the binding sites of immobilized antibodies. Thus, the multivalent modification of Nbs was demonstrated to be a promising means to enhance the performance of Nbs-based immunoassays for foodborne pathogens.

Anti-Idiotypic Nanobody Alkaline Phosphatase Fusion Protein-Triggered On-Off-On Fluorescence Immunosensor for Aflatoxin in Cereals

Nanobodies (Nbs) are widely used in immunoassays with the advantages of small size and high stability. Here, the nanobody employed as the surrogate of aflatoxin antigen and the recognition mechanism of antiaflatoxin mAb with nanobody was studied by molecular modeling, which verified the feasibility of Nbs as antigen substitutes. On this basis, a nanobody-alkaline phosphatase fusion protein (Nb-AP) was constructed, and a highly sensitive "on-off-on" fluorescent immunosensor (OFO-FL immunosensor) based on the calcein/Ce3+ system was developed for aflatoxin quantification. Briefly, calcein serves as a signal transducer, and its fluorescence can be quenched after it is bound with Ce3+. In the presence of Nb-AP, AP catalyzed p-nitrophenyl phosphate to generate orthophosphate, which competes in binding with Ce3+, leading to fluorescence recovery. The method has a linearity range of 0.005-100 ng/mL, and the IC50 of the OFO-FL immunosensor was 0.063 ng/mL, which was 18-fold lower than that of conventional enzyme-linked immunosorbent assay. The assay was successfully applied in food samples with a recovery of 88-121%.

Phage-Displayed Nanobody as a Sensitive Nanoprobe to Enhance Chemiluminescent Immunoassay for Cronobacter sakazakii Detection in Dairy Products

The exploitation of stable, high-affinity, and low-cost nanoprobes is essential to develop immunoassays for real-time monitoring of foodborne pathogens, so as to safeguard human health. The possible interaction of the Fc fragment of antibodies with spA protein on Staphylococcus aureus will result in unexpected interference. To address this consideration, we described herein for the first time the development of nanobodies that by definition are devoid of the Fc fraction. These nanobodies directed against Cronobacter sakazakii (C. sakazakii) were retrieved from a dedicated immune phage-displayed nanobody library. The binders showed superiority of low cost, strong stability, high binding affinity, and adequate load capacity. Thereafter, a phage-mediated sandwich enzyme-linked immunosorbent assay (ELISA) was constructed by using Cs-Nb2 as an antigen-capturing antibody and phage-displayed Cs-Nb1 as a detection probe. To further enhance the sensitivity, a chemiluminescent enzyme immunoassay (CISA) was established by replacing the substrate from 3,3',5,5'-tetramethylbenzidine (TMB) to luminol, providing a limit of detection of 1.04 × 104 CFU/mL, with a recovery of 98.15-114.63% for the detection of C. sakazakii in dairy products. The proposed nanobody-based phage-mediated sandwich CLISA shows various advantages, including high sensitivity, cost effectiveness, enhanced loading capacity of the enzyme, and high resistance to the matrix effect, providing a strategy for the design of immunoassays toward foodborne pathogens.

Nanobodies: the Potential Application in Bacterial Treatment and Diagnosis

An infection caused by bacteria is one of the main factors that poses a threat to human health. A recent report from the World Health Organization (WHO) has highlighted that bacteria that cause blood infections have become increasingly drug-resistant. Therefore, it is crucial to research and develop new techniques for detecting and treating these infections. Since their discovery, nanobodies have exhibited numerous outstanding biological properties. They are easy to express, modify, and have high stability, robust permeability and low immunogenicity, all of which indicate their potential as a substitute. Nanobodies have been utilized in a variety of studies on viruses and cancer. This article primarily focuses on nanobodies and introduces their characteristics and application in the diagnosis and treatment of bacterial infections.

A characteristic bacterial SERS marker for direct identification of Salmonella in real samples assisted by a high-performance SERS chip and a selective culture medium

Salmonella should be absent in pharmaceutical preparations and foods according to the regulations. However, up to now, rapid and convenient identification of Salmonella is still full of challenge. Herein, we reported a label-free surface-enhanced Raman scattering (SERS) method for direct identification of Salmonella spiked in drug samples based on a characteristic bacterial SERS marker assisted by a high-performance SERS chip and a selective culture medium. The SERS chip being fabricated through in situ growth of bimetallic Au-Ag nanocomposites on silicon wafer within 2 h, featured a high SERS activity (EF > 10⁷), good uniformity and batch-to-batch consistency (RSD < 10 %), and satisfactory chemical stability. The directly-visualized SERS marker at 1222 cm^-1 originated from bacterial metabolite hypoxanthine was robust and exclusive for discrimination of Salmonella with other bacterial species. Moreover, the method was successfully used for direct discrimination of Salmonella in mixed pathogens by using a selective culture medium, and could identify Salmonella contaminant at ∼1 CFU spiked level in a real sample (Wenxin granule, a botanical drug) after 12 h of enrichment. The combined results showed that developed SERS method is practical and reliable, and could be a promising alternative for rapid identification of Salmonella contamination in pharmaceutical and foods industries.

Nanobody-based immunomagnetic separation platform for rapid isolation and detection of Salmonella enteritidis in food samples

Rapid separation and identification of Salmonella enteritidis (S. enteritidis) in food is of great importance to prevent outbreaks of foodborne diseases. Herein, by using O and H antigens as targets, an epitope-based bio-panning strategy was applied to isolate specific nanobodies towards S. enteritidis. This method constitutes an efficient way to obtain specific antibody fragments and test pairwise nanobodies. On this basis, a double nanobody-based sandwich enzyme-linked immunosorbent assay (ELISA) coupled with immunomagnetic separation (IMS) was developed to rapid enrich and detect S. enteritidis in food. The detection limit of the IMS-ELISA was 3.2 × 10³ CFU/mL. In addition, 1 CFU of S. enteritidis in food samples can be detected after 4-h cultivation, which was shortened by 2 h after IMS. The IMS-ELISA strategy could avoid matrix interference and shorten the enrichment culture time, which has great potential for application in monitoring bacterial contamination in food.

An Overview of the Public Health Challenges in Diagnosing and Controlling Human Foodborne Pathogens

Pathogens found in food are believed to be the leading cause of foodborne illnesses; and they are considered a serious problem with global ramifications. During the last few decades, a lot of attention has been paid to determining the microorganisms that cause foodborne illnesses and developing new methods to identify them. Foodborne pathogen identification technologies have evolved rapidly over the last few decades, with the newer technologies focusing on immunoassays, genome-wide approaches, biosensors, and mass spectrometry as the primary methods of identification. Bacteriophages (phages), probiotics and prebiotics were known to have the ability to combat bacterial diseases since the turn of the 20th century. A primary focus of phage use was the development of medical therapies; however, its use quickly expanded to other applications in biotechnology and industry. A similar argument can be made with regards to the food safety industry, as diseases directly endanger the health of customers. Recently, a lot of attention has been paid to bacteriophages, probiotics and prebiotics most likely due to the exhaustion of traditional antibiotics. Reviewing a variety of current quick identification techniques is the purpose of this study. Using these techniques, we are able to quickly identify foodborne pathogenic bacteria, which forms the basis for future research advances. A review of recent studies on the use of phages, probiotics and prebiotics as a means of combating significant foodborne diseases is also presented. Furthermore, we discussed the advantages of using phages as well as the challenges they face, especially given their prevalent application in food safety.

Magnetic microbead enzyme-linked immunoassay based on phage encoded protein RBP 41-mediated for rapid and sensitive detection of Salmonella in food matrices

Rapid and sensitive quantitative detection methods are required to monitor and detect Salmonella throughout the food supply chain and early prevention of foodborne disease outbreaks. In this study, a magnetic microbead enzyme-linked immunoassay (MELISA) based on phage receptor binding protein was developed for rapid enrichment and detection of Salmonella in complex food matrices. RBP 41 from phage T102 acted as a species-specific recognition element for Salmonella by exploiting its strong binding capacity to Salmonella surface receptors. RBP 41-MBs were prepared by coupling recombinant RBP 41 with MBs and used to separate and enrich Salmonella cells from spiked food samples. The captured complexes were further integrated with ELISA procedures by HRP-labeled anti-Salmonella antibody for rapid and accurate detection of Salmonella. The whole method took <1.5 h and the detection limit was 10 CFU/mL. Therefore, MELISA was successfully developed for the detection of Salmonella in various spiked food samples (skim milk, lettuce, and chicken breast). The ELISA reaction process of this method was carried out on magnetic beads. It simplified the process of the traditional ELISA method and reduces the reaction time. This study expanded the use of phage-associated proteins and demonstrated the promising prospects for practical applications in the detection of foodborne pathogens.

Recent Progress in Nanotechnology-Based Approaches for Food Monitoring

Throughout the food supply chain, including production, storage, and distribution, food can be contaminated by harmful chemicals and microorganisms, resulting in a severe threat to human health. In recent years, the rapid advancement and development of nanotechnology proposed revolutionary solutions to solve several problems in scientific and industrial areas, including food monitoring. Nanotechnology can be incorporated into chemical and biological sensors to improve analytical performance, such as response time, sensitivity, selectivity, reliability, and accuracy. Based on the characteristics of the contaminants and the detection methods, nanotechnology can be applied in different ways in order to improve conventional techniques. Nanomaterials such as nanoparticles, nanorods, nanosheets, nanocomposites, nanotubes, and nanowires provide various functions for the immobilization and labeling of contaminants in electrochemical and optical detection. This review summarizes the recent advances in nanotechnology for detecting chemical and biological contaminations in the food supply chain.

PEGylated Ni Single-Atom Catalysts as Ultrasensitive Electrochemiluminescent Probes with Favorable Aqueous Dispersibility for Assaying Drug-Resistant Pathogens

Ni single-atom catalysts (SACs) were synthesized by high-temperature calcination of nickel ions and 1,10-phenanthroline on carbon black as a carrier. Benefiting from the ultrahigh atom utilization efficiency, Ni SACs can significantly accelerate decay of dissolved oxygen to generate abundant reactive oxygen species through an oxygen reduction reaction occurring on cathodes. The generated reactive oxygen species can vastly enhance the electrochemiluminescent (ECL) signal of luminol without participation of exogenous co-reactants. To overcome the inherent unfavorable aqueous dispersibility of Ni SACs prepared by the calcination protocol, they were functionalized with highly hydrophilic PEG 2000. Thanks to the abundant carboxyl groups on PEG 2000, the PEGylated Ni SACs (Ni@PEG) can be used as ECL probes to tag biorecognition molecules. In this proof-of-principle work, an ECL biosensor for assaying methicillin-resistant Staphylococcus aureus was developed by using porcine IgG as capture molecule and phage cell-binding domain tagged with Ni@PEG as signal tracer. It shows a broad linear range of 73-7.3 × 10⁶ CFU/mL and a low detection limit of 25 CFU/mL. The recovery values for assaying spiked samples are between 80.8 and 119.2%. It was also utilized to assess MRSA susceptibility to four antibiotics, with results consistent with those obtained by the standard broth microdilution technique. To the best of our knowledge, it is the first time to utilize aqueous dispersible SACs as highly sensitive ECL probes for developing biosensors.

Research progress on detection techniques for point-of-care testing of foodborne pathogens

The global burden of foodborne disease is enormous and foodborne pathogens are the leading cause of human illnesses. The detection of foodborne pathogenic bacteria has become a research hotspot in recent years. Rapid detection methods based on immunoassay, molecular biology, microfluidic chip, metabolism, biosensor, and mass spectrometry have developed rapidly and become the main methods for the detection of foodborne pathogens. This study reviewed a variety of rapid detection methods in recent years. The research advances are introduced based on the above technical methods for the rapid detection of foodborne pathogenic bacteria. The study also discusses the limitations of existing methods and their advantages and future development direction, to form an overall understanding of the detection methods, and for point-of-care testing (POCT) applications to accurately and rapidly diagnose and control diseases.

A New Biorecognition-Element-Free IDμE Sensor for the Identification and Quantification of E. coli

The label-free biosensor has emerged as an effective tool for the purpose of early detection of causative pathogens such as Escherichia coli as a preventive measure. In this study, a biorecognition-element-free interdigitated microelectrode (IDμE) sensor is designed and developed with this in mind, with good reliability and affordability. Results show that the designed sensor can identify E. coli with good selectivity using an impedance and capacitance of 7.69 MHz. At its optimum impedance of 1.3 kHz, the IDμE sensor can reliably quantify E. coli in a range of measurement (10^3.2~10⁶ cfu/mL), linearity (R² = 0.97), sensitivity (18.15 kΩ/log (cfu/mL)), and limit of detection (10^3.2 cfu/mL). In summary, the IDμE sensor developed possesses high potential for industrial and clinical applications.

Source Tracker Modeling Based on 16S rDNA Sequencing and Analysis of Microbial Contamination Sources for Pasteurized Milk

Milk is rich in fat, protein, minerals, vitamins, peptides, immunologically active substances, and other nutrients, and it plays an important role in satisfying human nutrition and health. However, dairy product safety incidents caused by microbial contamination have occurred. We found that the total bacterial numbers in the pasteurized product were low and far below the limit requirements of the food safety standards of the European Union, the United States, and China. At the genus level, the primary microbial groups found in milk samples were Acinetobacter, Macrococcus, Pseudomonas, and Lactococcus, while in the equipment rinse water and air samples there was contamination by Stenotrophomonas and Acinetobacter. The Source Tracker model analysis indicated that the microorganisms in the final milk products were significantly related to the contamination in product tanks and raw milk. Therefore, it is the hope that this work can provide guidance to pinpoint contamination problems using the proper quality control sampling at specific stages in the pasteurization process.

Nanobody and Nanozyme-Enabled Immunoassays with Enhanced Specificity and Sensitivity

Immunoassay as a rapid and convenient method for detecting a variety of targets has attracted tremendous interest with its high specificity and sensitivity. Among the commonly used immunoassays, enzyme-linked immunosorbent assay has been widely used as a gold standard method in various fields that consists of two main components including a recognition element and an enzyme label. With the rapid advances in nanotechnology, nanobodies and nanozymes enable immunoassays with enhanced specificity and sensitivity compared with conventional antibodies and natural enzymes. This review is focused on the applications of nanobodies and nanozymes in immunoassays. Nanobodies advantage lies in their small size, high specificity, mass expression, and high stability. Nanozymes with peroxidase, phosphatase, and oxidase activities and their applications in immunoassays are highlighted and discussed in detail. In addition, the challenges and outlooks in terms of the use of nanobodies and the development of novel nanozymes in practical applications are discussed.

Development of nanobody-horseradish peroxidase-based sandwich ELISA to detect Salmonella Enteritidis in milk and in vivo colonization in chicken

Background

Salmonella Enteritidis (S. Enteritidis) being one of the most prevalent foodborne pathogens worldwide poses a serious threat to public safety. Prevention of zoonotic infectious disease and controlling the risk of transmission of S. Enteriditidis critically requires the evolution of rapid and sensitive detection methods. The detection methods based on nucleic acid and conventional antibodies are fraught with limitations. Many of these limitations of the conventional antibodies can be circumvented using natural nanobodies which are endowed with characteristics, such as high affinity, thermal stability, easy production, especially higher diversity. This study aimed to select the special nanobodies against S. Enteriditidis for developing an improved nanobody-horseradish peroxidase-based sandwich ELISA to detect S. Enteritidis in the practical sample. The nanobody-horseradish peroxidase fusions can help in eliminating the use of secondary antibodies labeled with horseradish peroxidase, which can reduce the time of the experiment. Moreover, the novel sandwich ELISA developed in this study can be used to detect S. Enteriditidis specifically and rapidly with improved sensitivity.

Results

This study screened four nanobodies from an immunized nanobody library, after four rounds of screening, using the phage display technology. Subsequently, the screened nanobodies were successfully expressed with the prokaryotic and eukaryotic expression systems, respectively. A sandwich ELISA employing the SE-Nb9 and horseradish peroxidase-Nb1 pair to capture and to detect S. Enteritidis, respectively, was developed and found to possess a detection limit of 5 × 10⁴ colony forming units (CFU)/mL. In the established immunoassay, the 8 h-enrichment enabled the detection of up to approximately 10 CFU/mL of S. Enteriditidis in milk samples. Furthermore, we investigated the colonization distribution of S. Enteriditidis in infected chicken using the established assay, showing that the S. Enteriditidis could subsist in almost all parts of the intestinal tract. These results were in agreement with the results obtained from the real-time PCR and plate culture. The liver was specifically identified to be colonized with quite a several S. Enteriditidis, indicating the risk of S. Enteriditidis infection outside of intestinal tract.

Conclusions

This newly developed a sandwich ELISA that used the SE-Nb9 as capture antibody and horseradish peroxidase-Nb1 to detect S. Enteriditidis in the spike milk sample and to analyze the colonization distribution of S. Enteriditidis in the infected chicken. These results demonstrated that the developed assay is to be applicable for detecting S. Enteriditidis in the spiked milk in the rapid, specific, and sensitive way. Meanwhile, the developed assay can analyze the colonization distribution of S. Enteriditidis in the challenged chicken to indicate it as a promising tool for monitoring S. Enteriditidis in poultry products. Importantly, the SE-Nb1-vHRP as detection antibody can directly bind S. Enteritidis captured by SE-Nb9, reducing the use of commercial secondary antibodies and shortening the detection time. In short, the developed sandwich ELISA ushers great prospects for monitoring S. Enteritidis in food safety control and further commercial production.

Graphic Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01376-y.

A rapid and facile analytical approach to detecting Salmonella Enteritidis with aptamer-based surface-enhanced Raman spectroscopy

Salmonella should be absence in pharmaceutical preparations and foods according to regulations in many countries. Up to now, rapidly detecting Salmonella at 1 CFU·[10 g (mL) ]^-1 in pharmaceutical preparation or 1 CFU·[25 g (mL) ]^-1 in food samples is still a challenge. Herein, we present an aptamer-based surface-enhanced Raman spectroscopy (SERS) method for rapidly detecting Salmonella Enteritidis by using a handheld Raman instrument. The aptamer could specifically recognize S. Enteritidis, and 4-MBA self-assembled on the surface of Au@Ag NPs was used as a Raman reporter molecule. The method was validated to be high specific with no interference from other five pathogenic bacteria. It could identify S. Enteritidis contaminant at ∼ 1 CFU·(10 g)^-1 spiked level in a real sample (Wenxin granule, a botanical drug) after 6 h of enrichment. The detection time was much shorter than that of the methods (more than 54 ∼ 96 h) in the standards of pharmaceutical preparations and foods. In addition, the method could quantitatively determinate S. Enteritidis with satisfactory results. The SERS peak intensities of 4-MBA at 1072 cm^-1 showed a good linear correlation (R² = 0.9873) with the logarithms of S. Enteritidis concentrations ranging from 4.17 × 10² to 1.39 × 10⁷ CFU·mL^-1. T-test result (P = 0.425) revealed that there was no significant difference between the determination results obtained by the SERS method and the plate counting method. Therefore, the study indicated that the method was practical and reliable, and it could be a promising alternative for the on-site detection of S. Enteritidis.

Bacterial coloration immunofluorescence strip for ultrasensitive rapid detection of bacterial antibodies and targeted antibody-secreting hybridomas

The indirect enzyme-linked immunosorbent assay (ELISA) is the gold standard method for monoclonal antibody (McAb) detection and plays a unique role in the preparation of bacterial antibodies. To solve the laborious issues associated with indirect ELISA, a novel bacterial coloration immunofluorescence strip (BCIFS) for antibody detection using colored bacteria instead of a labeled antibody as the antigen and tracer simultaneously and goat anti-mouse IgG as the test line was developed. The affinity range survey of BCIFS indicated that hybridoma cell cultures of E. coli O157:H7 (D3, E7) and Vibrio parahemolyticus (H7, C9) were detected, which complied with the results of indirect ELISA. Compared with the traditional indirect ELISA, the BCIFS sensitivity for E7 cell cultures, ascites, and purified antibodies was at least 4-fold more sensitive, and the BCIFS cross-reactivity for E7 cell cultures was almost consistent with that of indirect ELISA. In addition, the BCIFS isotypes for E. coli O157:H7 cell cultures and Vibrio parahemolyticus were IgG2a and IgG1, respectively, which were identical to the indirect ELISA. Furthermore, the BCIFS method was confirmed by McAb preparation, effective antibody use, and targeted antibody-secreted hybridoma preparation and screening, which showed excellent performance and substitution of the indirect ELISA method. Combined with methylcellulose semisolid medium, BCIFS offers a novel, easy to operate, rapid preparation method for antigen-specific hybridomas. This is the first report using BCIFS instead of indirect ELISA for bacterial antibody detection and application in different samples, which demonstrates a rapid and powerful tool for antibody engineering.

Fluonanobody-based nanosensor via fluorescence resonance energy transfer for ultrasensitive detection of ochratoxin A

Ochratoxin A (OTA) contamination in food is a serious threat to public health. There is an urgent need for development of rapid and sensitive methods for OTA detection, to minimize consumer exposure to OTA. In this study, we constructed two OTA-specific fluonanobodies (FluoNbs), with a nanobody fused at the carboxyl-terminal (SGFP-Nb) or the amino-terminal (Nb-SGFP) of superfolder green fluorescence protein. SGFP-Nb, which displayed better fluorescence performance, was selected as the tracer for OTA, to develop a FluoNb-based nanosensor (FN-Nanosens) via the fluorescence resonance energy transfer, where the SGFP-Nb served as the donor and the chemical conjugates of OTA-quantum dots served as the acceptor. After optimization, FN-Nanosens showed a limit of detection of 5 pg/mL, with a linear detection range of 5-5000 pg/mL. FN-Nanosens was found to be highly selective for OTA and showed good accuracy and repeatability in recovery experiments using cereals with various complex matrix environments. Moreover, the contents of OTA in real samples measured using FN-Nanosens correlated well with those from the liquid chromatography with tandem mass spectrometry. Therefore, this work illustrated that the FluoNb is an ideal immunosensing tool and that FN-Nanosens is reliable for rapid detection of OTA in cereals with ultrahigh sensitivity.

Recombinant antibodies and their use for food immunoanalysis

Antibodies are widely employed as biorecognition elements for the detection of a plethora of compounds including food and environmental contaminants, biomarkers, or illicit drugs. They are also applied in therapeutics for the treatment of several disorders. Recent recommendations from the EU on animal protection and the replacement of animal-derived antibodies by non-animal-derived ones have raised a great controversy in the scientific community. The application of recombinant antibodies is expected to achieve a high growth rate in the years to come thanks to their versatility and beneficial characteristics in comparison to monoclonal and polyclonal antibodies, such as stability in harsh conditions, small size, relatively low production costs, and batch-to-batch reproducibility. This review describes the characteristics, advantages, and disadvantages of recombinant antibodies including antigen-binding fragments (Fab), single-chain fragment variable (scFv), and single-domain antibodies (VHH) and their application in food analysis with especial emphasis on the analysis of biotoxins, antibiotics, pesticides, and foodborne pathogens. Although the wide application of recombinant antibodies has been hampered by a number of challenges, this review demonstrates their potential for the sensitive, selective, and rapid detection of food contaminants.

A gold nanoparticle based colorimetric sensor for the rapid detection of Yersinia enterocolitica serotype O:8 in food samples

Foodborne diseases from Yersinia enterocolitica serotype O:8 represent global public health problems.

Trigging Isothermal Circular Amplification-Based Tuning of Rigorous Fluorescence Quenching into Complete Restoration on a Multivalent Aptamer Probe Enables Ultrasensitive Detection of Salmonella

Detection of pathogenic bacteria is of vital significance for combating and preventing infectious diseases. In this work, we developed a multivalent aptamer probe (Multi-VAP)-based trigging isothermal circular amplification (TICA) for rapidly and ultrasensitively detecting Salmonella. In this sensing system, the fluorescence of Multi-VAP was strongly quenched via the dual effect of FRET. Introduction of Salmonella to the system forced the configuration change of Multi-VAP, leading to the occurrence of a TICA responsible for tuning all of the fluorescence-quenched Multi-VAP into a complete restoration state. This prominent feature allows the reasonable combination of a strong background restraint and great target signal amplification into one sensing system, which in turn benefits the improvement of the signal-to-noise ratio to ensure that the system has an ultrahigh sensitivity. Combined with the employment of an aptamer to ensure that it has excellent specificity, the Salmonella can be quantitatively and qualitatively analyzed even from human serum. The total processing merely requires sample addition and incubation. The turnaround time of the complete analysis from "sample-to-result" was within 30 min. With the method to decrease the time to detect and simplify the process to operate, the assay was successfully used as a sensing platform for specific detection of as few as 9 CFU/mL Salmonella.

Generation of a nanobody-alkaline phosphatase fusion and its application in an enzyme cascade-amplified immunoassay for colorimetric detection of alpha fetoprotein in human serum

Sensitive detection of liver disease biomarkers can facilitate the diagnosis of primary hepatoma and other benign liver diseases, and the alpha fetoprotein (AFP) was selected as the model macromolecule in this work. Herein an enzyme cascade-amplified immunoassay (ECAIA) based on the nanobody-alkaline phosphatase fusion (Nb-ALP) and MnO₂ nanoflakes was developed for detecting AFP. The bifunctional biological macromolecule Nb-ALP serves as the detection antibody and the reporter molecule. The MnO₂ nanoflakes mimic the oxidase for catalyzing the 3,3',5,5'-tetramethylbenzidine (TMB) into the blue oxidized TMB, which has a quantitative signal at the wavelength of 650 nm. Moreover, the Nb-ALP could dephosphorylate the ascorbic acid-2-phosphate (AAP) to form the ascorbic acid (AA) that can disintegrate the nanoflakes to reduce their oxidation capacity with the content decrease of the oxidized TMB. Using the constructed TMB-MnO₂ colorimetric sensing system for Nb-ALP and the optimized experimental parameters, the ECAIA has a limit of detection (LOD) of 0.148 ng/mL which is 18.7-fold lower than that of the p-nitrophenylphosphate (pNPP)-based method (LOD = 2.776 ng/mL). The ECAIA showed good selectivity for AFP with observed negligible cross-reactions with several common cancer biomarkers. The recovery rate for AFP spiked in human serum ranged from 94.8% to 113% with the relative standard deviation from 0.3% to 6.5%. For analysis of the actual human serum samples, a good linear correlation was found between the results tested by the ECAIA and the automatic chemiluminescence analyzer. Thus, the ECAIA was demonstrated to be a promising tool for highly sensitive and selective detection of AFP, providing a reference for analysis of other macromolecule biomarkers.

Development of a Double Nanobody-Based Sandwich Immunoassay for the Detecting Staphylococcal Enterotoxin C in Dairy Products

Staphylococcal enterotoxins (SEs) represent the leading reason for staphylococcal food poisoning (SFP) and various other diseases. Reports often indicate Staphylococcal enterotoxin C (SEC) as the most frequently found enterotoxin in dairy products. To minimize consumer exposure to SEC, this paper aimed to create a sandwich enzyme-linked immunosorbent assay (ELISA) based on nanobodies (sandwich Nbs-ELISA) to accurately detect SEC in dairy products without the influence of staphylococcal protein A (SpA). Therefore, after inoculating a Bactrian camel with SEC, a phage display Nb library was created. Eleven Nbs against SEC were identified in three biopanning steps. Based on their affinity and pairing level, a sandwich Nbs-ELISA was developed using the C6 anti-SEC Nb as the capture antibody, while the detection antibody was represented by the C11 phage display anti-SEC Nb. In optimal conditions, the quantitative range of the present sandwich ELISA was 4-250 ng/mL with a detection limit (LOD) of 2.47 ng/mL, obtained according to the blank value plus three standard deviations. The developed technique was subjected to specific measurements, revealing minimal cross-reactivity with Staphylococcus aureus (S. aureus), Staphylococcal enterotoxin A (SEA), Staphylococcal enterotoxin B (SEB), and SpA. The proposed method exhibited high specificity and an excellent recovery rate of 84.52~108.06% in dairy products. Therefore, the sandwich Nbs-ELISA showed significant potential for developing a specific, sensitive technique for SEC detection in dairy products.

Overview of Rapid Detection Methods for Salmonella in Foods: Progress and Challenges

Salmonella contamination in food production and processing is a serious threat to consumer health. More and more rapid detection methods have been proposed to compensate for the inefficiency of traditional bacterial cultures to suppress the high prevalence of Salmonella more efficiently. The contamination of Salmonella in foods can be identified by recognition elements and screened using rapid detection methods with different measurable signals (optical, electrical, etc.). Therefore, the different signal transduction mechanisms and Salmonella recognition elements are the key of the sensitivity, accuracy and specificity for the rapid detection methods. In this review, the bioreceptors for Salmonella were firstly summarized and described, then the current promising Salmonella rapid detection methods in foodstuffs with different signal transduction were objectively summarized and evaluated. Moreover, the challenges faced by these methods in practical monitoring and the development prospect were also emphasized to shed light on a new perspective for the Salmonella rapid detection methods applications.

Enzyme cascade-amplified immunoassay based on the nanobody-alkaline phosphatase fusion and MnO2 nanosheets for the detection of ochratoxin A in coffee

Ochratoxin A (OTA) is a common food contaminant with multiple toxicities and thus rapid and accurate detection of OTA is indispensable to minimize the threat of OTA to public health. Herein a novel enzyme cascade-amplified immunoassay (ECAIA) based on the mutated nanobody-alkaline phosphatase fusion (mNb-AP) and MnO2 nanosheets was established for detecting OTA in coffee. The detection principle is that the dual functional mNb-AP could specifically recognize OTA and dephosphorylate the ascorbic acid-2-phosphate (AAP) into ascorbic acid (AA), and the MnO2 nanosheets mimicking the oxidase could be reduced by AA into Mn2+ and catalyze the 3,3',5,5'-tetramethyl benzidine into blue oxidized product for quantification. Using the optimal conditions, the ECAIA could be finished within 132.5 min and shows a limit of detection of 3.38 ng mL-1 (IC10) with an IC50 of 7.65 ng mL-1 and a linear range (IC20-IC80) of 4.55-12.85 ng mL-1. The ECAIA is highly selective for OTA. Good recovery rates (84.3-113%) with a relative standard deviation of 1.3-3% were obtained and confirmed by high performance liquid chromatography with a fluorescence detector. The developed ECAIA was demonstrated to be a useful tool for the detection of OTA in coffee which provides a reference for the analysis of other toxic small molecules.

Direct Transverse Relaxation Time Biosensing Strategy for Detecting Foodborne Pathogens through Enzyme-Mediated Sol-Gel Transition of Hydrogels

In this work, we develop a direct transverse relaxation time (T₂) biosensing strategy and employ it for assaying foodborne pathogens relying on the alkaline phosphatase (ALP)-mediated sol-gel transition of hydrogels. ALP can catalyze the reaction to generate an acidic environment to transform the sol-state alginate solution to hydrogel, and this hydrogelation process can directly regulate the diffusion rate of water protons that results in a T₂ change of water molecules. By means of enzyme-modulated sol-gel transition and antigen-antibody interactions, this T₂ biosensor displays high sensitivity for detecting 50 CFU/mL S. enteritidis within 2 h. This biosensing strategy directly modulates the water molecules rather than magnetic probes in traditional methods, offering a straightforward, novel, and sensitive platform for pathogen detection.

A novel antigen immunochromatography fluorometric strip for rapid detection and application of pathogenic bacterial high-quality antibody

Unlike traditional immunoassay strips, a novel antigen immunechromatography fluorometric strip (AICFS) using inactivated bacterial antigen instead of an antibody as a test line and goat anti-mouse IgG-FITC as a tracer was developed. The applicability survey of AICFS indicated that E. coli O157:H7 (D3) and Acidovorax citrulli (6F) hybridoma cell cultures could be detected, but Vibrio parahemolyticus (H7, C9) hybridoma cell cultures were missed compared with the indirect enzyme-linked immunosorbent assay (ELISA). The four antibody affinity constants (Ka) were measured and compared, and AICFS could be suitable for high-affinity antibody detection. Compared with the traditional indirect ELISA, the AICFS sensitivity for D3 cell cultures, ascites, and purified antibodies was at least 2-fold more sensitive, the AICFS specific for D3 cell cultures by comparative interpretation was compliant except for the strain ATCC 43895, and the indirect ELISA missed it. More importantly, the AICFS method was confirmed by various real samples that it could be used in different scenarios regarding the antibody, including McAb preparation, the effective antibody use, and high-affinity antibody-secreted hybridoma auxiliary preparation and screening. It could be an excellent alternative method with less than 5% corresponding processing time for indirect ELISA method for pathogenic bacterial high-quality antibody detection. This is the first report of using AICFS for bacterial high-quality antibody detection and application in different samples, which demonstrates a rapid auxiliary tool for high-affinity antibody secreted-hybridoma screening and an excellent alternative method for high-quality antibody application.

Selection of specific nanobodies to develop an immuno-assay detecting Staphylococcus aureus in milk

The interaction between conventional immunoglobulins (Igs) and the Ig-binding surface proteins of Staphylococcus aureus (S. aureus) have obstructed the development of immuno-assays to detect these bacteria. The current study aimed to select nanobodies (Nbs) recognizing specifically S. aureus and to establish an immuno-assay to uncover S. aureus contaminations in foods. An alpaca was immunized with an inactivated S. aureus strain followed by the construction of a Nb library from which four target-specific Nbs were retrieved. Subsequently, a sandwich ELISA employing the Nb147 and biotinylated-Nb147 pair to capture and to detect S. aureus, respectively, was established to possess a detection limit of 1.4 × 10⁵ colony forming units (CFU)/mL. The dedicated immuno-assay has been verified by detecting 10 CFU/mL of S. aureus in milk samples after an 8 h-enrichment step. This study provides the basis of an easy, reproducible and effective immuno-assay to screen for S. aureus contaminations in foods.

Immunoassay for foodborne pathogenic bacteria using magnetic composites Ab@Fe3O4, signal composites Ap@PtNp, and thermometer readings

A point-of-care (POC) immunoassay was established for the sensitive and rapid detection of pathogenic Escherichia coli O157:H7, using magnetic Fe₃O₄ organic-inorganic composites (Ab@Fe₃O₄) for immunomagnetic separation, nanozyme platinum nanoparticle (PtNp) organic-inorganic composites (Ap@PtNp) for signal amplification, and thermometer readings. Antibodies and Fe₃O₄ were incubated in Cu²⁺ phosphate buffer to synthesize the magnetic composite Ab@Fe₃O₄ with antibodies, to specifically capture E. coli O157:H7. Antimicrobial peptides and PtNp were incubated in Cu²⁺ phosphate buffer to synthesize the signal composites Ap@PtNp with antimicrobial peptides (magainin I), recognizing and labeling E. coli O157:H7. In the presence of E. coli O157:H7, magnetic microcomposites targeted bacteria and signal microcomposites to form the sandwich structure: Ab@Fe₃O₄-bacteria-Ap@PtNp for magnetic separation. Ap@PtNp of signal composites catalyzed H₂O₂ to generate thermo-signals (temperature rise), which were determined by a thermometer. This point-of-care bioassay detected E. coli O157:H7 in the linear range of 10¹-10⁷ CFU mL^-1 and with a detection limit of 14 CFU mL^-1. One-pot process magnetic Fe₃O₄ organic-inorganic composites (Ab@Fe₃O₄, magnetic microcomposites, MMC) for immunomagnetic separation and nanozyme platinum nanoparticle (PtNp) organic-inorganic composites (Ap@PtNp, signal microcomposites, SMC) were used as signal amplification and thermometer readings for E. coli O157:H7 detection.

Biosensors for rapid detection of Salmonella in food: A review

Salmonella is one of the main causes of foodborne infectious diseases, posing a serious threat to public health. It can enter the food supply chain at various stages of production, processing, distribution, and marketing. High prevalence of Salmonella necessitates efficient and effective approaches for its identification, detection, and monitoring at an early stage. Because conventional methods based on plate counting and real-time polymerase chain reaction are time-consuming and laborious, novel rapid detection methods are urgently needed for in-field and on-line applications. Biosensors provide many advantages over conventional laboratory assays in terms of sensitivity, specificity, and accuracy, and show superiority in rapid response and potential portability. They are now recognized as promising alternative tools and one of the most on-site applicable and end user-accessible methods for rapid detection. In recent years, we have witnessed a flourishing of studies in the development of robust and elaborate biosensors for detection of Salmonella in food. This review aims to provide a comprehensive overview on Salmonella biosensors by highlighting different signal-transducing mechanisms (optical, electrochemical, piezoelectric, etc.) and critically analyzing its recent trends, particularly in combination with nanomaterials, microfluidics, portable instruments, and smartphones. Furthermore, current challenges are emphasized and future perspectives are discussed.

Ultrasensitive label-free immunochromatographic strip sensor for Salmonella determination based on salt-induced aggregated gold nanoparticles

Here we present an innovative label-free immunochromatographic strip (ICTS) sensor, in which salt-induced aggregated gold nanoparticles (SIA-AuNPs) act as the signal probe, allowing in 14 min the identification and sensitive quantification of Salmonella as model targets. It has been evidenced that SIA-AuNPs could be absorbed on the surface of bacteria based on van der Waals forces. The SIA-AuNPs@Salmonella complex was captured by anti-Salmonella polyclonal antibody deposited on the test zone. With the label-free ICTS sensor, we successfully detected Salmonella in a concentration range of 10³-10⁸ CFU/mL and a visual detection limit of 1 × 10³ CFU/mL. The band of test zone could be distinguished at a concentration of 10³ CFU/mL by naked eye, which is 100-fold lower than the cationic AuNPs based method. The strip sensor was further validated with real samples including cabbage and drinking water with excellent precision and showed to provide excellent recovery.