Sensitive and highly specific detection ofCronobacter sakazakiibased on monoclonal sandwich ELISA

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.

An ultrasensitive colorimetric biosensor for the detection of Gram-positive bacteria by integrating paper-based enrichment and carbon dot-based selective recognition

Rapid screening of bacteria by low-cost and eco-friendly material-based approaches is still a major challenge. Herein, a colorimetric biosensor was designed for the ultrasensitive and rapid detection of Gram-positive bacteria. The biosensor exploited polydopamine and polyethyleneimine (PDA-PEI)-modified papers for separating bacteria and carbon dots (CDs) for selective colorimetric detection of Gram-positive bacteria. Noble metal-free CDs can target Gram-positive bacteria by binding with peptidoglycan and possess peroxidase-like activity. Thus, they can avert the step of modifying recognition probes, facilitating biosensor fabrication, and reducing the cost. This biosensor can detect S. aureus as low as 1 cfu mL^-1, L. monocytogenes as low as 5 cfu mL^-1, and B. subtilis as low as 9 cfu mL^-1 within 55 min. In addition, a portable device was constructed to enable convenient and on-site quantitative detection of Gram-positive bacteria. The feasibility of the biosensor was verified by detecting Gram-positive bacteria in eggshell and sausage samples with recoveries ranging from 91.2% to 110%.

Development of multi-reactive aptamers for Cronobacter spp. using the sequential partitioning method to detect them in powdered infant formula

Cronobacter spp. are opportunistic foodborne pathogens typically detected in contaminated powdered infant formula (PIF). Thus, the rapid detection and control of Cronobacter spp. are required to prevent outbreaks, necessitating the development of specific aptamers. In this study, we isolated aptamers specific to all seven species of Cronobacter (C. sakazakii, C. malonaticus, C. turicensis, C. muytjensii, C. dublinensis, C. condimenti, and C. universalis) using a newly proposed sequential partitioning method. This method avoids the repeated enrichment steps, reducing the total aptamer selection time compared with the conventional systematic evolution of ligands by the exponential enrichment (SELEX) process. We isolated four aptamers showing high affinity and specificity for all seven species of Cronobacter, with dissociation constants of 3.7-86.6 nM. This represents the first successful isolation of aptamers for multiple targets using the sequential partitioning method. Further, the selected aptamers could effectively detect Cronobacter spp. in contaminated PIF.

Conventional and advanced detection techniques of foodborne pathogens: A comprehensive review

Foodborne pathogens are a major public health concern and have a significant economic impact globally. From harvesting to consumption stages, food is generally contaminated by viruses, parasites, and bacteria, which causes foodborne diseases such as hemorrhagic colitis, hemolytic uremic syndrome (HUS), typhoid, acute, gastroenteritis, diarrhea, and thrombotic thrombocytopenic purpura (TTP). Hence, early detection of foodborne pathogenic microbes is essential to ensure a safe food supply and to prevent foodborne diseases. The identification of foodborne pathogens is associated with conventional (e.g., culture-based, biochemical test-based, immunological-based, and nucleic acid-based methods) and advances (e.g., hybridization-based, array-based, spectroscopy-based, and biosensor-based process) techniques. For industrial food applications, detection methods could meet parameters such as accuracy level, efficiency, quickness, specificity, sensitivity, and non-labor intensive. This review provides an overview of conventional and advanced techniques used to detect foodborne pathogens over the years. Therefore, the scientific community, policymakers, and food and agriculture industries can choose an appropriate method for better results.

Detection of Microorganisms in Body Fluids via MTT-PMS Assay

Early detection of microorganisms is essential for the management of infectious diseases. However, this is challenging, as traditional culture methods are labor-intensive and time-consuming. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide-phenazine methosulfate (MTT-PMS) assay has been used to evaluate the metabolic activity in live cells and can thus be used for detecting living microorganisms. With the addition of NaOH and Tris-EDTA, the same approach can be accelerated (within 15 min) and used for the quick detection of common bacterial pathogens. The assay results can be evaluated colorimetrically or semi-quantitatively. Here, the quick detection by MTT-PMS assay was further investigated. The assay had a detection limit of approximately 104 CFU/mL. In clinical evaluations, we used the MTT-PMS assay to detect clinical samples and bacteriuria (>105 CFU/mL). The negative predictive value of the MTT-PMS assay for determining bacteriuria was 79.59% but was 100% when the interference of abnormal blood was excluded. Thus, the MTT-PMS assay might be a potential "rule-out" tool for bacterial detection in clinical samples, at a cost of approximately USD 1 per test. Owing to its low cost, rapid results, and easy-to-use characteristics, the MTT-PMS assay may be a potential tool for microorganism detection.

Specific detection of Cronobacter sakazakii in powdered infant formula using ssDNA aptamer

Cronobacter sakazakii (C. sakazakii) is a foodborne pathogen associated with bacterial meningitis, sepsis, and necrotizing enterocolitis in premature and immuno-compromised infants. C. sakazakii is typically acquired by ingesting contaminated powdered infant formula (PIF). The growing demand for a safe food supply requires rapid detection of foodborne pathogens for delivering safe-to-consume food to consumers. In the present study, we isolated C. sakazakii-specific aptamers using a centrifugation-based partitioning method (CBPM) instead of systematic evolution of ligands by exponential enrichment (SELEX) process. Unlike SELEX, the CBPM reduces the evolution-loop time to obtain enriched probes, allowing the isolation of target-specific aptamers in a shorter time. The two aptamers (SC25 and SC45) isolated using the CBPM showed high affinity and specificity for C. sakazakii (K_d: 34 and 66 nM). Among the two aptamers, SC25 aptamer detected efficiently C. sakazakii in PIF with less cross-reactivity. Our results indicate that the isolated aptamers could be used for detecting C. sakazakii in PIF and reducing the overall testing time compared with the conventional C. sakazakii detection method.

Portable Device for Quick Detection of Viable Bacteria in Water

(1) Background: Access to clean water is a very important factor for human life. However, pathogenic microorganisms in drinking water often cause diseases, and convenient/inexpensive testing methods are urgently needed. (2) Methods: The reagent contains 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and phenazine methosulfate (PMS) and can react with succinate dehydrogenase within bacterial cell membranes to produce visible purple crystals. The colorimetric change of the reagent after reaction can be measured by a sensor (AS7262). (3) Results: Compared with traditional methods, our device is simple to operate and can provide rapid (i.e., 5 min) semi-quantitative results regarding the concentration of bacteria within a test sample. (4) Conclusions: This easy-to-use device, which employs MTT-PMS reagents, can be regarded as a potential and portable tool for rapid water quality determination.

Labchip-based diagnosis system for on-site application: Sensitive and easy-to-implement detection of single recoverable Cronobacter in infant formula without post-enrichment treatment

Microfluidic labchips have achieved much advancement in the molecular diagnosis of foodborne pathogens. Whereas difficulties in the flow control during the transportation of liquid fluids can occur and should be overcome. Manipulations of reaction temperature and the complex procedures from sample pre-treatment to analysis in a single chip device are major obstacles for the on-site application. Thus, the efficient temperature control of samples without any flow of reaction fluids in microfluidic channels of plastic chip and the simplest protocol omitting post-enrichment processing steps may overcome these limitations represented by the stability and the complexity, respectively. This study aims to develop a novel type of labchip and thermocycler specialized for the gene amplification in microfluidic channels and to evaluate the detectability by sensing the minimum recoverable level of Cronobacter in powdered infant formula (PIF). We developed a thermocycling device accelerating reactions through dual heating-blocks optimized to control temperatures of samples in microfluidic-channels by direct contact with labchip sequentially and repetitively. The structural design of microfluidic channels was to eliminate interference factors associated with the optical detection of fluorescent signals (without distortion due to air bubbles in the reaction chamber). To improve the applicability, a portable device and simplified operation to allow direct loading of samples in the chip without post-enrichment procedures were also adopted. Detection performance was evaluated by a sensitivity/specificity tests using 50 isolates of Cronobacter. Cross-reactivity tests for non-Cronobacter organisms and gDNA [human, raw materials of PIF (cow, soybean)] showed that there was no interference-factor causing false-positive results. In terms of the applied research conducted by using PIF, the enrichment of samples without broth medium (distilled water) displayed outstanding performance and 12 h of incubation facilitated detecting target at concentration as low as 1 CFU/300 g PIF (as initial contamination level) without post-enrichment treatment. Validation of the operation conditions using 30 commercial PIF products was also consistent. The present study presents a novel approach of microfluidic technology with perspective to not only the performance and the practicability [easy-to-implement protocol, portable materials, cost-effectiveness (the use of a miniaturized plastic chip requires a minimum level of materials)] for on-site diagnosis.

Immunogold Nanoparticles for Rapid Plasmonic Detection of C. sakazakii

Cronobacter sakazakii is a foodborne pathogen that can cause a rare, septicemia, life-threatening meningitis, and necrotizing enterocolitis in infants. In general, standard methods for pathogen detection rely on culture, plating, colony counting and polymerase chain reaction DNA-sequencing for identification, which are time, equipment and skill demanding. Recently, nanoparticle- and surface-based immunoassays have increasingly been explored for pathogen detection. We investigate the functionalization of gold nanoparticles optimized for irreversible and specific binding to C. sakazakii and their use for spectroscopic detection of the pathogen. We demonstrate how 40-nm gold nanoparticles grafted with a poly(ethylene glycol) brush and functionalized with polyclonal antibodies raised against C. sakazakii can be used to specifically target C. sakazakii. The strong extinction peak of the Au nanoparticle plasmon polariton resonance in the optical range is used as a label for detection of the pathogens. Individual binding of the nanoparticles to the C. sakazakii surface is also verified by transmission electron microscopy. We show that a high degree of surface functionalization with anti-C. sakazakii optimizes the detection and leads to a detection limit as low as 10 CFU/mL within 2 h using a simple cuvette-based UV-Vis spectrometric readout that has great potential for further optimization.

Sensitive, fast, and specific immunoassays for methyltestosterone detection

An indirect competitive enzyme-linked immunosorbent assay (icELISA) and an immunochromatographic strip assay using a highly specific monoclonal antibody, were developed to detect methyltestosterone (MT) residues in animal feed. The optimized icELISA had a half-inhibition concentration value of 0.26 ng/mL and a limit of detection value of 0.045 ng/mL. There was no cross-reactivity with eight analogues, revealing high specificity for MT. Based on icELISA results, the recovery rate of MT in animal feed was 82.4%–100.6%. The results were in accordance with those obtained by gas chromatography-mass spectrometry. The developed immunochromatographic strip assay, as the first report for MT detection, had a visual cut-off value of 1 ng/mL in PBS, 2.5 ng/g in fish feed, and 2.5 ng/g in pig feed. Therefore, these immunoassays are useful and fast tools for MT residue detection in animal feed.