Immunomagnetic separation: An effective pretreatment technology for isolation and enrichment in food microorganisms detection

Phage based magnetic capture method as an aid for real time RPA detection of Salmonella spp. in milk

Salmonella is a major cause of foodborne diseases worldwide. Conventional rapid assays for detecting Salmonella in real samples often encounter severe matrix interference or detect the limited number of species of a genus, resulting the inaccuracy of detection. In this study, we developed a method that combined phage-based magnetic capture with real time recombinase polymerase amplification (RPA) for the rapid, highly sensitive, and specific detection of Salmonella in milk with an ultra-low detection limit. The Felix O-1 phage-conjugated magnetic beads (O-1 pMBs) synthesized in this method showed excellent capture ability for Salmonella spp. and ideal specificity for non-Salmonella strains. After O-1 pMBs-based magnetic separation, the limit of detection (LOD) of the real time RPA assay was 50 cfu/mL in milk samples, which was significantly increased by a magnitude of 3-4 orders. The method exhibited a high sensitivity (compatibility) of 100% (14/14) for all tested Salmonella serotype strains and an ideal specificity (exclusivity) of 100% (7/7) for the tested non-Salmonella strains. The entire detection process including Salmonella capture, DNA extraction, and real time RPA detection was completed within 1.5 h. Furthermore, milk samples spiked with 10 cfu/25 mL of Salmonella were detected positive after cultured in buffered peptone water for only 3 h. Therefore, the proposed method could be an alternative for the rapid and accurate detection of Salmonella.

A highly sensitive point-of-care detection platform for Salmonella typhimurium by integrating magnetic enrichment and fluorescent CsPbBr3@SiO2

A platform was designed based on Fe3O4 and CsPbBr3@SiO2 for integrated magnetic enrichment-fluorescence detection of Salmonella typhimurium, which significantly simplifies the detection process and enhances the working efficiency. Fe3O4 served as a magnetic enrichment unit for the capture of S. typhimurium. CsPbBr3@SiO2 was employed as a fluorescence-sensing unit for quantitative signal output, where SiO2 was introduced to strengthen the stability of CsPbBr3, improve its biomodificability, and prevent lead leakage. More importantly, the SiO2 shell shows neglectable absorption or scattering towards fluorescence, making the CsPbBr3@SiO2 exhibit a high quantum yield of 74.4%. After magnetic enrichment, the decreasing rate of the fluorescence emission intensity of the CsPbBr3@SiO2 supernatant at 527 nm under excitation light at UV 365 nm showed a strong linear correlation with S. typhimurium concentration of 1 × 102~1 × 108 CFU∙mL-1, and the limit of detection (LOD) reached 12.72 CFU∙mL-1. This platform has demonstrated outstanding stability, reproducibility, and resistance to interference, which provides an alternative for convenient and quantitative detection of S. typhimurium.

Development of an Open Droplet Microchannel-Based Magnetosensor for Immunofluorometric Assay of Trimethoprim in Chicken and Pork Samples with a Wide Linear Range

Trimethoprim (TMP), functioning as a synergistic antibacterial agent, is utilized in diagnosing and treating diseases affecting livestock and poultry. Human consumption of the medication indirectly may lead to its drug accumulation in the body and increase drug resistance due to its prolonged metabolic duration in livestock and poultry, presenting significant health hazards. Most reported immunoassay techniques, such as ELISA and immunochromatographic assay (ICA), find it challenging to achieve the dual advantages of high sensitivity, simplicity of operation, and a wide detection range. Consequently, an open droplet microchannel-based magnetosensor for immunofluorometric assay (OMM-IFA) of trimethoprim was created, featuring a gel imager to provide a signal output derived from the highly specific antibody (Ab) targeting trimethoprim. The method exhibited high sensitivity in chicken and pork samples, with LODs of 0.300 and 0.017 ng/mL, respectively, and a wide linear range, covering trimethoprim's total maximum residue limits (MRLs). Additionally, the spiked recoveries in chicken and pork specimens varied between 81.6% and 107.9%, maintaining an acceptable variation coefficient below 15%, aligning well with the findings from the ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) technique. The developed method achieved a much wider linear range of about 5 orders of magnitude of 10-2-103 levels with grayscale signals as the output signal, which exhibited high sensitivity, excellent applicability and simple operability based on magnetic automation.

Efficient magnetic enrichment cascade single-step RPA-CRISPR/Cas12a assay for rapid and ultrasensitive detection of Staphylococcus aureus in food samples

Staphylococcus aureus (S. aureus) is a prevalent foodborne pathogen that could cause severe food poisoning. Thus, rapid, efficient, and ultrasensitive detection of S. aureus in food samples is urgently needed. Here, we report an efficient magnetic enrichment cascade single-step recombinase polymerase amplification (RPA)-CRISPR/Cas12a assay for the ultrasensitive detection of S. aureus. Magnetic beads (MBs) functionalized with S. aureus-specific antibodies were initially used for S. aureus enrichment from the complex matrix, with 98% capture efficiency in 5 min and 100-fold sensitivity improvement compared with unenriched S. aureus. Next, a single-step RPA-CRISPR/Cas12a-based diagnostic system with optimized extraction-free bacteria lysis was constructed. This assay could detect as low as 1 copy/μL (five copies/reaction) of extracted DNA template and 10 CFU/mL of S. aureus within 40 min. Furthermore, the assay could effectively detect S. aureus in real food samples such as lake water, orange juice, pork, and lettuce, with concordant results to qPCR assays. The proposed cascade signal-amplification assay eliminates the need for lengthy bacterial culture and complex sample preparation steps. Hence, the proposed assay shows great application potential for rapid, efficient, and ultrasensitive detection of pathogens in real food samples.

Specific separation and sensitive detection of foodborne pathogens by phage-derived bacterial-binding protein-nano magnetic beads coupled with smartphone-assisted paper sensor

Foodborne pathogen infection poses a significant threat to public health and is considered as one of the most serious hazards in global food safety. Herein, a sensitive and efficient method for on-site monitoring of foodborne pathogens was developed by using a smartphone-assisted paper-sensor combined with phage-derived bacterial-binding proteins-nano magnetic beads (PBPs-MBs). PBPs including tail fiber protein (TFP:gp13), cell-wall binding domain (CBD) of endolysin and tailspike protein (TSP) coated on the surface of MBs were applied for rapid separation and enrichment of targeted bacteria (Escherichia coli O157:H7, Staphylococcus aureus and Salmonella typhimurium, respectively) from food samples in 20 min before detection on paper-based sensors. The paper-based sensor was loaded with the lytic agent (polymyxin B) to induce bacterial lysis and release specific endogenous enzymes. Subsequently, three distinct chromogenic substrates were hydrolyzed by their corresponding enzymes, resulting in characteristic color changes on the paper, respectively. In addition, a smartphone APP for red-green-blue (RGB) color analysis of paper was able to directly detect three foodborne pathogens. As a result, the limit of detection (LOD) values for three foodborne pathogens were found to be 2.44 × 102, 2.68 × 104 and 4.62 × 103 CFU/mL, respectively, which were much lower than other studies (106-108 CFU/mL) based on enzymes. Moreover, the feasibility of this approach was further assessed through the successful detection of targeted bacteria in real samples with satisfactory recovery rates. In conclusion, this smartphone-assisted biosensor offers promising application potential for point-of-care testing (POCT) of foodborne pathogens in resource-scarce areas.

Illuminating the Tiny World: A Navigation Guide for Proper Raman Studies on Microorganisms

Raman spectroscopy is an emerging method for the identification of bacteria. Nevertheless, a lot of different parameters need to be considered to establish a reliable database capable of identifying real-world samples such as medical or environmental probes. In this review, the establishment of such reliable databases with the proper design in microbiological Raman studies is demonstrated, shining a light into all the parts that require attention. Aspects such as the strain selection, sample preparation and isolation requirements, the phenotypic influence, measurement strategies, as well as the statistical approaches for discrimination of bacteria, are presented. Furthermore, the influence of these aspects on spectra quality, result accuracy, and read-out are discussed. The aim of this review is to serve as a guide for the design of microbiological Raman studies that can support the establishment of this method in different fields.

Automated sepsis detection with vancomycin- and allantoin-polydopamine magnetic nanoparticles

Rapid and accurate identification of the bacteria responsible for sepsis is paramount for effective patient care. Molecular diagnostic methods, such as polymerase chain reaction (PCR), encounter challenges in sepsis due to inhibitory compounds in the blood, necessitating their removal for precise analysis. In this study we present an innovative approach that utilizes vancomycin (Van) and allantoin (Al)-conjugated polydopamine (PDA)-coated magnetic nanoparticles (MNPs) for the rapid and automated enrichment of bacteria and their DNA extraction from blood without inducing clumping and aggregation of blood. Al/Van-PDA-MNPs, facilitated by IMS, eliminate the need for preliminary sample treatments, providing a swift and efficient method for bacterial concentration and DNA extraction within an hour. Employing Al/Van-PDA-MNPs within an automated framework has markedly improved our ability to pre-concentrate various Gram-negative and Gram-positive bacteria directly from blood samples. This advancement has effectively reduced the detection threshold to 102 colony-forming unit/mL by both PCR and quantitative PCR. The method's expedited processing time, combined with its precision, positions it as a feasible diagnostic tool for diverse healthcare settings, ranging from small clinics to large hospitals. Furthermore, the innovative application of nanoparticles for DNA extraction holds promising potential for advancing sepsis diagnostics, enabling earlier interventions and improving patient outcomes.

Magnetic capture device for large volume sample analysis

Immunomagnetic separation (IMS) techniques employing superparamagnetic particles can successfully isolate various components from mixtures. However, their utility can be limited for large-volume samples, viscous samples, or those containing a high density of particulate matter because of the need to generate high field gradients for particle recovery. Therefore, a new class of immunomagnetic particles was devised utilizing a single, macroscopic Pyrex spinbar conjugated with biorecognition elements to address these limitations. Advantages include an inherent capacity for effective mixing, an almost instantaneous recovery of the spinbar that can be performed without expensive equipment and with no loss of magnetic particles during processing, and reduced transfer of sample matrix. As a result, spinbars can provide an effective means for IMS with large-volume assays composed of complex matrices.

Rapid and simultaneous multiepitope antigen-based detection of Enterococcus by microscale thermophoresis and immunomagnetic separation

Background

Generally, enterococci bacteria cause nosocomial infections and are major indicators of bacterial contamination in marine bathing beach. However, a method for the rapid and simultaneous detection of multiple pathogenic enterococci has not been developed on account of the wide variety of pathogenic enterococci and their existence in complex matrices.

Methods

Immunoinformatics tools were used to design a multi-epitope antigen for the detection of various pathogenic enterococci by using the sequence of dltD gene on enterococci lipoteichoic acid (LTA) surface, which is associated with toxicological effects. The multi-epitopes included enterococci such as Enterococcus faecalis, E. gallinarum, E. raffinosus, E. durans, E. faecium, E. hirae, E. thailandicus, E. casseliflavus, E. avium, E. mundtii, E. lactis, E. solitarius, E. pseudoavium, and E. malodoratum. Microscale thermophoresis (MST) and western blot were carried out to detect the affinity between multi-epitope antigens and antibodies and between multi-epitope antibodies and bacteria. Furthermore, the detection of pathogenic enterococci was carried out by using immunomagnetic beads (IMBs) and immune chromatographic test strip (ICTS).

Results

The multi-epitope antibody had a satisfactory affinity to the antigen and enterococci. IMBs and ICTS were detected with a minimum of 101 CFU/mL and showed incompatibility for Vibrio parahemolyticus, V. vulnifcus, V. harveyi, V. anguillarum, and Edwardsiella tarda.

Implication

The present study demonstrated that the multi-epitope antigens exhibited excellent specificity and sensitivity, making them highly suitable for efficient on-site screening of enterococci bacteria in marine bathing beaches.

The effect of magnetic bead size on the isolation efficiency of lung cancer cells in a serpentine microchannel with added cavities

An investigation was conducted to examine the effect of magnetic bead (MB) size on the effectiveness of isolating lung cancer cells using the immunomagnetic separation (IMS) method in a serpentine microchannel with added cavities (SMAC) structure. Carboxylated magnetic beads were specifically conjugated to target cells through a modification procedure using aptamer materials. Cells immobilized with different sizes (in micrometers) of MBs were captured and isolated in the proposed device for comparison and analysis. The study yields significance regarding the clarification of device working principles by using a computational model. Furthermore, an accurate evaluation of the MB size impact on capture efficiency was achieved, including the issue of MB-cell accumulation at the inlet-channel interface, despite it being overlooked in many previous studies. As a result, our findings demonstrated an increasing trend in binding efficiency as the MB size decreased, evidenced by coverages of 50.5%, 60.1%, and 73.4% for sizes of 1.36 μm, 3.00 μm, and 4.50 μm, respectively. Additionally, the overall capture efficiency (without considering the inlet accumulation) was also higher for smaller MBs. However, when accounting for the actual number of cells entering the channel (i.e., the effective capture), larger MBs showed higher capture efficiency. The highest effective capture achieved was 88.4% for the size of 4.50 μm. This research provides an extensive insight into the impact of MB size on the performance of IMS-based devices and holds promise for the efficient separation of circulating cancer cells (CTCs) in practical applications.

Rapid screening of electrochemically active bacteria based on a biocathode-functional bipolar electrode-electrochemiluminescence platform

A functional bipolar electrode-electrochemiluminescence (BPE-ECL) platform based on biocathode reducing oxygen was constructed for detecting electrochemically active bacteria (EAB) in this paper. Firstly, thiolated trimethylated chitosan quaternary ammonium salt (TMC-SH) layer was assembled on the gold-plated cathode of BPE. TMC-SH contains quaternary ammonium salt branch chain, which can inhibit the growth of microorganisms on the surface or in the surrounding environment while absorbing bacteria. Then, the peristaltic pump was used to flow all of the samples through the cathode, and the EAB was electrostatically adsorbed on the electrode surface. Finally, applying a constant potential to the BPE, bacteria can catalyze electrochemical reduction of O2, and decrease the overpotential of O2 reduction at the cathode, which in turn generates an ECL reporting intensity change at the anode. In this way, live and dead bacteria can be distinguished, and the influence of complex food substrates on detection can be greatly reduced.

Nanomaterial-based biosensors for the detection of foodborne bacteria: a review

Ensuring food safety is a critical concern for the development and well-being of humanity, as foodborne illnesses caused by foodborne bacteria have increasingly become a major public health concern worldwide. Traditional food safety monitoring systems are expensive and time-consuming, relying heavily on specialized equipment and operations. Therefore, there is an urgent need to develop low-cost, user-friendly and highly sensitive biosensors for detecting foodborne bacteria. In recent years, the combination of nanomaterials with optical biosensors has provided a prospective future platform for the detection of foodborne bacteria. By harnessing the unique properties of nanomaterials, such as their high surface area-to-volume ratio and exceptional sensitivity, in tandem with the precision of optical biosensing techniques, a new prospect has opened up for the rapid and accurate identification of potential bacterial contaminants in food. This review focuses on recent advances and new trends of nanomaterial-based biosensors for the detection of foodborne pathogens, which mainly include noble metal nanoparticles (NMPs), metal organic frameworks (MOFs), graphene nanomaterials, quantum dot (QD) nanomaterials, upconversion fluorescent nanomaterials (UCNPs) and carbon dots (CDs). Additionally, we summarized the research progress of color indicators, nanozymes, natural enzyme vectors and fluorescent dye biosensors, focusing on the advantages and disadvantages of nanomaterial-based biosensors and their development prospects. This review provides an outlook on future technological directions and potential applications to help identify the most promising areas of development in this field.

Enhanced detection of Listeria monocytogenes using tetraethylenepentamine-functionalized magnetic nanoparticles and LAMP-CRISPR/Cas12a-based biosensor

BACKGROUND

Listeria monocytogenes is a pathogenic bacterium that can lead to severe illnesses, especially among vulnerable populations. Therefore, the development of rapid and sensitive detection methods is vital to prevent and manage foodborne diseases. In this study, we used tetraethylenepentamine (TEPA)-functionalized magnetic nanoparticles (MNPs) and a loop-mediated isothermal amplification (LAMP)-based CRISPR/Cas12a-based biosensor to concentrate and detect, respectively, L. monocytogenes. LAMP enables DNA amplification at a constant temperature, providing a highly suitable approach for point-of-care testing (POCT). The ability of CRISPR/Cas12a to cleave ssDNA reporter, coupled with TEPA-functionalized MNPs effective attachment to negatively charged bacteria, forms a promising biosensor.

RESULTS

The LAMP assay was meticulously developed by selecting specific primers and designing crRNA sequences targeting a specific region within the hly gene of L. monocytogenes. We selected primer and refined the amplification conditions by systematically exploring a temperature range from 59 °C to 69 °C, ensuring the attainment of optimal performance. This process was complemented by systematic optimization of LAMP-CRISPR/Cas12a system parameters. In particular, we successfully established the optimal ssDNA reporter concentrations (0-1.2 μM) and Cas12a-mediated trans-cleavage times (0-20 min), crucial components that underpin the effectiveness of the LAMP-CRISPR/Cas12a-based biosensor. For optimizing parameters in capturing L. monocytogenes using TEPA-functionalized MNPs, capture efficiency was significantly enhanced through adjustments in TEPA-functionalized MNPs concentration, incubation times, and magnetic separation duration. Large-volume (20 mL) magnetic separation exhibited a 10-fold sensitivity improvement over conventional methods. Utilizing TEPA-functionalized MNPs, the LAMP-CRISPR/Cas12a-based biosensor achieved detection limits of 100 CFU mL-1 in pure cultures and 100 CFU g-1 in enoki mushrooms.

SIGNIFICANCE

The integration of this novel technique with the LAMP-CRISPR/Cas12a-based biosensor enhances the accuracy and sensitivity of L. monocytogenes detection in foods, and it can be a promising biosensor for POCT. The 10-fold increase in sensitivity compared to conventional methods makes this approach a groundbreaking advancement in pathogenic bacteria detection for food safety and public health.

Principles, Methods, and Real-Time Applications of Bacteriophage-Based Pathogen Detection

Bacterial pathogens in water, food, and the environment are spreading diseases around the world. According to a World Health Organization (WHO) report, waterborne pathogens pose the most significant global health risks to living organisms, including humans and animals. Conventional bacterial detection approaches such as colony counting, microscopic analysis, biochemical analysis, and molecular analysis are expensive, time-consuming, less sensitive, and require a pre-enrichment step. However, the bacteriophage-based detection of pathogenic bacteria is a robust approach that utilizes bacteriophages, which are viruses that specifically target and infect bacteria, for rapid and accurate detection of targets. This review shed light on cutting-edge technologies about the novel structure of phages and the immobilization process on the surface of electrodes to detect targeted bacterial cells. Similarly, the purpose of this study was to provide a comprehensive assessment of bacteriophage-based biosensors utilized for pathogen detection, as well as their trends, outcomes, and problems. This review article summaries current phage-based pathogen detection strategies for the development of low-cost lab-on-chip (LOC) and point-of-care (POC) devices using electrochemical and optical methods such as surface-enhanced Raman spectroscopy (SERS).

Rapid detection of Salmonella in food matrices by photonic PCR based on the photothermal effect of Fe3O4

Salmonella causes most deaths from diarrheal disease worldwide. Therefore, Salmonella must be accurately and quickly detected, even in complex food matrices, which is difficult to achieve using conventional culture methods. Here we propose a novel photonic polymerase chain reaction (PCR) method based on ferroferric oxide (Fe3O4) for the detection of Salmonella typhimurium in complex samples. Owing to the great photothermal conversion performance of Fe3O4, rapid thermal cycling could be accomplished. Our optimized photonic PCR system specifically detected Salmonella typhimurium in complex food matrices within 50 min. Quantitative data showed a limit of detection up to 102 CFU/mL in food samples. This method is suitable for the detection of all pathogenic microorganisms and is universal.

Smartphone-assisted biosensor based on broom-like bacteria-specific magnetic enrichment platform for colorimetric detection of Listeria monocytogenes

Pathogenic bacteria contamination poses a major threat to human health. The detection of low-abundance bacteria in complex samples has always been a knotty problem, and high-sensitivity bacterial detection remains challenging. In this work, a novel magnetic platform with high enrichment efficiency for L. monocytogenes was developed. The magnetic platform was designed by branched polyglutamic acid-mediated indirect coupling of cefepime on magnetic nanoparticles (Cefe-PGA-MNPs), and the specific enrichment of low-abundance L. monocytogenes in real samples was achieved by an external magnet, with a capture efficiency over 90%. A controllable and highly active platinum-palladium nanozyme was synthesized and further introduced in the magnetic nanoplatform for the construction of enzymatic colorimetric biosensor. The total detection time for L. monocytogenes was within 100 min. The colorimetric signals generated by labelled nanozyme were corresponding to different concentrations of L. monocytogenes, with a limit of detection (LOD) of 3.1 × 101 CFU/mL, and high reliability and accuracy (with a recovery rate ranging from 96.5% to 116.4%) in the test of real samples. The concept of the developed method is applicable to various fields of biosensing that rely on magnetic separation platforms.

Comparison of four commercial immunomagnetic separation kits for the detection of Cryptosporidium

Cryptosporidium spp. are protozoan parasites of significant health importance found in environmental waters globally. Four commercially available Cryptosporidium-specific immunomagnetic separation (IMS) kits used in various water sample matrices were analysed and compared. Beads were characterised by flow cytometry and tested for the recovery efficiencies for oocysts spiked into different matrices: river water sediment, clay sample, and filter backwash sample. Results showed that Dynabeads™ Cryptosporidium and Waterborne Crypto-Grab™ kits contained immunoglobulin IgM antibody-coated beads. In contrast, the BioPoint CryptoBead and the TCS Isolate kits contained immunoglobulin IgG antibody-coated beads. BioPoint CryptoBead was significantly coated with more antibodies and were able to capture oocysts more rapidly compared to the other beads. Recovery efficiencies of Dynabeads™, TCS Isolate® beads, and BioPoint CryptoBead ranged from 55 to 93% when tested against different sample matrices, with BioPoint CryptoBead resulting in the highest at 93% in reagent-grade water and Dynabeads™ at 55%, the lowest against clay samples. The Waterborne beads did not perform well on any samples, with recovery efficiencies ranging from 0 to 8%. Fluorescence microscopy analyses showed that both the IMS method and the sample matrix processed affect the quality of the membranes, with the cleanest samples for microscopy examination observed from BioPoint CryptoBead.

Rising to the surface: capturing and detecting bacteria by rationally-designed surfaces

Analytical microbiology has made substantial progress since its conception, starting from potato slices, through selective agar media, to engineered surfaces modified with capture probes. While the latter represents the dominant approach in designing sensors for bacteria detection, the importance of sensor surface properties is frequently ignored. Herein, we highlight their significant role in the complex process of bacterial transition from planktonic to sessile, representing the first and critical step in bacteria detection. We present the main surface features and discuss their effect on the bio-solid interface and the resulting sensing capabilities for both flat and particulate systems. The concepts of rationally-designed surfaces for enhanced bacterial detection are presented with recent examples of sensors (capture probe-free) relying solely on surface cues.

M13 Bacteriophage-Assisted Recognition and Signal Spatiotemporal Separation Enabling Ultrasensitive Light Scattering Immunoassay

The demand for the ultrasensitive and rapid quantitative analysis of trace target analytes has become increasingly urgent. However, the sensitivity of traditional immunoassay-based detection methods is limited due to the contradiction between molecular recognition and signal amplification caused by the size effect of nanoprobes. To address this dilemma, we describe versatile M13 phage-assisted immunorecognition and signal transduction spatiotemporal separation that enable ultrasensitive light-scattering immunoassay systems for the quantitative detection of low-abundance target analytes. The newly developed immunoassay strategy combines the M13 phage-assisted light scattering signal fluctuations of gold nanoparticles (AuNPs) with gold in situ growth (GISG) technology. Given the synergy of M13 phage-mediated leverage effect and GISG-amplified light scattering signal modulation, the practical detection capability of this strategy can achieve the ultrasensitive and rapid quantification of ochratoxin A and alpha-fetoprotein in real samples at the subfemtomolar level within 50 min, displaying about 4 orders of magnitude enhancement in sensitivity compared with traditional phage-based ELISA. To further improve the sensitivity of our immunoassay, the biotin-streptavidin amplification scheme is implemented to detect severe acute respiratory syndrome coronavirus 2 spike protein down to the attomolar range. Overall, this study offers a direction for ultrasensitive quantitative detection of target analytes by the synergistic combination of M13 phage-mediated leverage effect and GISG-amplified light scattering signal modulation.

Tannic acid-derived selective capture of bacteria from apple juice

Given the enormous burden pathogens pose on human health, rapid capture and removal of bacteria for sterilization or further bacterial detection is essential. Herein, tannic acid-functionalized virus-like Fe₃O₄ (vFe₃O₄-TA) was established for bacterial enrichment. We investigated the ability of vFe₃O₄-TA to capture Gram-negative bacteria (E. coli, S. flex and S. typhi) and Gram-positive bacteria (S. aureus, MRSA and LM), respectively. Compared to the capture efficiency of <15 % for Gram-negative bacteria, vFe₃O₄-TA showed excellent selectivity and efficiency in isolating Gram-positive bacteria with >87 % removal efficiency. GFN-xTB semiempirical quantum chemical calculations revealed that the selective recognition originates from the high affinity between TA and peptidoglycan. Without impacting ingredients, the TA-mediated trapper also shows excellent ability to distinguish Gram-positive bacteria in juice samples. These results are expected to reveal the interaction of TA with bacteria, and inaugurate a potential natural safe tool for food safety control, medical treatment and environmental remediation.

Hybridization chain reaction cascaded amplification platform for sensitive detection of pathogen

Rapid, sensitive, and accurate identification of pathogens is vital for preventing and controlling fish disease, reducing economic losses in aquaculture, and interrupting the spread of food-borne diseases in human populations. Herein, we proposed a hybridization chain reaction (HCR) cascaded dual-signal amplification platform for the ultrasensitive and specific detection of pathogenic microorganisms. A couple of specific primers for target bacterial 16S rRNAs were used to obtain amplified target single-stranded DNAs (AT-ssDNA). Then, AT-ssDNA initiated HCR amplification along with the opening of fluorophore (FAM) and a quencher (BHQ1) labeled hairpin reporter probe (H1), and the FAM fluorescence signal recovered. The proposed strategy could achieve a detection limit down to 0.31 CFU/mL for Staphylococcus aureus (S. aureus), 0.49 CFU/mL for Escherichia coli (E. coli) in buffer, and a linear range from 1 to 1 × 10⁶ CFU/mL for S. aureus, 1 to 1 × 10⁷ CFU/mL for E. coli. Furthermore, this platform enabled sensitive and precise detection of pathogenic microorganisms in complex samples such as fish blood and different organ tissues (large intestine, gallbladder, heart, liver, ren, gill, skin), which shows great potential in disease prevention and control in aquatic products.

Electrochemical aptasensor based on exonuclease III-mediated signal amplification for sensitive detection of vomitoxin in cornmeal

Vomitoxin (DON) residues in grains are of great concern to public health. Herein, a label-free aptasensor was constructed to detect DON distributed in grains. Cerium-based metal-organic framework composite gold nanoparticles (CeMOF@Au) were used as substrate materials to facilitate electron transfer and provided more binding sites for DNA. The separation of DON-aptamer (Apt) complex and cDNA was achieved by magnetic separation technique based on magnetic beads (MBs), ensuring the specificity of the aptasensor. Exonuclease III (Exo III)-assisted cDNA cycling process strategy would be triggered when cDNA was separated and introduced to the sensing interface for further signal amplification. Under optimal conditions, the constructed aptasensor presented a wide detection range from 1 × 10^-8 mg·mL^-1 to 5 × 10^-4 mg·mL^-1 for DON, and the detection limit was 1.79 × 10^-9 mg·mL^-1, including a satisfactory recovery in cornmeal sample spiked with DON. The results showed that the proposed aptasensor had high reliability and promising application potential in detecting DON.

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.

Oxygen Sensor-Based Respirometry and the Landscape of Microbial Testing Methods as Applicable to Food and Beverage Matrices

The current status of microbiological testing methods for the determination of viable bacteria in complex sample matrices, such as food samples, is the focus of this review. Established methods for the enumeration of microorganisms, particularly, the 'gold standard' agar plating method for the determination of total aerobic viable counts (TVC), bioluminescent detection of total ATP, selective molecular methods (immunoassays, DNA/RNA amplification, sequencing) and instrumental methods (flow cytometry, Raman spectroscopy, mass spectrometry, calorimetry), are analyzed and compared with emerging oxygen sensor-based respirometry techniques. The basic principles of optical O₂ sensing and respirometry and the primary materials, detection modes and assay formats employed are described. The existing platforms for bacterial cell respirometry are then described, and examples of particular assays are provided, including the use of rapid TVC tests of food samples and swabs, the toxicological screening and profiling of cells and antimicrobial sterility testing. Overall, O₂ sensor-based respirometry and TVC assays have high application potential in the food industry and related areas. They detect viable bacteria via their growth and respiration; the assay is fast (time to result is 2-8 h and dependent on TVC load), operates with complex samples (crude homogenates of food samples) in a simple mix-and-measure format, has low set-up and instrumentation costs and is inexpensive and portable.

Separation and colorimetric detection of Escherichia coli by phage tail fiber protein combined with nano-magnetic beads

A colorimetric detection method for Escherichia coli (E. coli) in water was established based on a T7 phage tail fiber protein-magnetic separation. Firstly, the tail fiber protein (TFP) was expressed and purified to specifically recognize E. coli, which was verified by using fusion protein GFP-tagged TFP (GFP-TFP) and fluorescence microscopy. Then TFP conjugated with magnetic beads were applied to capture and separate E. coli. The TFP was covalently immobilized on the surface of magnetic beads and captured E. coli as verified by scanning electron microscopy (SEM). Finally, polymyxin B was used to lyse E. coli in solution and the released intracellular β-galactosidase (β-gal) could hydrolyze the colorimetric substrate chlorophenol red-β-D-galactopyranoside (CPRG), causing color change from yellow to purple. The high capture efficiencies of E. coli ranged from 88.70% to 95.65% and E. coli could be detected at a concentration of 10² CFU/mL by naked eyes. The specificity of the chromogenic substrate was evaluated using five different pathogen strains as competitors and tests with four kinds of real water samples showed recoveries of 86.00% to 92.25%. The colorimetric changes determined by visual inspection can be developed as an efficient platform for point-of-care detection of E. coli in resource-limited regions.

An achromatic colorimetric nanosensor for sensitive multiple pathogen detection by coupling plasmonic nanoparticles with magnetic separation

Rapid screening of multiple pathogens will greatly improve the efficiency of pandemic prevention and control. Colorimetric methods exhibit the advantages of convenience, portability, low cost, time efficiency, and free of sophisticated instruments, yet usually have difficulties in simultaneous detection and suffer from monotonous color changes with low visual resolution and sensitivity. Hence, coupled three kinds of plasmonic nanoparticles (NPs) with magnetic separation, we developed an achromatic colorimetric nanosensor with highly enhanced visual resolution for simultaneous detection of SARS-CoV-2, Staphylococcus aureus, and Salmonella typhimurium. The achromatic nanosensor was composed of SARS-CoV-2-targeting red gold NPs, S. aureus-targeting yellow silver NPs and S. typhimurium-targeting blue silver triangle NPs mixed as black color. In the detection, three corresponding magnetic probes were added into the above mixture. In the presence of a target pathogen, it would be recognized and combined with corresponding colored reporters and magnetic probes to form sandwich complexes, which were removed by magnetic separation, and the sensor changed from black to a chromatic color (the color of the reporters remained in supernatant). Consequently, different target pathogen induced different color. For example, SARS-CoV-2, S. aureus, and S. typhimurium respectively produced green, purple, and orange. While coexistence of S. aureus and S. typhimurium produced red, and coexistence of S. aureus and SARS-CoV-2 produced blue, etc. Therefore, by observing the color change or measuring the absorption spectra, multiple pathogen detection was achieved conveniently. Compared with most colorimetric sensors, this achromatic nanosensor involved rich color change, thus significantly enhancing visual resolution and inspection sensitivity. Therefore, this sensor opened a promising avenue for efficient monitoring and early warning of food safety and quality.

Immunomagnetic-bead enriched culturomics (IMBEC) for isolating pathobionts from feces of colorectal cancer patients

Culturomics employs various cultivating conditions to obtain different types of bacteria and new species. However, current culturomics lacks a highly efficient method for isolating specific pathobionts. Immunomagnetic bead technology, which uses magnetic beads conjugated with antibodies for capturing the antigen to realize enrichment of the targets, has been employed as an alternative method. In this study, we developed a novel method, immunomagnetic bead-enriched culturomics (IMBEC), in which magnetic bead-conjugated antibodies purified from the fecal samples of patients with colorectal cancer (CRC) were used to enrich and isolate potential pathobionts. A protocol for enriching potential pathobionts via immunomagnetic capture was developed by optimizing the concentrations of coupling reagents, NaCl, and detergent. The efficacy of pathobiont enrichment was compared between antibody-coated magnetic beads (antibody group) and nonconjugated blank magnetic beads (blank group). To determine the proinflammatory potential of isolates from both groups, we investigated their ability to induce cytokine production in THP-1 macrophages. This protocol was employed for isolating bacteria from 10 fecal samples of patients with CRC, which were simultaneously compared with those isolated from the blank group. A total of 209 bacterial species were isolated from both groups, including 173 from the antibody group, 160 from the blank group, and 124 from both groups. Bacteria isolated from the antibody group produced more proinflammatory cytokines than those isolated from the blank group. IMBEC is a promising method for relatively specific isolation of potential pathobionts for a particular disease of interest.

Preparation of polyclonal antibody against a universal bacterial antigen OmpA deduced by bioinformatic analysis and preliminary evaluation of concentration effects on foodborne pathogens

Rapid and ultrasensitive microbial detection in actual samples have challenges because of target pathogen diversity and low abundance. In this study, we attempted to capture and concentrate multiple pathogens by combining magnetic beads with polyclonal antibodies against a universal antigen of ompA, LAMOA-1, before further detection. A protein sequence consisting of 241 amino acids with spatial conformation similar to E. coli ompA was identified and expressed as a recombinant protein in prokaryotes according to the results of sequence alignment among 432 sequences of ompA belonging to intestinal bacteria from gram-negative bacteria. Purified from immunized rabbits, the anti-LAMOA-1 antibody was shown to effectively recognize 12 foodborne bacterial species. Antibody-conjugated beads were used to concentrate the bacteria when the bacterial concentration in artificially contaminated samples is between 10 and 100 CFU/mL, which shortens detection duration by 8-24 h. The enrichment strategy is potentially beneficial for detection of foodborne pathogens.

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.

Colorimetric and photothermal dual-mode lateral flow immunoassay based on Au-Fe3O4 multifunctional nanoparticles for detection of Salmonella typhimurium

Au-Fe₃O₄ multifunctional nanoparticles (NPs) were synthesized and integrated with lateral flow immunoassay (LFIA) for dual-mode detection of Salmonella typhimurium. The Au-Fe₃O₄ NPs not only combined excellent local surface plasmon resonance characteristics and superparamagnetic properties, but also exhibited good photothermal effect. In the detection, antibody-conjugated Au-Fe₃O₄ NPs first captured S. typhimurium from complex matrix, which was then loaded on the LFIA strip and trapped by the T-line. By observing the color bands with the naked eyes, qualitative detection was performed free of instrument. By measuring the photothermal signal, quantification was achieved with a portable infrared thermal camera. The introduction of magnetic separation achieved the enrichment and purification of target bacteria, thus enhancing the detection sensitivity and reducing interference. This dual-mode LFIA achieved a visual detection limit of 5 × 10⁵ CFU/mL and a photothermal detection limit of 5 × 10⁴ CFU/mL. Compared with traditional Au-based LFIA, this dual-mode LFIA increased the detection sensitivity by 2 orders of magnitude and could be directly applied to unprocessed milk sample. Besides, this dual-mode LFIA showed good reproducibility and specificity. The intra-assay and inter-assay variation coefficients were 3.0% and 7.9%, and with this dual-mode LFIA, other bacteria hardly produced distinguishable signals. Thus, the Au-Fe₃O₄ NPs-based LFIA has potential to increase the efficiency of pandemic prevention and control. Au-Fe₃O₄ nanoparticle proved to be a promising alternative reporter for LFIA, achieving multifunctions: target purification, target enrichment, visual qualitation, and instrumental quantification, which improved the limitations of traditional LFIA.

Magnetic Nanoseparation Technology for Efficient Control of Microorganisms and Toxins in Foods: A Review

Outbreaks of foodborne diseases mediated by food microorganisms and toxins remain one of the leading causes of disease and death worldwide. It not only poses a serious threat to human health and safety but also imposes a huge burden on health care and socioeconomics. Traditional methods for the removal and detection of pathogenic bacteria and toxins in various samples such as food and drinking water have certain limitations, requiring a rapid and sensitive strategy for the enrichment and separation of target analytes. Magnetic nanoparticles (MNPs) exhibit excellent performance in this field due to their fascinating properties. The strategy of combining biorecognition elements with MNPs can be used for fast and efficient enrichment and isolation of pathogens. In this review, we describe new trends and practical applications of magnetic nanoseparation technology in the detection of foodborne microorganisms and toxins. We mainly summarize the biochemical modification and functionalization methods of commonly used magnetic nanomaterial carriers and discuss the application of magnetic separation combined with other instrumental analysis techniques. Combined with various detection techniques, it will increase the efficiency of detection and identification of microorganisms and toxins in rapid assays.

An automatic centrifugal system for rapid detection of bacteria based on immunomagnetic separation and recombinase aided amplification

This study reported an automatic centrifugal system for rapid quantification of foodborne pathogenic bacteria based on immunomagnetic separation (IMS) for target bacteria enrichment and recombinase aided amplification (RAA) for nucleic acid detection. First, target bacteria were captured by immune magnetic nanoparticles (MNPs) to form magnetic bacteria, which were purified and enriched by magnetic separation. Then, nucleic acid extraction buffer was used to extract genomic DNA of magnetic bacteria and dissolve lyophilized RAA reagent. Finally, isothermal amplification and fluorescent detection were conducted for bacteria quantification. Bacteria magnetic separation, nucleic acid extraction and fluorescent RAA detection were elaborately achieved in a centrifugal disc with unique functional chambers and multistage siphon channels. A supporting device was developed to automatically and successively perform the programmed centrifugal protocol, including temperature control for isothermal amplification and fluorescence detection for real-time RAA analysis. Under optimal conditions, this centrifugal system enabled Salmonella detection as low as 10 CFU mL^-1 in spiked chicken samples in 1 h with average recovery of 105.6% and average standard deviation of 8.4%. It has been demonstrated as an alternative for rapid detection of Salmonella.

Review in isothermal amplification technology in food microbiological detection

Food-borne diseases caused by microbial contamination have always been a matter of great concern to human beings. Hence, the research on these problems has never stopped. With the development of microorganism amplification technology, more and more detection methods have come into our vision. However, traditional detection technologies presents more or less drawbacks, such as complicated operation, low accuracy, low sensitivity, long-time detection, and so on. Therefore, more convenient, accurate, and sensitive measurement for the microorganism are needed. Isothermal amplification technology is one of the alternative approach containing the above mentioned advantages. This work mainly summarizes the principles of loop-mediated isothermal amplification (LAMP) and rolling circle amplification (RCA) which belong to isothermal amplification. Meanwhile, the application of LAMP and RCA in food microorganism detection is introduced.

Micro/nanomotor technology: the new era for food safety control

Food poisoning caused by eating contaminated food remains a threat to global public health. Making the situation even worse is the aggravated global environmental pollution, which poses a major threat to the safety of agricultural resources. Food adulteration has been rampant owing to negligent national food safety regulations. The speed at which contaminated food is detected and disposed of determines the extent to which consumers' lives are safeguarded and agricultural economic losses are prevented. Micro/nanomotors offer a high-speed mobile loading platform that substantially increases the chemical reaction rates and, accordingly, exhibit great potential as alternatives to conventional detection and degradation techniques. This review summarizes the propulsion modes applicable to micro/nanomotors in food systems and the advantages of using micro/nanomotors, highlighting examples of their potential use in recent years for the detection and removal of food contaminants. Micro/nanomotors are an emerging technology for food applications that is moving toward mass production, simple preparation, and important functions.

Combination of filtration and immunomagnetic separation based on real-time PCR to detect foodborne pathogens in fresh-cut apple

Rapid detection methods require pre-enrichment culture in order to detect low levels of foodborne pathogens. To rapidly detect foodborne pathogens, enrichment culture processes could be replaced. Filtration and immunomagnetic separation methods have been identified to effectively concentrate and separate target pathogens from foods. In this study, a combination of filtration and immunomagnetic separation (IMS) has enabled the rapid and sensitive detection of foodborne pathogens. The pretreatment method, including separation and concentration procedures, increased sensitivity 10-100-fold. The sensitivity of a combination method using filtration and IMS to detect Escherichia coli O157:H7 and Salmonella enterica subsp. enterica serovar Typhimurium was 10⁰-10¹ CFU/10 mL. In fresh-cut apples, IMS combined with filtration effectively improved the detection limit of real-time PCR to 2.70 × 10¹ CFU/g in E. coli O157:H7 and 1.80 × 10² CFU/g in Salmonella. The filtration simplified processing of large-volumes (250 mL) and effectively concentrated pathogens while decreasing immunomagnetic beads used in IMS. Bacterial concentration by IMS combined with filtration increased sensitivity 10-100-fold compared with control. In addition, the application of IMS effectively removed concentrated residual food material (10-15 mg/mL) after filtration, improving relative sensitivity. In conclusion, this method may detect foodborne pathogen in foods such as fresh-cut fruits in a more rapid and sensitive fashion than traditional culture-based methods.

Culture-Free Quantification of Bacteria Using Digital Fluorescence Imaging in a Tunable Magnetic Capturing Cartridge for Onsite Food Testing

Accurate, onsite detection of pathogenic bacteria from food matrices is required to rapidly respond to pathogen outbreaks. However, accurately detecting whole-cell bacteria in large sample volumes without an enrichment step remains a challenge. Therefore, bacterial samples must be concentrated, identified, and quantified. We developed a tunable magnetic capturing cartridge (TMCC) and combined it with a portable digital fluorescence reader for quick, onsite, quantitative detection of Staphylococcus aureus. The TMCC platform integrates an absorption pad impregnated with water-soluble polyvinyl alcohol (PVA) with an injection-molded polycarbonate (PC) plate that has a hard magnet on its back and an acrylonitrile-butadiene-styrene case. An S. aureus-specific antibody conjugated with magnetic nanoparticles was used to concentrate bacteria from a large-volume sample and capture bacteria within the TMCC. The retention time for capturing bacteria on the TMCC was adjusted by controlling the concentration and volume of the PVA solution. Concentrated bacterial samples bound to target-specific aptamer probes conjugated with quantum dots were loaded into the TMCC for a controlled time, followed by attachment of the bacteria to the PC plate and removal of unbound aptamer probes with wash buffer. The captured bacteria were quantified using a digital fluorescence reader equipped with an embedded program that automatically counts fluorescently tagged bacteria. The bacterial count made using the TMCC was comparable to a standard plate count (R² = 0.9898), with assay sensitivity and specificity of 94.3 and 100%, respectively.

Supercharged MPNs? Automated Determination of High-Throughput Most Probable Number (htMPN) Using Chip-Based 3D Digital PCR

Surface plating on agar and most probable number (MPN) are the standard methods for determining bacterial viability but both have limitations. Here we present a novel cell count method, high-throughput MPN (htMPN), that uses a chip-based digital PCR instrument to accelerate and to improve the quantification of viable or sublethally injured cells. This method tracks growth of up to 20,000 individual bacterial cells on a single chip. Single cells were grown in the individual wells of the chip at their optimal temperature until the cell density was high enough to detect the fluorescent signal with cell-permeant or cell-impermeant DNA-intercalating fluorescent dyes. This method based on microfluidic devices implemented in digital PCR equipment was equivalent to surface plating in determining cell counts of Escherichia coli, Salmonella enterica serovar Typhimurium, Fructilactobacillus sanfranciscensis, Pseudomonas putida, and vegetative cells but not spores of Bacillus subtilis. Viable E. coli could be enumerated within 7 h. Culture of strict aerobes was restricted to strains that are capable of nitrate respiration; organisms requiring complex media that also contain double-stranded DNA were detected after treatment of growth media with DNase before inoculation. Our approach not only monitors the frequency distribution of bacterial growth and determines cell counts with high reliability but also detected heat-injured cells of S. Typhimurium that escaped detection by the surface plating. Overall, the method accelerates detection of viable bacterial cells, facilitates automation, and offers new possibilities for the analysis of individual bacterial cells. IMPORTANCE htMPN uses chip-based fluorescence acquisition and is a simple and compact tool for automatic viable cell enumeration with applications in microbiological research. This method applies to a wide range of anaerobic or facultative anaerobic species and improves accuracy by reducing the number of pipetting steps. In addition, the method offers an additional tool for single-cell microbiology. The single cell time-to-detection times have been used as an important criterion for the physiological state of bacterial cells after sublethal stress, and htMPNs support the acquisition of such data with an unprecedented number of cells. In particular, htMPN provides an anaerobic environment and enables a long incubation time to increase the recovery rate of sublethally injured cells. Given its reproducibility and reliability, our approach can potentially be applied to quantify viable cells in samples from environmental, clinical, or food samples to reduce the risk of underestimation of the number of viable bacterial cells.

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.

Giant Magnetoresistance Biosensors for Food Safety Applications

Nowadays, the increasing number of foodborne disease outbreaks around the globe has aroused the wide attention of the food industry and regulators. During food production, processing, storage, and transportation, microorganisms may grow and secrete toxins as well as other harmful substances. These kinds of food contamination from microbiological and chemical sources can seriously endanger human health. The traditional detection methods such as cell culture and colony counting cannot meet the requirements of rapid detection due to some intrinsic shortcomings, such as being time-consuming, laborious, and requiring expensive instrumentation or a central laboratory. In the past decade, efforts have been made to develop rapid, sensitive, and easy-to-use detection platforms for on-site food safety regulation. Herein, we review one type of promising biosensing platform that may revolutionize the current food surveillance approaches, the giant magnetoresistance (GMR) biosensors. Benefiting from the advances of nanotechnology, hundreds to thousands of GMR biosensors can be integrated into a fingernail-sized area, allowing the higher throughput screening of food samples at a lower cost. In addition, combined with on-chip microfluidic channels and filtration function, this type of GMR biosensing system can be fully automatic, and less operator training is required. Furthermore, the compact-sized GMR biosensor platforms could be further extended to related food contamination and the field screening of other pathogen targets.

A novel lateral flow immunoassay strip based on a label-free magnetic Fe3O4@UiO-66-NH2 nanocomposite for rapid detection of Listeria monocytogenes

Listeria monocytogenes (L. monocytogenes) is one of the most lethal pathogenic bacteria. Although the traditional microbial culture method has high sensitivity and selectivity for the diagnosis of L. monocytogenes, it is time-consuming and not suitable for on-site detection. A rapid, convenient and visualized on-site detection method is particularly needed. In this work, Fe₃O₄@UiO-66-NH₂ was prepared for both magnetic separation and lateral flow immunoassay (LFIA) for the detection of L. monocytogenes by taking advantage of the easy separation of the magnetic core Fe₃O₄ and the high surface area of the outer layer UiO-66-NH₂. Fe₃O₄@UiO-66-NH₂ with a high surface area and good water-dispersibility and optical properties was synthesized by a simple hydrothermal process. It could directly adsorb on the surface of target bacteria and form Fe₃O₄@UiO-66-NH₂-bacteria conjugates, without the labeling of an antibody. After magnetic separation and concentration, the Fe₃O₄@UiO-66-NH₂-bacteria conjugates were detected by the antibody on the test line of the LFIA strip, resulting in a visible orange band. The capture efficiency and LFIA detection of Fe₃O₄@UiO-66-NH₂ were optimized in this study. Under the optimal conditions, a good linear correlation between the test line intensity and the concentration of L. monocytogenes was obtained in the range of 10⁵-10⁸ CFU mL^-1, and the limit of detection was 2.2 × 10⁶ CFU mL^-1 by the naked eye. The Fe₃O₄@UiO-66-NH₂-based LFIA strip showed strong specificity for L. monocytogenes, and the detection took 45 min without culture enrichment. Therefore, the proposed Fe₃O₄@UiO-66-NH₂-based strip showed the advantages of simple synthesis, being label-free, low cost, good selectivity and convenience.

Development of a fluorescent test strip sensor based on surface positively-charged magnetic bead separation for the detection of Listeria monocytogenes

Listeria monocytogenes is one of the major foodborne pathogens, which may cause serious food safety problems and illnesses in humans and animals. Consequently, sensitive, fast and reliable detection methods, as well as effective sample preparation methods are in great demand. In this study, a magnetic separation method based on a aptamer functionalized positively-charged magnetic beads (Fe₃O₄@aptamer) was established and a fluorescent test strip sensor was constructed for the rapid, sensitive and specific detection of Listeria monocytogenes. Benefiting from the dual recognition and signal amplification process of Fe₃O₄@aptamer enrichment and the polymerase chain reaction of the hly gene, the fluorescent strip sensor for the detection of Listeria monocytogenes was determined to be reliable and sensitive, with a linear curve obtained in the range of 1.0 × 10² to 1.9 × 10⁸ CFU mL^-1, and a detection limit of 1.0 × 10² CFU mL^-1. The detection was achieved in 3 h without culture enrichment. Furthermore, the developed method was successfully applied for the detection of Listeria monocytogenes in pork tenderloin, with the recoveries ranging from 91.1% to 97.1%, and a coefficient of variation of less than 23.4%, revealing the feasible and reliable application of this method in practical samples. The proposed fluorescent strip sensor is rapid, sensitive and specific, giving it great application prospects for use in the field of pathogenic bacterium detection.

Power-free microfluidic biosensing of Salmonella with slide multivalve and disposable syringe

In this study, a power-free biosensor was presented to detect Salmonella typhimurium on a microfluidic chip using a slide multivalve for channel selection and a disposable syringe for fluidic transfer. First, bacterial sample with immunomagnetic nanoparticles (IMNPs) and glucose oxidase (GOx) modified immune polystyrene nanoparticles (IPNPs), washing buffer, glucose, and peroxide test strip (PTS) were preloaded in their respective chambers at the periphery of chip. After the slide multivalve was selected to connect sample chamber with common separation chamber, which was connected with a syringe, the mixture of Salmonella, IMNPs and IPNPs was back and forth moved through 3D Tesla-structure micromixer using the syringe, resulting in the formation of IMNP-Salmonella-IPNP complexes, which were captured in the separation chamber using a magnet. Then, two washing chambers were selectively connected respectively to remove sample background and excessive IPNPs, and glucose chamber was connected, allowing the GOx to catalyze glucose to produce hydrogen peroxide in the separation chamber. Finally, PTS chamber was connected and the catalysate was transferred from the separation chamber to the PTS chamber, leading to the color change of PTS, followed by using smartphone App to collect and analyze the image of PTS for bacterial determination. The simple biosensor enabled simple detection of Salmonella as few as 130 CFU/mL within 60 min and is promising for practical applications in the resource-limited regions due to its low cost, simple operation, and small size.

Advances in bacterial concentration methods and their integration in portable detection platforms: A review

Early detection and identification of microbial contaminants is crucial in many sectors, including clinical diagnostics, food quality control and environmental monitoring. Biosensors have recently gained attention among other bacterial detection technologies due to their simplicity, rapid response, selectivity, and integration/miniaturization potential in portable microfluidic platforms. However, biosensors are limited to the analysis of small sample volumes, and pre-concentration steps are necessary to reach the low sensitivity levels of few bacteria per mL required in the analysis of real clinical, industrial or environmental samples. Many platforms already exist where bacterial detection and separation/accumulation systems are integrated in a single platform, but they have not been compiled and critically analysed. This review reports on most recent advances in bacterial concentration/detection platforms with emphasis on the concentration strategy. Systems based on five concentration strategies, i.e. centrifugation, filtration, magnetic separation, electric separation or acoustophoresis, are here presented and compared in terms of processed sample volume, concentration efficiency, concentration time, ability to work with different types of samples, and integration potential, among others. The critical evaluation presented in the review is envision to facilitate the development of future platforms for fast, sensitive and in situ bacterial detection in real sample.