Methods for rapid separation and concentration of bacteria in food that bypass time-consuming cultural enrichment

Carbapenem-Resistant E. coli Adherence to Magnetic Nanoparticles

Carbapenem-resistant Enterobacterales (CRE) is an emerging global concern. Specifically, carbapenemase-producing (CP) E. coli strains in CRE have recently been found in clinical, environmental, and food samples worldwide, causing many hospitalizations and deaths. Their rapid identification and characterization are paramount in control, management options, and treatment choices. Thus, this study aimed to characterize the cell surface properties of carbapenem-resistant (R) E. coli isolates and their interaction with glycan-coated magnetic nanoparticles (gMNPs) compared with carbapenem-susceptible (S) E coli. This study used two groups of bacteria: The first group included E. coli (R) isolates harboring carbapenemases and had no antibiotic exposure. Their initial gMNP-cell binding capacity, with cell surface characteristics, was assessed. In the second group, one of the E. coli (R) isolates and E. coli (S) had long-term serial antibiotic exposure, which we used to observe their cell surface characteristics and gMNP interactions. Initially, cell surface characteristics (cell morphology and cell surface charge) of the E. coli isolates were evaluated using confocal laser scanning microscope (LSCM) and a Zetasizer, respectively. The interaction of gMNPs with the E. coli isolates was assessed through LSCM and transmission electron microscope (TEM). Further, the gMNP-cell attachment was quantified as a concentration factor (CF) through the standard plating method. The results showed that the CF values of all E. coli (R) were significantly different from those of E. coli (S), which could be due to the differences in cell characteristics. The E. coli (R) isolates displayed heterogeneous cell shapes (rod and round cells) and lower negative zeta potential (cell surface charge) values compared to E. coli (S). Further, this research identified the differences in the cell surface characteristics of E. coli (S) under carbapenem exposure, compared to unexposed E. coli (S) that impact their attachment capacity. The gMNPs captured more E. coli (S) cells compared to carbapenem-exposed E. coli (S) and all E. coli (R) isolates. This study clearly found that differences in cell surface characteristics impact their interaction with magnetic nanoparticles. The gained insights aid in further understanding adhesion mechanisms to develop or improve bacterial isolation techniques and diagnostic and treatment methods for CRE.

Research progress on detection of foodborne pathogens: The more rapid and accurate answer to food safety

In recent years, foodborne diseases have posed a serious threat to human health, and rapid detection of foodborne pathogens is particularly crucial for the prevention and control of such diseases. This article offers a detailed overview of the development of detection techniques for foodborne pathogens, transitioning from traditional microbiological culture methods to the current array of techniques, including immunological, molecular biological, and biosensor-based methods. It summarizes the technical principles, advantages, disadvantages, and research progress of these diverse methods. Furthermore, the article demonstrates that the combination of different methods enhances the efficiency and accuracy of pathogens detection. Specifically, the article focuses on the application and advantages of combining CRISPR/Cas systems with other detection methods in the detection of foodborne pathogens. CRISPR/Cas systems, with their high specificity, sensitivity, and ease of operation, show great potential in the field of foodborne pathogens detection. When integrated with other detection techniques such as immunological detection techniques, molecular biology detection techniques, and biosensors, the accuracy and efficiency of detection can be further improved. By fully utilizing these tools, early detection and control of foodborne diseases can be achieved, enhancing public health and preventing disease outbreaks. This article serves as a valuable reference for exploring more convenient, accurate, and sensitive field detection methods for foodborne pathogens, promoting the application of rapid detection techniques, and ensuring food safety and human health.

Rapid Isolation of Low-Level Carbapenem-Resistant E. coli from Water and Foods Using Glycan-Coated Magnetic Nanoparticles

Carbapenem-resistant Enterobacterales (CRE) are one of the major global issues needing attention. Among them, carbapenemase-producing (CP) E. coli strains are commonly found in clinical and biological samples. Rapid and cost-effective detection of such strains is critical in minimizing their deleterious impact. While promising progress is being made in rapid detection platforms, separation and enrichment of bacteria are required to ensure the detection of low bacterial counts. The current separation methods, such as centrifugation, filtration, electrophoresis, and immunomagnetic separation, are often tedious, expensive, or ineffective for clinical and biological samples. Further, the extraction and concentration of antimicrobial-resistant bacteria (ARB) are not well documented. Thus, this study assessed the applicability of cost-effective glycan-coated magnetic nanoparticles (gMNPs) for simple and rapid extraction of CP E. coli. The study included two resistant (R)strains: Klebsiella pneumoniae carbapenemase (KPC)-producing E. coli (R: KPC) and New Delhi metallo-β-lactamase (NDM)-producing E. coli (R: NDM). A susceptible E. coli (S) strain was used as a control, a reference bacterium. The gMNPs successfully extracted and concentrated E. coli (R) and E. coli (S) at low concentrations from large volumes of buffer solution, water, and food samples. The gMNPs concentrated up to two and five times their initial concentration for E. coli (R) and E. coli (S) in the buffer solution, respectively. In water and food samples, the concentration of E. coli (S) and E. coli (R) were similar and ranged 1-3 times their initial inoculation. A variation in the concentration from different food samples was seen, displaying the impact of food microstructure and natural microflora. The cost-effective and rapid bacterial cell capture by gMNPs was achieved in 15 min, and its successful binding to the bacterial cells in the buffer solution and food matrices was also confirmed using Transmission Electron Microscopy (TEM). These results show promising applications of gMNPs to extract pathogens and ARB from biological samples.

Aggregation-Based Bacterial Separation with Gram-Positive Selectivity by Using a Benzoxaborole-Modified Dendrimer

Antimicrobial-resistant (AMR) bacteria have become a critical global issue in recent years. The inefficacy of antimicrobial agents against AMR bacteria has led to increased difficulty in treating many infectious diseases. Analyses of the environmental distribution of bacteria are important for monitoring the AMR problem, and a rapid as well as viable pH- and temperature-independent bacterial separation method is required for collecting and concentrating bacteria from environmental samples. Thus, we aimed to develop a useful and selective bacterial separation method using a chemically synthesized nanoprobe. The metal-free benzoxaborole-based dendrimer probe BenzoB-PAMAM(+), which was synthesized from carboxy-benzoxaborole and a poly(amidoamine) (PAMAM) dendrimer, could help achieve Gram-positive bacterial separation by recognizing Gram-positive bacterial surfaces over a wide pH range, leading to the formation of large aggregations. The recognition site of benzoxaborole has a desirable high acidity and may therefore be responsible for the improved Gram-positive selectivity. The Gram-positive bacterial aggregation was then successfully collected by using a 10 μm membrane filter, with Gram-negative bacteria remaining in the filtrate solution. BenzoB-PAMAM(+) will thus be useful for application in biological analyses and could contribute to further investigations of bacterial distributions in environmental soil or water.

Influence of Biological and Environmental Factors in the Extraction and Concentration of Foodborne Pathogens using Glycan-Coated Magnetic Nanoparticles

Rapid detection of foodborne pathogens is essential to preventing foodborne illness outbreaks. Before detection can occur, however, it is often necessary to extract and concentrate bacteria. Conventional methods such as centrifugation, filtration, and immunomagnetic separation can often be time-consuming, ineffective, or costly when working with complex food matrices. This work used cost-effective glycan-coated magnetic nanoparticles (MNPs) for rapid concentration of Escherichia coli O157, Listeria monocytogenes, and Staphylococcus aureus. Glycan-coated MNPs were used to concentrate bacteria from both buffer solution and food matrices while examining the effect of factors including solution pH, bacterial concentration, and target bacterial species. In both pH 7 and reduced pH experiments, successful extraction of bacterial cells occurred in all food matrices and bacteria tested. In neutral pH buffer solution, bacteria were concentrated to 4.55 ± 1.17, 31.68 ± 6.10 and 64.27 ± 16.78 times their initial concentration (mean ± standard deviation) for E. coli, L. monocytogenes and S. aureus, respectively. Successful bacterial concentration occurred in several food matrices, including S. aureus in milk (pH 6), L. monocytogenes in sausage (pH 7), and E. coli O157 in flour (pH 7). The insights gained may facilitate future applications of glycan-coated MNPs to extract foodborne pathogens.

Volatile Metabolite Profiling of Wheat Kernels Contaminated by Fusarium graminearum

Traditional methods used to detect fungi or mycotoxins are time-consuming and prevent real-time monitoring. In this study, solid-phase microextraction combined with full two-dimensional gas chromatography time-of-flight mass spectrometry was utilized to detect volatile organic compounds (VOCs) produced by fungi during grain infestation predictive F. graminearum PH-1 infestation in wheat. The results show that the VOCs emitted by F. graminearum can distinguish strains at different growth stages. The growth matrices (potato dextrose agar medium and wheat kernels) play a large role in VOC production. The infection of wheat sample F. graminearum showed that a specific relationship between VOCs and the composition of fungal flora, for example, 5-pentyl-cyclohexa-1,3-diene, 3-hexanone, and 1,3-octadiene, was positively correlated with the infection rate of PH-1. In the correlation study of fungal mycotoxins and VOCs, zearalenone produced by F. graminearum was predicted based on the VOCs released. Further analysis determined the correlation of three VOCs, 6-butyl-1,4-cycloheptadiene, hexahydro-3-methylenebenzofuran-2(3H)-one, and (E,E)-3,5-octadien-2-one, with zearalenone production, confirming the ability of VOCs as characteristic markers of mycotoxins.

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.

Current Methods for Extraction and Concentration of Foodborne Bacteria with Glycan-Coated Magnetic Nanoparticles: A Review

Rapid and accurate food pathogen detection is an essential step to preventing foodborne illnesses. Before detection, removal of bacteria from the food matrix and concentration to detectable levels are often essential steps. Although many reviews discuss rapid concentration methods for foodborne pathogens, the use of glycan-coated magnetic nanoparticles (MNPs) is often omitted. This review seeks to analyze the potential of this technique as a rapid and cost-effective solution for concentration of bacteria directly from foods. The primary focus is the mechanism of glycan-coated MNP binding, as well as its current applications in concentration of foodborne pathogens. First, a background on the synthesis, properties, and applications of MNPs is provided. Second, synthesis of glycan-coated particles and their theorized mechanism for bacterial adhesion is described. Existing research into extraction of bacteria directly from food matrices is also analyzed. Finally, glycan-coated MNPs are compared to the magnetic separation technique of immunomagnetic separation (IMS) in terms of cost, time, and other factors. At its current state, glycan-coated MNPs require more research to fully identify the mechanism, potential for optimization, and extraction capabilities directly in food matrices. However, current research indicates glycan-coated MNPs are an incredibly cost-effective method for rapid food pathogen extraction and concentration.

Application of Engineered Bacteriophage T7 in the Detection of Bacteria in Food Matrices

Detection of pathogens in a food matrix is challenging due to various constraints including complexity and the cost of sample preparation for microbial analysis from food samples, time period for the detection of pathogens, and high cost and specialized resources required for advanced molecular assays. To address some of these key challenges, this study illustrates a simple and rapid colorimetric detection of target bacteria in distinct food matrices, including fresh produce, without prior isolation of bacteria from a food matrix. This approach combines bacteriophage-induced expression of an exogenous enzyme, alkaline phosphatase, the specific colorimetric substrate that generates insoluble color products, and a simple filtration method to localize the generation of colored signal. Using this approach, this study demonstrates the specific detection of inoculated Escherichia coli in coconut water and baby spinach leaves. Without isolating bacteria from the selected food matrices and using a food sample size that is representative of industrial samples, the inoculated samples were added to the enrichment broth for a short period (5 h) and incubated with an engineered bacteriophage T7 with a phoA gene. The incubation period with the engineered bacteriophage was 30 min for liquid samples and 2 h for fresh produce samples. The samples were then filtered through a 0.2-micron polycarbonate membrane and incubated with a colorimetric substrate, i.e., nitro blue tetrazolium/5-bromo-4-chloro-3-indolyl phosphate (NBT/BCIP). This substrate forms a dark purple precipitate upon interactions with the released enzyme on a filter membrane. This approach successfully detected 10 CFU/ml of E. coli in coconut water and 10² CFU/g of E. coli on baby spinach leaves with 5 h of enrichment. Success of this approach illustrates potential for detecting target bacteria in food systems using a simple visual assay and/or quantitative colorimetric measurements.

Recognition of pathogens in food matrixes based on the untargeted in vivo microbial metabolite profiling via a novel SPME/GC × GC-QTOFMS approach

Foodborne diseases incurred by pathogenic bacteria are one of the major threats in food safety, and thus it is important to develop facile and effective recognition methodology of pathogens in food. Herein, a new automatic approach for detection of in vivo volatile metabolites emitted from foodborne pathogens was proposed by coupling solid phase microextraction (SPME) technique with a comprehensive two-dimensional gas chromatography quadrupole time-of-flight mass spectrometry (GC × GC-QTOFMS). A novel polymer composite based SPME probe which possessed high-coverage of microbial metabolites was utilized in this contribution to realize the sensitive extraction of untargeted metabolites. As a result, a total of 126 in vivo metabolites generated by the investigated pathogens were detected and identified, with 33, 29, 25, 21 and 18 volatile metabolites belonging to Shigella sonnei, Escherichia coli, Salmonella typhimurium, Vibrio parahaemolyticus and Staphylococcus aureus, respectively. Multivariate statistical analyses were applied for further analysis of metabolic data and separation of responsive metabolic features among different microbial systems were found, which were also successfully verified in foodstuffs contaminated by microorganisms. The growth trend of the potential volatile markers of each pathogen in food samples kept consistent with that of the pure strain incubated in medium during the whole incubation time. This study promotes the application of SPME technology in microbial volatile metabolomics and contributes to the development of new approaches for foodborne pathogens recognition.

Hydrodynamic cavitation for the rapid separation and electrochemical detection of Cryptosporidium parvum and Escherichia coli O157:H7 in ground beef

Foodborne illnesses are a major contributor to misery and health challenges in both rich and poor nations. Illnesses from pathogens such as Escherichia coli and Cryptosporidium parvum oocysts account for most of the cases of diarrhea in the world. Many standard methods exist for detecting these pathogens in water. However, these standard methods do not readily translate to the detection of the same pathogens in food. Detection techniques for pathogens in food are often inadequate, due to their inability to completely separate pathogens from food matrices. In this paper, we present a technique to separate and detect both Escherichia coli cells and Cryptosporidium parvum oocysts that have been embedded in ground meat. We achieve this objective by combining enzymatic digestion of the meat, hydrodynamic cavitation to disassemble pathogens from the meat, immunomagnetic separation to purify meat samples and indirect electrochemical detection of the target pathogens. Our use of hydrodynamic cavitation to separate pathogens is compared against an industry standard separation technique. Results indicate that the use of hydrodynamic cavitation amplifies the detection capabilities of our sensing technique and is overall comparable to or better than conventional stomacher sample preparation.

Direct Detection of Escherichia coli, Staphylococcus aureus, and Salmonella spp. in Animal-derived Foods Using a Magnetic Bead-based Immunoassay

In this study, an immuno-magnetic bead (IMB)-based assay was developed to simultaneously detect Escherichia coli, Staphylococcus aureus, and Salmonella spp. and was tested in four animal-derived foods: beef, ham, egg, and ricotta cheese. The IMB-based assay exhibited good specificity by binding to five E. coli serotypes [capture efficiency (CE) average (avg.) 90.4%], five S. aureus strains (CE avg. 91.4%), and five Salmonella serotypes (CE avg. 95.4%) but not binding to non-target bacteria (CE<10%). Furthermore, the assay detected all three pathogens with a detection limit of 10 CFU/g without the need for enrichment or additional platforms. Since the results demonstrated that the IMB-based assay can effectively separate and enrich target bacteria from a variety of animal-derived food matrixes, the assay exhibits good specificity for potential use in providing rapid, immunological, presumptive identification of pathogenic bacteria.

Comparison of subtypes of Listeria monocytogenes isolates from naturally contaminated watershed samples with and without a selective secondary enrichment

Two enrichment methods for Listeria monocytogenes using Immuno Magnetic Separation (IMS) were tested to determine if they selected the same subtypes of isolates. Both methods used a non-selective primary enrichment and one included subculture in Fraser Broth, while the other involved direct plating of IMS beads. Sixty-two naturally contaminated watershed samples from the Central California Coast were used as a source of L. monocytogenes, and subtype diversity was measured by serotype and Multiple Number Variable Tandem Repeat Analysis (MLVA). Three different serotypes were detected from both methods with serotype 4b strains making up 87% of the isolates, serotype 1/2a making up 8%, and serotype 1/2b making up 5%. The data suggest that serotype 1/2a strains were more likely to be isolated from the Fraser Broth culture method. Sixty-two different MLVA types were detected and the more common MLVA types were detected by both culture methods. Forty-three MLVA types were detected only from one culture method or the other, while 19 types were detected from both culture methods. The most common MLVA type-12 was detected in 33 of the 62 water samples, and represented 31% of the isolates from both culture methods. This limited study provides evidence that using both enrichment culture methods allowed for detection of a greater diversity of isolates among the samples than the use of one method alone, and that a wide diversity of L. monocytogenes strains exist in this watershed.

Rapid and sensitive detection of Escherichia coli O157:H7 in milk and ground beef using magnetic bead-based immunoassay coupled with tyramide signal amplification

Escherichia coli O157:H7 is a major foodborne pathogen that has posed serious problems for food safety and public health. Recent outbreaks and recalls associated with various foods contaminated by E. coli O157:H7 clearly indicate its deleterious effect on food safety. A rapid and sensitive detection assay is needed for this harmful organism to prevent foodborne illnesses and control outbreaks in a timely manner. We developed a magnetic bead-based immunoassay for detection of E. coli O157:H7 (the most well-known Shiga toxigenic E. coli strain) with a 96-well microplate as an assay platform. Immunomagnetic separation (IMS) and tyramide signal amplification were coupled to the assay to increase its sensitivity and specificity. This immunoassay was able to detect E. coli O157:H7 in pure culture with a detection limit of 50 CFU/ml in less than 3 h without an enrichment step. The detection limit was decreased 10-fold to 5 CFU/ml with addition of a 3-h enrichment step. When this assay was tested with other nontarget foodborne pathogens and common enteric bacteria, no cross-reactivity was found. When tested with artificially contaminated ground beef and milk samples, the assay sensitivity decreased two- to fivefold, with detection limits of 250 and 100 CFU/ml, respectively, probably because of the food matrix effect. The assay results also were compared with those of a sandwich-type enzyme-linked immunosorbent assay (ELISA) and an ELISA coupled with IMS; the developed assay was 25 times and 4 times more sensitive than the standard ELISA and the IMS-ELISA, respectively. Tyramide signal amplification combined with IMS can improve sensitivity and specificity for detection of E. coli O157:H7. The developed assay could be easily adapted for other foodborne pathogens and will contribute to improved food safety and public health.

Automated dead-end ultrafiltration for concentration and recovery of total coliform bacteria and laboratory-spiked Escherichia coli O157:H7 from 50-liter produce washes to enhance detection by an electrochemiluminescence immunoassay

An automated concentration system (ACS) based on dead-end ultrafiltration was used in this study to concentrate bacteria, including Escherichia coli O157:H7, from 50-liter produce washes (PWs, sieved produce wash). Cells trapped in the filters were recovered in approximately 400 ml of buffer to create PW retentates (PWRs). Extent of concentration was determined by analyzing PWs and PWRs for total coliform bacteria and E. coli O157:H7 using standard methods. In addition, an electrochemiluminescence immunoassay was evaluated for detection of E. coli O157:H7 in spiked PWs and PWRs to demonstrate usefulness of the ACS for same-day detection. The levels of total coliform bacteria and E. coli O157:H7 in PWRs were higher than those in PWs by 1.85 ± 0.41 log most probable number per 100 ml and 1.82 ± 0.24 log CFU/ml, respectively. Electrochemiluminescence detection of E. coli O157:H7 was accomplished within 2 h using ACS concentration of lettuce and spinach wash water artificially spiked with the pathogen at levels as low as 0.36 log CFU/ml and 1.39 log CFU/ml, respectively. Detection of E. coli O157:H7 at -0.93 ± 0.15 log CFU/ml in lettuce wash occurred within approximately 6 h when a 4-h enrichment step was added to the procedure. Use of dead-end ultrafiltration increased bacterial concentrations in PWR and allowed same-day detection of low levels of E. coli O157:H7 in PW. This concentration system could be useful to improve the sensitivity of current rapid methods for detection of low levels of foodborne pathogens in PW water.

Novel genomic tools for specific and real-time detection of biothreat and frequently encountered foodborne pathogens

The bacterial genera Escherichia, Salmonella, Shigella, Vibrio, Yersinia, and Francisella include important food safety and biothreat agents. By extensive mining of the whole genome and protein databases of diverse, closely and distantly related bacterial species and strains, we have identified novel genome regions, which we utilized to develop a rapid detection platform for these pathogens. The specific genomic targets we have identified to design the primers in Francisella tularensis subsp. tularensis, F. tularensis subsp. novicida, Shigella dysenteriae, Salmonella enterica serovar Typhimurium, Vibrio cholerae, Yersinia pestis, and Yersinia pseudotuberculosis contained either known genes or putative proteins. Primer sets were designed from the target regions for use in real-time PCR assays to detect specific biothreat pathogens at species or strain levels. The primer sets were first tested by in silico PCR against whole-genome sequences of different species, subspecies, or strains and then by in vitro PCR against genomic DNA preparations from 23 strains representing six biothreat agents (Escherichia coli O157:H7 strain EDL 933, Shigella dysenteriae, S. enterica serovar Typhi, F. tularensis subsp. tularensis, V. cholerae, and Y. pestis) and six foodborne pathogens (Salmonella Typhimurium, Salmonella Saintpaul, Shigella sonnei, F. tularensis subsp. novicida, Vibrio parahaemolyticus, and Y. pseudotuberculosis). Each pathogen was specifically identifiable at the genus and species levels. Sensitivity assays performed with purified DNA showed the lowest detection limit of 128 fg of DNA/μl for F. tularensis subsp. tularensis. A preliminary test to detect Shigella organisms in a milk matrix also enabled the detection of 6 to 60 CFU/ml. These new tools could ultimately be used to develop platforms to simultaneously detect these pathogens.

Surface plasmon resonance detection using antibody-linked magnetic nanoparticles for analyte capture, purification, concentration, and signal amplification

Rapid, sensitive, and accurate detection of analytes present in low concentrations in complex matrixes is a critical challenge. One issue that affects many biosensor protocols is the number and nature of the interferences present in complex matrixes such as plasma, urine, stool, and environmental samples, resulting in loss of sensitivity and specificity. We have developed a method for rapid purification, concentration, and detection of target analytes from complex matrixes using antibody-coated superparamagnetic nanobeads (immunomagnetic beads, or IMBs). The surface plasmon resonance (SPR) detection signal from staphylococcal enterotoxin B (SEB) was dramatically increased when the IMBs were used as detection amplifiers. When SEB detection included a 10-fold concentration/purification IMB protocol, a substantial increase in detection sensitivity was observed. This procedure was used to successfully purify and concentrate SEB from serum and stool samples, then amplify the SPR detection signal. SEB at a concentration of 100 pg/mL was easily detected in both buffer and stool samples using this procedure. The IMB protocol also served to verify the analyte detection by using two different anti-SEB antibodies, mouse monoclonal antibodies attached to the magnetic nanobeads and rabbit polyclonal antibodies on the SPR sensor surface. Multiple detections of SEB in stool were performed using the same sensor surface by regenerating the sensor surfaces with a pH 2.2 buffer wash.

Use of Ionic Liquid–Based Extraction for Recovery of Salmonella Typhimurium and Listeria monocytogenes from Food Matrices

Methods for rapid separation (&lt;5 h) and concentration of bacteria based on solubilization of complex food matrices have been developed recently to facilitate rapid molecular detection methods. However, a major disadvantage of these protocols is the resulting lack of viability of the microorganisms under study due to extensive use of chemicals and enzymes, which can inhibit subsequent quantitative microbiological analyses. In this study, a new class of organic salts, ionic liquids, were used for solubilization of various foodstuffs, with subsequent molecular and microbiological quantification methods. This approach was applied to gram-positive Listeria monocytogenes and gram-negative Salmonella enterica serovar Typhimurium. By introducing the ionic liquid 1-ethyl-3-methylimidazolium thiocyanate into an existing food solubilization protocol, both molecular and microbiological quantification methods could be used subsequently without losing performance or prolonging the analysis. These experiments resulted in an average recovery of 87% of inoculated bacterial cells with real-time PCR, 85% recovery on nonselective agar plates, and 43% on selective medium. These results illustrate the feasibility of applying ionic liquids in sample pretreatment steps for rapid detection and quantification of bacterial pathogens.

Sample preparation: the forgotten beginning

Advances in molecular technologies and automated instrumentation have provided many opportunities for improved detection and identification of microorganisms; however, the upstream sample preparation steps needed to apply these advances to foods have not been adequately researched or developed. Thus, the extent to which these advances have improved food microbiology has been limited. The purpose of this review is to present the current state of sample preparation, to identify knowledge gaps and opportunities for improvement, and to recognize the need to support greater research and development efforts on preparative methods in food microbiology. The discussion focuses on the need to push technological developments toward methods that do not rely on enrichment culture. Among the four functional components of microbiological analysis (i.e., sampling, separation, concentration, detection), the separation and concentration components need to be researched more extensively to achieve rapid, direct, and quantitative methods. The usefulness of borrowing concepts of separation and concentration from other disciplines and the need to regard the microorganism as a physicochemical analyte that may be directly extracted from the food matrix are discussed. The development of next-generation systems that holistically integrate sample preparation with rapid, automated detection will require interdisciplinary collaboration and substantially increased funding.

Plastic-adherent DNA aptamer-magnetic bead and quantum dot sandwich assay for Campylobacter detection

DNA aptamers were developed against MgCl(2)-extracted surface proteins from Campylobacter jejuni. The two highest affinity aptamers were selected for use in a magnetic bead (MB) and red quantum dot (QD)-based sandwich assay scheme. The assay was evaluated using both heat-killed and live C. jejuni and exhibits detection limits as low as an average of 2.5 colony forming unit (cfu) equivalents in buffer and 10-250 cfu in various food matrices. The assay exhibits low cross-reactivity with bacterial species outside the Campylobacter genus, but exhibits substantial cross-reactivity with C. coli and C. lari. The assay was evaluated with a spectrofluorometer and a commercially available handheld fluorometer, which yielded comparable detection limits and ranges. Remarkably, the sandwich assay components adhere to the inside face of polystyrene cuvettes even in food matrices near neutral pH, thereby enabling a rapid homogeneous assay, because fluorescence is concentrated to a small, thin planar area and background fluorescence from the bulk solution is minimized. The plastic cuvette-adherent technology coupled to a sensitive handheld fluorometer may enable rapid (15-20 min), portable detection of foodborne pathogens from "farm-to-fork" by obviating the slow enrichment culture phase used by other food safety tests.

Immunocapture and real-time PCR to detect Campylobacter spp

In this work, the feasibility of using a large-volume immunocapture system as a sample pretreatment before detection of Campylobacter was studied. Real-time PCR was used for detection of captured cells after immunocapture. This immunocapture system is able to process high-volume samples by recirculation, increasing the possibility of capturing cells in low numbers. After 30 min of recirculation, the sample is concentrated from 250 ml to 200 microl. In this study, different parameters were compared in order to improve cell capture. The analysis of inoculated chicken skin showed that detection of Campylobacter at levels of 10(3) CFU/25 g was possible after 8 h of enrichment. The low recovery of Campylobacter cells (< 1%) makes this separation method qualitative rather than quantitative. The detection limit of the entire protocol was increased due to the low cell recovery of the sample pretreatment. Therefore, this immunoseparation is able to recover cells present in high concentration after enrichment but not cells present in low concentration. Isolation of Campylobacter cells is achievable using this separation method rather than rapid detection.

Rapid separation and concentration of food-borne pathogens in food samples prior to quantification by viable-cell counting and real-time PCR

Buoyant density gradient centrifugation has been used to separate bacteria from complex food matrices, as well as to remove compounds that inhibit rapid detection methods, such as PCR, and to prevent false-positive results due to DNA originating from dead cells. Applying a principle of buoyant density gradient centrifugation, we developed a method for rapid separation and concentration following filtration and low- and high-speed centrifugation, as well as flotation and sedimentation buoyant density centrifugation, for 12 food-borne pathogens (Salmonella enterica, Escherichia coli, Yersinia enterocolitica, Campylobacter jejuni, Vibrio cholerae O139, Vibrio parahaemolyticus O3K6, Vibrio vulnificus, Providencia alcalifaciens, Aeromonas hydrophila, Bacillus cereus, Staphylococcus aureus, and Clostridium perfringens) in 13 different food homogenates. This method can be used prior to real-time quantitative PCR (RTi-qPCR) and viable-cell counting. Using this combined method, the target organisms in the food samples theoretically could be concentrated 250-fold and detected at cell concentrations as low as 10(1) to 10(3) CFU/g using the RTi-qPCR assay, and amounts as small as 10(0) to 10(1) CFU/g could be isolated using plate counting. The combined separation and concentration methods and RTi-qPCR confirmed within 3 h the presence of 10(1) to 10(2) CFU/g of Salmonella and C. jejuni directly in naturally contaminated chicken and the presence of S. aureus directly in remaining food items in a poisoning outbreak. These results illustrated the feasibility of using these assays for rapid inspection of bacterial food contamination during a real-world outbreak.

Detection and quantification of Listeria monocytogenes by 5'-nuclease polymerase chain reaction targeting the actA gene

AIMS

The aim of this study was to develop a 5'-nuclease polymerase chain reaction (PCR) for the rapid detection and quantification of Listeria monocytogenes.

METHODS AND RESULTS

Specific primers and a fluorogenic probe were designed, which target a specific sequence of the actA gene encoding for a protein involved in the actin filament assembly. The PCR system was highly sensitive and specific for L. monocytogenes (inclusivity, 100%; exclusivity, 100%), which was determined using 46 L. monocytogenes and 28 non-L. monocytogenes strains. Detection limits of 10(4) cfu ml(-1) after 35 cycles and 10(2) cfu ml(-1) after 45 cycles were achieved by PCR in both real-time and end-point fluorescence measurement modes. Linear calibration lines were obtained in the range from 10(2) to 10(9) cfu ml(-1) for three L. monocytogenes strains in real-time PCR with 45 cycles.

CONCLUSIONS

The developed 5'-nuclease PCR of the actA gene provides a new target for the rapid detection and quantification of L. monocytogenes.

SIGNIFICANCE AND IMPACT OF THE STUDY

In conjunction with enrichment or with an appropriate quantitative sample preparation technique, the method is suitable for food safety applications.

Immobilization and detection of Listeria monocytogenes

A procedure was developed for immobilization of Listeria monocytogenes cells on metal hydroxides coupled with detection and enumeration using an automated optical system. The results of the immobilization procedure (<1 h) and detection during overnight incubation agreed with calculated plate counts, and this technique is simple and rapid and provides samples that are ready for confirmation of the presence of the pathogen by rapid methods.

Current and developing technologies for monitoring agents of bioterrorism and biowarfare

Recent events have made public health officials acutely aware of the importance of rapidly and accurately detecting acts of bioterrorism. Because bioterrorism is difficult to predict or prevent, reliable platforms to rapidly detect and identify biothreat agents are important to minimize the spread of these agents and to protect the public health. These platforms must not only be sensitive and specific, but must also be able to accurately detect a variety of pathogens, including modified or previously uncharacterized agents, directly from complex sample matrices. Various commercial tests utilizing biochemical, immunological, nucleic acid, and bioluminescence procedures are currently available to identify biological threat agents. Newer tests have also been developed to identify such agents using aptamers, biochips, evanescent wave biosensors, cantilevers, living cells, and other innovative technologies. This review describes these current and developing technologies and considers challenges to rapid, accurate detection of biothreat agents. Although there is no ideal platform, many of these technologies have proved invaluable for the detection and identification of biothreat agents.

Landscape phage probes for Salmonella typhimurium

We selected from landscape phage library probes that bind preferentially Salmonella typhimurium cells compared with other Enterobacteriaceae. The specificity of the phage probes for S. typhimurium was analyzed by the phage-capture test, the enzyme-linked immunosorbent assay (ELISA), and the precipitation test. Interaction of representative probes with S. typhimurium was characterized by fluorescence-activated cell sorting (FACS), and fluorescent, optical and electron microscopy. The results show that the landscape phage library is a rich source of specific and robust probes for S. typhimurium suitable for long-term use in continuous monitoring devices and biosorbents.

Simulating the sensitivity of pooled-sample herd tests for fecal Salmonella in cattle

Samples from livestock or food items are often submitted to microbiological analysis to determine whether or not the group (herd, flock or consignment) is shedding or is contaminated with a bacterial pathogen. This process is known as 'herd testing' and has traditionally involved subjecting each sample to a test on an individual basis. Alternatively one or more pools can be formed by combining and mixing samples from individuals (animals or items) and then each pool is subjected to a test for the pathogen. I constructed a model to simulate herd-level sensitivity of the individual-sample approach (HSe) and the herd-level sensitivity of the pooled-sample approach (HPSe) of tests for detecting pathogen. The two approaches are compared by calculating the relative sensitivity (RelHSe = HPSe/HSe). An assumption is that microbiological procedures had 100% specificity. The new model accounts for the potential for HPSe and RelHSe to be reduced by the dilution of pathogen that occurs when contaminated samples are blended with pathogen-free samples. Key inputs include a probability distribution describing the concentration of the pathogen of interest in samples, characteristics of the pooled-test protocol, and a 'test-dose-response curve' that quantifies the relationship between concentration of pathogen in the pool and the probability of detecting the target organism. The model also compares the per-herd cost of the pooled-sample and individual-sample approaches to herd testing. When applied to the example of Salmonella spp. in cattle feces it was shown that a reduction in the assumed prevalence of shedding can cause a substantial fall in HPSe and RelHSe. However, these outputs are much less sensitive to changes in prevalence when the number of samples per pool is high, or when the number of pools per herd-test is high, or both. By manipulating the number of pools per herd and the number of samples per pool HPSe can be optimized to suit the range of values of true prevalence of shedding of Salmonella that are likely to be encountered in the field.

The relationship between concentration of a dual marker strain of Salmonella Typhimurium in bovine faeces and its probability of detection by immunomagnetic separation and culture

AIMS

To modify a strain of Salmonella serotype Typhimurium to express unique marker traits and then define how the concentration of the marker in bovine faeces affects the probability of its detection by culture preceded by immunomagnetic separation (IMS).

METHODS AND RESULTS

DNA encoding for the production of green fluorescent protein (gfp) and resistance to kanamycin was inserted into the bacterial chromosome of Salm. Typhimurium. Transposon insertion was demonstrated by Southern blot hybridization. Varying amounts of one electroporant (gfpSal-1) were inoculated into suspensions of bovine faeces and attempts made to isolate gfpSal-1 using a protocol based on pre-enrichment incubation, IMS and enrichment in selective media. Isolates of gfpSal-1 were differentiated from wild strains of Salmonella using fluorescence under u.v. light and expression of kanamycin resistance. A logistic and Gompertz function each derived from the dose-response data partially explained the observations with the fit of the Gompertz function judged to be superior. The 10, 50 and 90% limits of detection from the Gompertz function were estimated to be 1.92, 2.03 and 2.27 CFU g(-1) respectively.

CONCLUSIONS

Reliance on the traditional concept of 'limit of detection' could introduce unacceptable errors in the interpretation of test findings when the concentration of Salm. Typhimurium in bovine faeces (pooled or individual) is below ca 3 CFU g(-1) of faeces.

SIGNIFICANCE AND IMPACT OF THE STUDY

The dose-response curve can be used to aid the design of protocols for detecting Salmonella in individual and pooled faecal specimens. The experiments demonstrate that both reporter genes in tandem are useful for studying the performance of culture-based methods for detecting pathogens in faeces.

Method for assessment of functional affinity of antibodies for live bacteria

A rapid and convenient method for assessment of functional affinity of antibodies against live bacteria is described. When a combination of immunomagnetic separation (IMS) with bioluminescent or fluorescent genetic labelling of the cells was employed, the method showed good correlation with plate count. However, the use of reporter bacteria allowed results to be obtained within 1 h compared with days using conventional methods. Due to its lower detection limit, the bioluminescent assay performed better than the fluorescent assay. Antibody affinities for Escherichia coli O157:H7 and Salmonella enteritidis were examined at different environmental conditions such as pH 3-7, temperature 4-25 degrees C, and sodium chloride concentrations 0-5% and compared with sensitivities of ELISA.