Detection of Salmonella enterica serovar typhimurium by using a rapid, array-based immunosensor

The plastisphere can protect Salmonella Typhimurium from UV stress under simulated environmental conditions

Plastic waste is found with increasing frequency in the environment, in low- and middle-income countries. Plastic pollution has increased concurrently with both economic development and rapid urbanisation, amplifying the effects of inadequate waste management. Distinct microbial communities can quickly colonise plastic surfaces in what is collectively known as the 'plastisphere'. The plastisphere can act as a reservoir for human pathogenic bacteria, including Salmonella enterica sp. (such as S. Typhimurium), which can persist for long periods, retain pathogenicity, and pose an increased public health risk. Through employing a novel mesocosm setup, we have shown here that the plastisphere provides enhanced protection against environmental pressures such as ultraviolet (UV) radiation and allows S. Typhimurium to persist at concentrations (>1 × 103 CFU/ml) capable of causing human infection, for up to 28 days. Additionally, using a Galleria Mellonella model of infection, S. Typhimurium exhibits greater pathogenicity following recovery from the UV-exposed plastisphere, suggesting that the plastisphere may select for more virulent variants. This study demonstrates the protection afforded by the plastisphere and provides further evidence of environmental plastic waste acting as a reservoir for dangerous clinical pathogens. Quantifying the role of plastic pollution in facilitating the survival, persistence, and dissemination of human pathogens is critical for a more holistic understanding of the potential public health risks associated with plastic waste.

Enduring pathogenicity of African strains of Salmonella on plastics and glass in simulated peri-urban environmental waste piles

In low- and middle-income countries, plastic has become a major constituent of landfills and urban dump sites. Environmental plastic pollution can also provide a novel surface for the formation of microbial biofilm, which often includes pathogenic bacteria and viruses. Here, under conditions simulating a peri-urban waste pile typical of an African informal settlement, we aimed to determine if pathogenic Salmonella spp. can retain their virulence following a prolonged period of desiccation on the surfaces of environmental plastic and glass. We show that clinically (and environmentally) relevant strains of Salmonella including S. Enteritidis, S. Typhimurium and S. Typhi can persist on plastic and glass for at least 28-days and that temperature (which increases with the depth of an urban waste pile) is a key determinant of this survival. All three strains of Salmonella retained their pathogenicity (determined by using a Galleria mellonella model of infection) following their recovery from the plastisphere indicating that plastics in the environment can act as reservoirs for human pathogens and could facilitate their persistence for extended periods of time. Pathogens colonising environmental plastic waste therefore pose a heightened public health risk, particularly in areas where people are frequently exposed to plastic pollution.

Direct Detection of Foodborne Pathogens via a Proximal DNA Probe-Based CRISPR-Cas12 Assay

Foodborne pathogens can cause illnesses. Existing tools for detecting foodborne pathogens are typically time-consuming or require complex protocols. Here, we report an assay to directly analyze pathogenic genes based on CRISPR-Cas12. This new test, termed proximal DNA probe-based CRISPR-Cas12 (PPCas12), facilitates the detection of foodborne pathogens without amplification steps. The elimination of the nucleic acid amplification process dramatically reduced the processing time, complexity, and costs in the analysis of foodborne pathogens. The substitution of the frequently used dually labeled DNA reporter with a proximal DNA probe in the PPCas12 assay led to a 4-fold sensitivity enhancement. PPCas12 offered a limit of detection of 619 colony-forming units in the detection of Salmonella enterica (S. enterica) without the nucleic acid amplification process. The specific recognition of genes via PPCas12 allowed distinguishing S. enterica from other foodborne pathogens. The PPCas12 assay was applied in the screening of S. enterica contamination on fresh eggs with high precision. Hence, the new PPCas12 assay will be a valuable tool for on-site monitoring of foodborne pathogens.

A review of Salmonella enterica with particular focus on the pathogenicity and virulence factors, host specificity and antimicrobial resistance including multidrug resistance

Salmonella genus represents the most common foodborne pathogens frequently isolated from food-producing animals that is responsible for zoonotic infections in humans and animal species including birds. Thus, Salmonella infections represent a major concern to public health, animals, and food industry worldwide. Salmonella enterica represents the most pathogenic specie and includes > 2600 serovars characterized thus far. Salmonella can be transmitted to humans along the farm-to-fork continuum, commonly through contaminated foods of animal origin, namely poultry and poultry-related products (eggs), pork, fish etc. Some Salmonella serovars are restricted to one specific host commonly referred to as "host-restricted" whereas others have broad host spectrum known as "host-adapted" serovars. For Salmonella to colonize its hosts through invading, attaching, and bypassing the host's intestinal defense mechanisms such as the gastric acid, many virulence markers and determinants have been demonstrated to play crucial role in its pathogenesis; and these factors included flagella, capsule, plasmids, adhesion systems, and type 3 secretion systems encoded on the Salmonella pathogenicity island (SPI)-1 and SPI-2, and other SPIs. The epidemiologically important non-typhoidal Salmonella (NTS) serovars linked with a high burden of foodborne Salmonella outbreaks in humans worldwide included Typhimurium, Enteritidis, Heidelberg, and Newport. The increased number of NTS cases reported through surveillance in recent years from the United States, Europe and low- and middle-income countries of the world suggested that the control programs targeted at reducing the contamination of food animals along the food chain have largely not been successful. Furthermore, the emergence of several clones of Salmonella resistant to multiple antimicrobials worldwide underscores a significant food safety hazard. In this review, we discussed on the historical background, nomenclature and taxonomy, morphological features, physical and biochemical characteristics of NTS with a particular focus on the pathogenicity and virulence factors, host specificity, transmission, and antimicrobial resistance including multidrug resistance and its surveillance.

CE method for analyzing Salmonella typhimurium in water samples

Salmonella typhimurium is commonly described as a food-borne pathogen. However, natural and drinking water are known to be important sources for the transmission of this pathogen in developing and developed countries. The standard method to determine Salmonella is laborious and many false positives are detected. To solve this, the present work was focused on the development of a capillary zone electrophoresis method coupled to ultraviolet detection for determination of Salmonella typhimurium in water (mineral and tap water). Separations were performed in less than 11 minutes using 4.5 mM Tris (hydroxymethyl)-aminomethane, 4.5 mM boric acid and 0.1 mM ethylene diamine tetraacetate (pH 8.4) with 0.1% v/v poly ethylene oxide as separation buffer. The precision of the method was evaluated in terms of repeatability obtaining a relative standard deviation of 10.5%. Using the proposed method Salmonella typhimurium could be separated from other bacteria that could be present in water such as Escherichia coli. Finally, the proposed methodology was applied to determine Salmonella typhimurium in tap and mineral water.

Sulfur and Its Role In Modern Materials Science

Although well-known and studied for centuries, sulfur continues to be at the center of an extensive array of scientific research topics. As one of the most abundant elements in the Universe, a major by-product of oil refinery processes, and as a common reaction site within biological systems, research involving sulfur is both broad in scope and incredibly important to our daily lives. Indeed, there has been renewed interest in sulfur-based reactions in just the past ten years. Sulfur research spans the spectrum of topics within the physical sciences including research on improving energy efficiency, environmentally friendly uses for oil refinery waste products, development of polymers with unique optical and mechanical properties, and materials produced for biological applications. This Review focuses on some of the latest exciting ways in which sulfur and sulfur-based reactions are being utilized to produce materials for application in energy, environmental, and other practical areas.

Label-free mapping of single bacterial cells using surface-enhanced Raman spectroscopy

Here we presented a simple, rapid and label-free surface-enhanced Raman spectroscopy (SERS) based mapping method for the detection and discrimination ofSalmonella entericaandEscherichia coli onsilver dendrites.

Facile synthesis of multifunctional multi-walled carbon nanotube for pathogen Vibrio alginolyticus detection in fishery and environmental samples

Interest in carbon nanotubes for detecting the presence of pathogens arises because of developments in chemical vapor deposition synthesis and progresses in biomolecular modification. Here we reported the facile synthesis of multi-walled carbon nanotubes (MWCNTs), which functioned as immuno-, magnetic, fluorescent sensors in detecting Vibrio alginolyticus (Va). The structures and properties of functionalized MWCNTs were characterized by ultraviolet (UV), Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), magnetic property measurement system (MPMS) and fluorescent spectra (FL). It was found that the functionalized MWCNTs showed: (1) low nonspecific adsorption for antibody-antigen, (2) strong interaction with antibody, and (3) high immune-magnetic activity for pathogenic cells. Further investigations revealed a strong positive linear relationship (R=0.9912) between the fluorescence intensity and the concentration of Va in the range of 9.0 × 10(2) to 1.5 × 10(6) cfum L(-1). Moreover, the relative standard deviation for 11 replicate detections of 1.0 × 10(4) cfum L(-1) Va was 2.4%, and no cross-reaction with the other four strains was found, indicating a good specificity for Va detection. These results demonstrated the remarkable advantages of the multifunctional MWCNTs, which offer great potential for the rapid, sensitive and quantitative detection of Va in fishery and environmental samples.

Capillary Array Waveguide Amplified Fluorescence Detector for mHealth

Mobile Health (mHealth) analytical technologies are potentially useful for carrying out modern medical diagnostics in resource-poor settings. Effective mHealth devices for underserved populations need to be simple, low cost, and portable. Although cell phone cameras have been used for biodetection, their sensitivity is a limiting factor because currently it is too low to be effective for many mHealth applications, which depend on detection of weak fluorescent signals. To improve the sensitivity of portable phones, a capillary tube array was developed to amplify fluorescence signals using their waveguide properties. An array configured with 36 capillary tubes was demonstrated to have a ~100X increase in sensitivity, lowering the limit of detection (LOD) of mobile phones from 1000 nM to 10 nM for fluorescein. To confirm that the amplification was due to waveguide behavior, we coated the external surfaces of the capillaries with silver. The silver coating interfered with the waveguide behavior and diminished the fluorescence signal, thereby proving that the waveguide behavior was the main mechanism for enhancing optical sensitivity. The optical configuration described here is novel in several ways. First, the use of capillaries waveguide properties to improve detection of weak florescence signal is new. Second we describe here a three dimensional illumination system, while conventional angular laser waveguide illumination is spot (or line), which is functionally one-dimensional illumination, can illuminate only a single capillary or a single column (when a line generator is used) of capillaries and thus inherently limits the multiplexing capability of detection. The planar illumination demonstrated in this work enables illumination of a two dimensional capillary array (e.g. x columns and y rows of capillaries). In addition, the waveguide light propagation via the capillary wall provides a third dimension for illumination along the axis of the capillaries. Such an array can potentially be used for sensitive analysis of multiple fluorescent detection assays simultaneously. The simple phone based capillary array approach presented in this paper is capable of amplifying weak fluorescent signals thereby improving the sensitivity of optical detectors based on mobile phones. This may allow sensitive biological assays to be measured with low sensitivity detectors and may make mHealth practical for many diagnostics applications, especially in resource-poor and global health settings.

High-throughput biosensors for multiplexed food-borne pathogen detection

Incidental contamination of foods by pathogenic bacteria and/or their toxins is a serious threat to public health and the global economy. The presence of food-borne pathogens and toxins must be rapidly determined at various stages of food production, processing, and distribution. Producers, processors, regulators, retailers, and public health professionals need simple and cost-effective methods to detect different species or serotypes of bacteria and associated toxins in large numbers of food samples. This review addresses the desire to replace traditional microbiological plate culture with more timely and less cumbersome rapid, biosensor-based methods. Emphasis focuses on high-throughput, multiplexed techniques that allow for simultaneous testing of numerous samples, in rapid succession, for multiple food-borne analytes (primarily pathogenic bacteria and/or toxins).

Detection of pathogens in foods: the current state-of-the-art and future directions

Over the last fifty years, microbiologists have developed reliable culture-based techniques to detect food borne pathogens. Although these are considered to be the "gold-standard," they remain cumbersome and time consuming. Despite the advent of rapid detection methods such as ELISA and PCR, it is clear that reduction and/or elimination of cultural enrichment will be essential in the quest for truly real-time detection methods. As such, there is an important role for bacterial concentration and purification from the sample matrix as a step preceding detection, so-called pre-analytical sample processing. This article reviews recent advancements in food borne pathogen detection and discusses future methods with a focus on pre-analytical sample processing, culture independent methods, and biosensors.

Multiplexed magnetic microsphere immunoassays for detection of pathogens in foods

Foodstuffs have traditionally been challenging matrices for conducting immunoassays. Proteins, carbohydrates, and other macromolecules present in food matrices may interfere with both immunoassays and PCR-based tests, and removal of particulate matter may also prove challenging prior to analyses. This has been found true when testing for bacterial contamination of foods using the standard polystyrene microspheres utilized with Luminex flow cytometers. Luminex MagPlex microspheres are encoded with the same dyes as standard xMAP microspheres, but have superparamagnetic properties to aid in preparation of samples in complex matrices. In this work, we present results demonstrating use of MagPlex for sample preparation and identification of bacteria and a toxin spiked into a variety of food samples. Fluorescence-coded MagPlex microsphere sets coated with antibodies for Salmonella, Campylobacter, Escherichia coli, Listeria, and staphylococcal enterotoxin B (SEB) were used to capture these bacteria and toxin from spiked foodstuffs and then evaluated by the Luminex system in a multiplex format; spiked foods included apple juice, green pepper, tomato, ground beef, alfalfa sprouts, milk, lettuce, spinach, and chicken washes. Although MagPlex microspheres facilitated recovery of the microspheres and targets from the complex matrices, assay sensitivity was sometimes inhibited by up to one to three orders of magnitude; for example the detection limits E. coli spiked into apple juice or milk increased 100-fold, from 1000 to 100,000 cfu/mL. Thus, while the magnetic and fluorescent properties of the Luminex MagPlex microspheres allow for rapid, multiplexed testing for bacterial contamination in typically problematic food matrices, our data demonstrate that achieving desired limits of detection is still a challenge.

Bioaffinity detection of pathogens on surfaces

The demand for improved technologies capable of rapidly detecting pathogens with high sensitivity and selectivity in complex environments continues to be a significant challenge that helps drive the development of new analytical techniques. Surface-based detection platforms are particularly attractive as multiple bioaffinity interactions between different targets and corresponding probe molecules can be monitored simultaneously in a single measurement. Furthermore, the possibilities for developing new signal transduction mechanisms alongside novel signal amplification strategies are much more varied. In this article, we describe some of the latest advances in the use of surface bioaffinity detection of pathogens. Three major sections will be discussed: (i) a brief overview on the choice of probe molecules such as antibodies, proteins and aptamers specific to pathogens and surface attachment chemistries to immobilize those probes onto various substrates, (ii) highlighting examples among the current generation of surface biosensors, and (iii) exploring emerging technologies that are highly promising and likely to form the basis of the next generation of pathogenic sensors.

Effect of physicochemical anomalies of soda-lime silicate slides on biomolecule immobilization

Glass microscope slides are considered by many as the substrate of choice for microarray manufacturing due to their amenability to various surface chemistry modifications. The use of silanes to attach various functional groups onto glass slides has provided a versatile tool for the covalent immobilization of many diverse biomolecules of interest. We recently noted a dramatic reduction in biomolecule immobilization efficiency on standard microscope slides prepared using a well-characterized silanization method. A survey of commercial soda-lime slides yielded the surprising result that slides purchased prior to 2008 had superior immobilization efficiencies when compared to those purchased after 2008. Characterization of the slides by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM), revealed a significant correlation (R > 0.9) between magnesium content, surface roughness, and bioimmobilization efficiency. High performance slides had higher magnesium content and higher root-mean-square (rms) roughness (P < 0.005) than slides with lower bioimmobilization efficiencies. Although the exact mechanism of how magnesium content and surface roughness affect silane deposition has not yet been defined, we show that recent changes in the chemical and physical properties of commercial soda-lime slides affect the ability of these slides to be covalently modified.

[Salmonella spp., important pathogenic agent transmitted through foodstuffs]

The occurrence of foodborne diseases has been a matter of discussion over the last years due to the worldwide concern with strategies for their control and for ensuring that safe food products reach the consumer. Salmonella spp. is among the most widespread microorganisms in nature, having man and animals as main natural reservoirs. With occurrence of regional serotypes causing salmonellosis, this pathogen is considered one of the main agents responsible for outbreaks of foodborne disease in the developing countries. The increasing incidence of salmonellosis caused by contaminated food has shown that, despite the recent technological improvements, this problem still occurs in all countries. Cattle and poultry are the main responsible for the transmission of this pathogenic agent. Due to its wide distribution in animals, the existence of asymptomatic carriers and its presence in foodstuff and in the environment Salmonella spp. represents a significant public health problem worldwide calling for permanent control programs and eradication strategies.

Fluoroimmunoassays using the NRL array biosensor

Array-based biosensor technology offers the user the ability to detect and quantify multiple targets in multiple samples simultaneously (Analytical Sciences 23:5-10, 2007). The NRL Array Biosensor has been developed with the aim of creating a system for sensitive, rapid, on-site screening for multiple targets of interest. This system is fluorescence-based, using evanescent illumination of a waveguide, and has demonstrated the use of both sandwich and competitive immunoassays for the detection of both high and low molecular weight targets, respectively. The current portable, automated system has demonstrated detection of a wide variety of analytes ranging from simple chemical compounds to entire bacterial cells, with applications in food safety, disease diagnosis, homeland security and environmental monitoring.

Comparison of classical serotyping and PremiTest assay for routine identification of common Salmonella enterica serovars

The commercial PremiTest Salmonella kit uses a multiplexed DNA typing test aimed at identifying common serovars of Salmonella enterica. It was used in assays over a 9-month period in the Belgian reference laboratory that performs the routine identification of Salmonella strains of animal origin. A blind analysis of 754 strains was conducted in parallel by classical serotyping and the PremiTest assay. Full results were available for 685 strains (90.8%) by serotyping, while the remaining 69 strains were found to be nontypeable due to either a lack of surface antigen expression or autoagglutination properties. When the PremiTest assay (version 4.2) was performed with crude bacterial extracts, it identified 658 strains (87.3%), including most strains found to be nontypeable by serotyping. In contrast, it gave no, wrong, dual, or noninterpretable results for 96 strains, for which 23 were caused by assay failures. When purified DNA instead of crude extracts were tested, the number of strains successfully identified to the serovar level increased to 714 (94.7%), while all assay failures were cleared. Our conclusion is that, in its actual development stage, the application of the investigated kit to purified DNA samples offers a valuable alternative to classical serotyping for laboratories performing the routine identification of Salmonella strains belonging to commonly encountered serovars and isolated from a given geographical area, assuming that the system has been validated beforehand with a significant number of strains originating from that particular area.

Automated analytical microarrays: a critical review

Microarrays provide a powerful analytical tool for the simultaneous detection of multiple analytes in a single experiment. The specific affinity reaction of nucleic acids (hybridization) and antibodies towards antigens is the most common bioanalytical method for generating multiplexed quantitative results. Nucleic acid-based analysis is restricted to the detection of cells and viruses. Antibodies are more universal biomolecular receptors that selectively bind small molecules such as pesticides, small toxins, and pharmaceuticals and to biopolymers (e.g. toxins, allergens) and complex biological structures like bacterial cells and viruses. By producing an appropriate antibody, the corresponding antigenic analyte can be detected on a multiplexed immunoanalytical microarray. Food and water analysis along with clinical diagnostics constitute potential application fields for multiplexed analysis. Diverse fluorescence, chemiluminescence, electrochemical, and label-free microarray readout systems have been developed in the last decade. Some of them are constructed as flow-through microarrays by combination with a fluidic system. Microarrays have the potential to become widely accepted as a system for analytical applications, provided that robust and validated results on fully automated platforms are successfully generated. This review gives an overview of the current research on microarrays with the focus on automated systems and quantitative multiplexed applications.

The array biosensor: portable, automated systems

With recent advances in surface chemistry, microfluidics, and data analysis, there are ever increasing reports of array-based methods for detecting and quantifying multiple targets. However, only a few systems have been described that require minimal preparation of complex samples and possess a means of quantitatively assessing matrix effects. The NRL Array Biosensor has been developed with the goal of rapid and sensitive detection of multiple targets from multiple samples analyzed simultaneously. A key characteristic of this system is its two-dimensional configuration, which allows controls and standards to be analyzed in parallel with unknowns. Although the majority of our work has focused on instrument automation and immunoassay development, we have recently initiated efforts to utilize alternative recognition molecules, such as peptides and sugars, for detection of a wider variety of targets. The array biosensor has demonstrated utility for a variety of applications, including food safety, disease diagnosis, monitoring immune response, and homeland security, and is presently being transitioned to the commercial sector for manufacturing.

Array biosensor for detection of ochratoxin A in cereals and beverages

Contamination of food by mycotoxins occurs in minute quantities, and therefore, there is a need for a highly sensitive and selective device that can detect and quantify these organic toxins. We report the development of a rapid and highly sensitive array biosensor for the detection and quantitation of ochratoxin A (OTA). The array biosensor utilizes a competitive immunoassay format. Immobilized OTA derivatives compete with toxin in solution for binding to fluorescent anti-OTA antibody spiked into the sample. This competition is quantified by measuring the formation of the fluorescent immunocomplex on the waveguide surface. The fluorescent signal is inversely proportional to the concentration of OTA in the sample. Analyses for OTA in buffer and a variety of food and beverage samples were performed. Samples were extracted with methanol, without any sample cleanup or preconcentration step prior to analysis. The limit of detection for OTA in several cereals ranged from 3.8 to 100 ng/g, while in coffee and wine, detection limits were 7 and 38 ng/g, respectively.

Quantitative and simultaneous detection of four foodborne bacterial pathogens with a multi-channel SPR sensor

We report the quantitative and simultaneous detection of four species of bacteria, Escherichia coli O157:H7, Salmonella choleraesuis serotype typhimurium, Listeria monocytogenes, and Campylobacter jejuni, using an eight-channel surface plasmon resonance (SPR) sensor based on wavelength division multiplexing. Detection curves showing SPR response versus analyte concentration were established for each species of bacteria in buffer at pH 7.4, apple juice at native pH 3.7, and apple juice at an adjusted pH of 7.4, as well as for a mixture containing all four species of bacteria in buffer. Control experiments were performed to show the non-fouling characteristics of the sensor surface as well as the specificity of the amplification antibodies used in this study. The limit of detection (LOD) for each of the four species of bacteria in the tested matrices ranges from 3.4 x 10(3) to 1.2 x 10(5) cfu/ml. Detection curves in buffer of an individual species of bacteria in a mixture of all four species of bacteria correlated well with detection curves of the individual species of bacteria alone. SPR responses were higher for bacteria in apple juice at pH 7.4 than in apple juice at pH 3.7. This difference in sensor response could be partly attributed to the pH dependence of antibody-antigen binding.

Antimicrobial peptide-based array for Escherichia coli and Salmonella screening

Numerous bacteria, plants, and higher organisms produce antimicrobial peptides (AMPs) as part of their innate immune system, providing a chemical defense mechanism against microbial invasion. Many AMPs exert their antimicrobial activity by binding to components of the microbe's surface and disrupting the membrane. The goal of this study was to incorporate AMPs into screening assays for detection of pathogenic species. Surface-immobilized AMPs such as polymyxins B and E could be used to detect Salmonella typhimurium and Escherichia coli O157:H7 in two assay formats: direct and sandwich. Both types of assay confirmed that the peptides were immobilized in active form and could bind cells in a concentration-dependent manner. Cell binding to the AMPs was peptide-density dependent. This method for monitoring pathogen binding was extended to include other cationic AMPs such as cecropin A, magainin I and parasin. Detection limits (LODs) for E. coli O157:H7 and S. typhimurium obtained with AMPs during sandwich assays were in the ranges of 5x10(4) to 5x10(5) and 1x10(5) to 5x10(6)cells mL(-1), respectively. The different AMPs showed significantly different affinities for the two bacterial species; the potential for classification of pathogens based on different binding patterns to AMPs is discussed.

Indirect competitive immunoassay for detection of aflatoxin B1 in corn and nut products using the array biosensor

Because of the potential health risks of aflatoxin B1 (AFB1), it is essential to monitor the level of this mycotoxin in a variety of foods. An indirect competitive immunoassay has been developed using the NRL array biosensor, offering rapid, sensitive detection and quantification of AFB1 in buffer, corn and nut products. AFB1-spiked foods were extracted with methanol and Cy5-anti-AFB1 added to the resulting sample. The extracted sample/antibody mix was passed over a waveguide surface patterned with immobilized AFB1. The resulting fluorescence signal decreased as the concentration of AFB1 in the sample increased. The limit of detection for AFB1 in buffer, 0.3 ng/ml, was found to increase to between 1.5 and 5.1 ng/g and 0.6 and 1.4 ng/g when measured in various corn and nut products, respectively.

Effect of environmental stresses on antibody-based detection of Escherichia coli O157:H7, Salmonella enterica serotype Enteritidis and Listeria monocytogenes

AIMS

To study the reaction patterns of selected antibodies to Escherichia coli O157:H7, Salmonella enterica serotype Enteritidis and Listeria monocytogenes cells exposed to various environmental stresses.

METHODS AND RESULTS

Escherichia coli O157:H7, Salmonella Enteritidis and L. monocytogenes cells subjected to different environmental stress of temperatures (4 and 45 degrees C), NaCl (5.5%), oxidative stress (15 mmol(-1) H2O2), acidic pH (5.5) and ethanol (5%) for 3 h (short-term stress) or for 5 days (long-term stress) were analysed by ELISA and Western blotting. The ELISA results indicated that most stresses caused 12-16% reductions in reaction for anti-E. coli O157:H7 and 20-48% reductions for anti-Salmonella polyclonal antibodies during short-term stress, whereas the most stresses exhibited enhanced reaction (44-100% increase) with the anti-L. monocytogenes polyclonal antibody. During long-term stress exposure to combined stress conditions of pH 5.5, 3.5% NaCl at 12 degrees C or at 4 degrees C, antibody reactions to the three pathogens were highly variable with the combined stress at 4 degrees C showing the most reductions (8-40%). Likewise, there were about 18-59% reductions in antibody reactions with pathogens when cultured in hotdog samples with the combined stress conditions. Western blot analyses of crude cell surface antigens from both short- and long-term stressed cells revealed that the changes in antibody reactions observed in ELISA were either because of repression, expression or possible denaturation of antigens on the surface of cells.

CONCLUSIONS

Overall, the antibody reactions were significantly reduced in pathogens exposed to both short- and long-term environmental stresses in culture medium or in meat sample because of expression, repression or denaturation of specific antigens in cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

In order to ensure the reliable detection of foodborne pathogens using antibody-based methods, the influence of stress on antibody reactions should be thoroughly examined and understood first as the physiological activities in cells are often altered in response to a stress.

Diagnostic and analytical applications of protein microarrays

DNA microarrays have changed the field of biomedical sciences over the past 10 years. For several reasons, antibody and other protein microarrays have not developed at the same rate. However, protein and antibody arrays have emerged as a powerful tool to complement DNA microarrays during the past 5 years. A genome-scale protein microarray has been demonstrated for identifying protein-protein interactions as well as for rapid identification of protein binding to a particular drug. Furthermore, protein microarrays have been shown as an efficient tool in cancer profiling, detection of bacteria and toxins, identification of allergen reactivity and autoantibodies. They have also demonstrated the ability to measure the absolute concentration of small molecules. Besides their capacity for parallel diagnostics, microarrays can be more sensitive than traditional methods such as enzyme-linked immunosorbent assay, mass spectrometry or high-performance liquid chromatography-based assays. However, for protein and antibody arrays to be successfully introduced into diagnostics, the biochemistry of immunomicroarrays must be better characterized and simplified, they must be validated in a clinical setting and be amenable to automation or integrated into easy-to-use systems, such as micrototal analysis systems or point-of-care devices.

Multiplexed measurement of serum antibodies using an array biosensor

The array biosensor provides the capability for simultaneously measuring titers of antibody against multiple antigens. Human antibodies against four different targets, tetanus toxin, diphtheria toxin, staphylococcal enterotoxin B (SEB) and hepatitis B, were measured simultaneously in sera from eight different donors in a single assay and titers were determined. The assays could measure amounts of bound antibody as low as approximately 100 fg. Each individual serum exhibited a different pattern of reactivity against the four target antigens. Applications of this biosensor capability include monitoring for exposure to pathogens and for efficacy of vaccination.

Biosensor detection of botulinum toxoid A and staphylococcal enterotoxin B in food

Immunoassays were developed for the simultaneous detection of staphylococcal enterotoxin B and botulinum toxoid A in buffer, with limits of detection of 0.1 ng/ml and 20 ng/ml, respectively. The toxins were also spiked and measured in a variety of food samples, including canned tomatoes, sweet corn, green beans, mushrooms, and tuna.

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.

A “do-it-yourself” array biosensor

We have developed an array biosensor for the simultaneous detection of multiple targets in multiple samples within 15-30 min. The biosensor is based on a planar waveguide, a modified microscope slide, with a pattern of small (mm2) sensing regions. The waveguide is illuminated by launching the emission of a 635 nm diode laser into the proximal end of the slide via a line generator. The evanescent field excites fluorophores bound in the sensing region and the emitted fluorescence is measured using a Peltier-cooled CCD camera. Assays can be performed on the waveguide in multichannel flow chambers and then interrogated using the detection system described here. This biosensor can detect many different targets, including proteins, toxins, cells, virus, and explosives with detection limits rivaling those of the ELISA detection system.

Antimicrobial Peptides for Detection of Bacteria in Biosensor Assays

Bacteria, plants, and higher and lower animals have evolved an innate immune system as a first line of defense against microbial invasion. Some of these organisms produce antimicrobial peptides (AMPs) as a part of this chemical immune system. AMPs exert their antimicrobial activity by binding to components of the microbe's surface and disrupting the membrane. The overall goal of this study was to apply the AMP magainin I as a recognition element for Escherichia coli O157:H7 and Salmonella typhimurium detection on an array-based biosensor. We immobilized magainin I on silanized glass slides using biotin-avidin chemistry, as well as through direct covalent attachment. Cy5-labeled, heat-killed cells were used to demonstrate that the immobilized magainin I can bind Salmonella with detection limits similar to analogous antibody-based assays. Detection limits for E. coli were higher than in analogous antibody-based assays, but it is expected that other AMPs may possess higher affinities for this target. The results showed that both specific and nonspecific binding strongly depend on the method used for peptide immobilization. Direct attachment of magainin to the substrate surface not only decreased nonspecific cell binding but also resulted in improved detection limits for both Salmonella and E. coli.