Development and characterization of a monoclonal antibody specific for Listeria monocytogenes and Listeria innocua

Cell-surface anchoring of Listeria adhesion protein on L. monocytogenes is fastened by internalin B for pathogenesis

Listeria adhesion protein (LAP) is a secreted acetaldehyde alcohol dehydrogenase (AdhE) that anchors to an unknown molecule on the Listeria monocytogenes (Lm) surface, which is critical for its intestinal epithelium crossing. In the present work, immunoprecipitation and mass spectrometry identify internalin B (InlB) as the primary ligand of LAP (K_D ∼ 42 nM). InlB-deleted and naturally InlB-deficient Lm strains show reduced LAP-InlB interaction and LAP-mediated pathology in the murine intestine and brain invasion. InlB-overexpressing non-pathogenic Listeria innocua also displays LAP-InlB interplay. In silico predictions reveal that a pocket region in the C-terminal domain of tetrameric LAP is the binding site for InlB. LAP variants containing mutations in negatively charged (E₅₂₃S, E₆₂₁S) amino acids in the C terminus confirm altered binding conformations and weaker affinity for InlB. InlB transforms the housekeeping enzyme, AdhE (LAP), into a moonlighting pathogenic factor by fastening on the cell surface.

Selection of Listeria monocytogenes InlA-Binding Peptides Using Phage Display—Novel Compounds for Diagnostic Applications?

Listeria monocytogenes is a pathogenic, gram-positive bacterium causing foodborne infections and listeriosis, an infection responsible for serious medical conditions, especially for pregnant women, newborns, or people with a weak immune system. Even after antibiotic treatment, 30% of clinical infections result in death. L. monocytogenes is able to enter and multiply in mammalian cells. Invasion into epithelial cells in the human intestine is mediated by the interaction of the bacterial surface protein internalin A (InlA) with the host cell receptor E-cadherin (E-cad). We have used phage display to select InlA-specific peptides consisting of 12 amino acids using a randomized, recombinant peptide library. We could demonstrate that the selected peptides bound to recombinant InlA protein as well as to L. monocytogenes cells. In vitro, some of the peptides inhibited the interaction between recombinant InlA and human E-cad. As far as we know, this is the first publication on the development of InlA-specific peptide ligands. In the future, our peptides might be used for the development of innovative diagnostic tools or even therapeutic approaches.

Phage Display-Derived Monoclonal Antibodies Against Internalins A and B Allow Specific Detection of Listeria monocytogenes

Listeria monocytogenes is the causative agent of listeriosis, a highly lethal disease initiated after the ingestion of Listeria-contaminated food. This species comprises different serovars, from which 4b, 1/2a, and 1/2b cause most of the infections. Among the different proteins involved in pathogenesis, the internalins A (InlA) and B (InlB) are the best characterized, since they play a major role in the enterocyte entry of Listeria cells during early infection. Due to their covalent attachment to the cell wall and location on the bacterial surface, along with their exclusive presence in the pathogenic L. monocytogenes, these proteins are also used as detection targets for this species. Even though huge advancements were achieved in the enrichment steps for subsequent Listeria detection, few studies have focused on the improvement of the antibodies for immunodetection. In the present study, recombinant InlA and InlB produced in Escherichia coli were used as targets to generate antibodies via phage display using the human naïve antibody libraries HAL9 and HAL10. A set of five recombinant antibodies (four against InlA, and one against InlB) were produced in scFv-Fc format and tested in indirect ELISA against a panel of 19 Listeria strains (17 species; including the three main serovars of L. monocytogenes) and 16 non-Listeria species. All five antibodies were able to recognize L. monocytogenes with 100% sensitivity (CI 29.24–100.0) and specificity (CI 88.78–100.0) in all three analyzed antibody concentrations. These findings show that phage display-derived antibodies can improve the biological tools to develop better immunodiagnostics for L. monocytogenes.

Listeria monocytogenes: review of pathogenesis and virulence determinants-targeted immunological assays

Listeria monocytogenes is one of the most invasive foodborne pathogens and is responsible for numerous outbreaks worldwide. Most of the methods to detect this bacterium in food require selective enrichment using traditional bacterial culture techniques that can be time-consuming and labour-intensive. Moreover, molecular methods are expensive and need specific technical knowledge. In contrast, immunological approaches are faster, simpler, and user-friendly alternatives and have been developed for the detection of L. monocytogenes in food, environmental, and clinical samples. These techniques are dependent on the constitutive expression of L. monocytogenes antigens and the specificity of the antibodies used. Here, updated knowledge on pathogenesis and the key immunogenic virulence determinants of L. monocytogenes that are used for the generation of monoclonal and polyclonal antibodies for the serological assay development are summarised. In addition, immunological approaches based on enzyme-linked immunosorbent assay, immunofluorescence, lateral flow immunochromatographic assays, and immunosensors with relevant improvements are highlighted. Though the sensitivity and specificity of the assays were improved significantly, methods still face many challenges that require further validation before use.

Pyruvate dehydrogenase complex-enzyme 2, a new target for Listeria spp. detection identified using combined phage display technologies

The genus Listeria comprises ubiquitous bacteria, commonly present in foods and food production facilities. In this study, three different phage display technologies were employed to discover targets, and to generate and characterize novel antibodies against Listeria: antibody display for biomarker discovery and antibody generation; ORFeome display for target identification; and single-gene display for epitope characterization. With this approach, pyruvate dehydrogenase complex—enzyme 2 (PDC-E2) was defined as a new detection target for Listeria, as confirmed by immunomagnetic separation-mass spectrometry (IMS-MS). Immunoblot and fluorescence microscopy showed that this protein is accessible on the bacterial cell surface of living cells. Recombinant PDC-E2 was produced in E. coli and used to generate 16 additional antibodies. The resulting set of 20 monoclonal scFv-Fc was tested in indirect ELISA against 17 Listeria and 16 non-Listeria species. Two of them provided 100% sensitivity (CI 82.35–100.0%) and specificity (CI 78.20–100.0%), confirming PDC-E2 as a suitable target for the detection of Listeria. The binding region of 18 of these antibodies was analyzed, revealing that ≈ 90% (16/18) bind to the lipoyl domains (LD) of the target. The novel target PDC-E2 and highly specific antibodies against it offer new opportunities to improve the detection of Listeria.

Multifuntional Gold Nanoparticles for the SERS Detection of Pathogens Combined with a LAMP-in-Microdroplets Approach

We developed a droplet-based optofluidic system for the detection of foodborne pathogens. Specifically, the loop-mediated isothermal amplification (LAMP) technique was combined with surface-enhanced Raman scattering (SERS), which offers an excellent method for DNA ultradetection. However, the direct SERS detection of DNA compromises the simplicity of data interpretation due to the variability of its SERS fingerprints. Therefore, we designed an indirect SERS detection method using multifunctional gold nanoparticles (AuNPs) based on the formation of pyrophosphate generated during the DNA amplification by LAMP. Towards this goal, we prepared multifunctional AuNPs involving three components with key roles: (1) thiolated poly(ethylene glycol) as stabilizing agent, (2) 1-naphthalenethiol as Raman reporter, and (3) glutathione as a bioinspired chelating agent of magnesium (II) ions. Thus, the variation in the SERS signal of 1-naphthalenethiol was controlled by the aggregation of AuNPs triggered by the complexation of pyrophosphate and glutathione with free magnesium ions. Using this strategy, we detected Listeria monocytogenes, not only in buffer, but also in a food matrix (i.e., ultra-high temperaturemilk) enabled by the massive production of hotspots as a result of the self-assemblies that enhanced the SERS signal. This allowed the development of a microdroplet-LAMP-SERS platform with isothermal amplification and real-time identification capabilities.

Bio-Plex suspension array immuno-detection of Listeria monocytogenes from cantaloupe and packaged salad using virulence protein inducing activated charcoal enrichment media

Listeria monocytogenes, the causative agent of listeriosis in humans, is a Gram-positive bacterium that is contracted via the ingestion of contaminated foods. Two of the largest outbreaks of listeriosis occurred following consumption of tainted cantaloupe and packaged salads. Molecular methods and immuno-based techniques for detection of L. monocytogenes in these food matrices can be difficult due to the presence of assay inhibiting elements. In this study, we utilized a novel enrichment media containing activated charcoal as the key ingredient that induces hyperactive expression and secretion of L. monocytogenes virulence proteins. The Bio-Plex suspension array system, based on Luminex xMAP technology, was subsequently employed to specifically detect accumulated L. monocytogenes secreted and membrane bound proteins via paramagnetic microsphere-antibody complexes. Cantaloupe and packaged salad samples were treated with a dilution series of L. monocytogenes and incubated in activated charcoal media following a short pre-enrichment step in Buffered Listeria Enrichment Broth. Secreted L. monocytogenes lysteriolysin O was captured using magnetic microsphere-antibody conjugates and measured using the Bio-Ple×200 analyzer. As few as 10⁰ CFU/g of L. monocytogenes was detected from both spiked cantaloupe and packaged salad samples. In addition, antibody conjugated microspheres targeting a membrane protein present on both pathogenic and nonpathogenic Listeria species was used to identify as few as 10⁰ CFU/g of both pathogenic and nonpathogenic species in cantaloupe and packaged salad. This method presumptively identifies L. monocytogenes from cantaloupe and packaged salad in less than 24 h and non-pathogenic Listeria species within 22 h.

Identification of two novel fowl adenovirus C-specific B cell epitopes using monoclonal antibodies against the capsid hexon protein

The diseases associated with fowl adenovirus (FAdV) infection, such as inclusion body hepatitis (IBH), hepatitis-hydropericardium syndrome (HPS), and gizzard erosion (GE), were first reported in Pakistan in 1987, and subsequent outbreaks have been reported worldwide, especially in China, where severe outbreaks of HPS with high mortality from 30 to 100% were recently reported and resulted in significant economic losses to the poultry industry. The diagnosis methods of FAdVs were mostly limited to the nucleotide sequence of hexon by PCR and DNA sequencing. The aim of this study was to generate B cell epitope maps of the species- and serotype-specific hexon L1 using monoclonal antibodies (mAbs) and bioinformatics tools for the development of novel diagnostic methods. In this study, the hexon L1 (230 amino acids) was expressed and used to generate 10 mAb-expressing hybridoma cell lines against the relative protein peptide. Subsequently, we defined the linear peptide epitopes recognized by these mAbs using a series of partially overlapping peptides derived from the FAdV-C hexon protein amino acid sequence to map mAbs reactivity. Finally, a common B cell epitope (₃₁PLAPKESMFN₄₀) for all species FAdVs and two FAdV-C-specific epitopes (₇₉KISGVFPNP₈₇ and ₁₈₁DYDDYNIGTT₁₉₀) were identified. These mAbs and their defined epitopes may support the development of the universal or species-specific differential diagnostic methods of FAdVs.

Conjugation of Specifically Developed Antibodies for High- and Low-Molecular-Weight Glutenins with Fluorescent Quantum Dots as a Tool for Their Detection in Wheat Flour Dough

The importance of gluten proteins, gliadins and glutenins, is well-known in the quality of wheat products. To gain more specific information about the role of glutenins in wheat dough, the two major subunits of glutenin, high- and low-molecular-weight (HMW and LMW) glutenins, were extracted, isolated, and identified by mass spectrometry. Antibodies for HMW and LMW glutenins were developed using the proteomic information on the characterized glutenin subunits. The antibodies were found to be specific to each subunit by western immunoblots and were then conjugated to quantum dots (QDs) using site-click conjugation, a new method to keep antibody integrity. A fluorescence-link immunosorbent assay tested the successful QD conjugation. The QD-conjugated antibodies were applied to dough samples, where they recognized glutenin subunits and were visualized using a confocal laser scanning microscope.

Expression of Surface Protein LapB by a Wide Spectrum of Listeria monocytogenes Serotypes as Demonstrated with Anti-LapB Monoclonal Antibodies

Protein antigens expressed on the surface of all strains of Listeria monocytogenes and absent from nonpathogenic Listeria spp. are presumably useful targets for pathogen identification, detection, and isolation using specific antibodies (Abs). To seek such surface proteins expressed in various strains of L. monocytogenes for diagnostic applications, we focused on a set of surface proteins known to be involved or putatively involved in L. monocytogenes virulence and identified Listeria adhesion protein B (LapB) as a candidate based on the bioinformatics analysis of whole-genome sequences showing that the gene coding for LapB was present in L. monocytogenes strains and absent from strains of other Listeria spp. Immunofluorescence microscopy (IFM), performed with rabbit polyclonal antibodies against the recombinant LapB protein (rLapB) of L. monocytogenes serotype 4b strain L10521, confirmed expression of LapB on the surface. A panel of 48 mouse monoclonal antibodies (MAbs) to rLaB was generated, and 7 of them bound strongly to the surface of L. monocytogenes cells as demonstrated using IFM. Further characterization of these 7 anti-LapB MAbs, using an enzyme-linked immunosorbent assay (ELISA), revealed that 6 anti-LapB MAbs (M3484, M3495, M3500, M3509, M3517, and M3519) reacted strongly with 46 (86.8%) of 53 strains representing 10 of the 12 serotypes tested (1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4ab, 4b, 4d, and 4e). These results indicate that LapB, together with companion anti-LapB MAbs, can be targeted as a biomarker for the detection and isolation of various L. monocytogenes strains from contaminated foods.

IMPORTANCE

Strains of L. monocytogenes are traditionally grouped into serotypes. Identification of a surface protein expressed in all or the majority of at least 12 serotypes would aid in the development of surface-binding monoclonal antibodies (MAbs) for detection and isolation of L. monocytogenes from foods. Bioinformatics analysis revealed that the gene coding for Listeria adhesion protein B (LapB), a surface protein involved in L. monocytogenes virulence, was present in L. monocytogenes strains and absent from other Listeria spp. Polyclonal antibodies against recombinant LapB (rLapB) detected the exposed epitopes on the surface of L. monocytogenes Production and extensive assessment of 48 MAbs to rLapB showed that 6 anti-LapB MAbs (M3484, M3495, M3500, M3509, M3517, and M3519) detected the expression of LapB in a wide range of L. monocytogenes isolates representing 10 of 12 serotypes tested, suggesting that LapB, together with specific MAbs, can be targeted as a biomarker for pathogen detection and isolation.

Fructose 1,6-Bisphosphate Aldolase, a Novel Immunogenic Surface Protein on Listeria Species

Listeria monocytogenes is a ubiquitous food-borne pathogen, and its presence in food or production facilities highlights the importance of surveillance. Increased understanding of the surface exposed antigens on Listeria would provide potential diagnostic and therapeutic targets. In the present work, using mass spectrometry and genetic cloning, we show that fructose-1,6-bisphosphate aldolase (FBA) class II in Listeria species is the antigen target of the previously described mAb-3F8. Western and dot blot assays confirmed that the mAb-3F8 could distinguish all tested Listeria species from close-related bacteria. Localization studies indicated that FBA is present in every fraction of Listeria cells, including supernatant and the cell wall, setting Listeria spp. as one of the few bacteria described to have this protein on their cell surface. Epitope mapping using ORFeome display and a peptide membrane revealed a 14-amino acid peptide as the potential mAb-3F8 epitope. The target epitope in FBA allowed distinguishing Listeria spp. from closely-related bacteria, and was identified as part of the active site in the dimeric enzyme. However, its function in cell surface seems not to be host cell adhesion-related. Western and dot blot assays further demonstrated that mAb-3F8 together with anti-InlA mAb-2D12 could differentiate pathogenic from non-pathogenic Listeria isolated from artificially contaminated cheese. In summary, we report FBA as a novel immunogenic surface target useful for the detection of Listeria genus.

Virulence Gene-Associated Mutant Bacterial Colonies Generate Differentiating Two-Dimensional Laser Scatter Fingerprints

In this study, we investigated whether a laser scatterometer designated BARDOT (bacterial rapid detection using optical scattering technology) could be used to directly screen colonies of Listeria monocytogenes, a model pathogen, with mutations in several known virulence genes, including the genes encoding Listeria adhesion protein (LAP; lap mutant), internalin A (ΔinlA strain), and an accessory secretory protein (ΔsecA2 strain). Here we show that the scatter patterns of lap mutant, ΔinlA, and ΔsecA2 colonies were markedly different from that of the wild type (WT), with >95% positive predictive values (PPVs), whereas for the complemented mutant strains, scatter patterns were restored to that of the WT. The scatter image library successfully distinguished the lap mutant and ΔinlA mutant strains from the WT in mixed-culture experiments, including a coinfection study using the Caco-2 cell line. Among the biophysical parameters examined, the colony height and optical density did not reveal any discernible differences between the mutant and WT strains. We also found that differential LAP expression in L. monocytogenes serotype 4b strains also affected the scatter patterns of the colonies. The results from this study suggest that BARDOT can be used to screen and enumerate mutant strains separately from the WT based on differential colony scatter patterns.

IMPORTANCE

In studies of microbial pathogenesis, virulence-encoding genes are routinely disrupted by deletion or insertion to create mutant strains. Screening of mutant strains is an arduous process involving plating on selective growth media, replica plating, colony hybridization, DNA isolation, and PCR or immunoassays. We applied a noninvasive laser scatterometer to differentiate mutant bacterial colonies from WT colonies based on forward optical scatter patterns. This study demonstrates that BARDOT can be used as a novel, label-free, real-time tool to aid researchers in screening virulence gene-associated mutant colonies during microbial pathogenesis, coinfection, and genetic manipulation studies.

Respiratory syncytial virus detection in cells and clinical samples by using three new monoclonal antibodies

Acute respiratory infections caused by the respiratory syncytial virus (RSV) are important health burdens that affect infants worldwide. RSV is also an important cause of morbidity and disease in adults, which causes enormous economic losses. At the present time, RSV infection is diagnosed by immunofluorescence, test pack and/or PCR, obtaining better results with PCR than with any other technique. The production of new monoclonal antibodies (mAbs) capable of detecting RSV in clinical samples is necessary to generate better and faster diagnosis tools for RSV. In this study, three new mAbs, directed against the RSV N and M2-1 proteins, were evaluated for the detection of RSV in clinical samples. Nasopharyngeal swabs were obtained from: 27 RSV-positive patients; 15 human metapneumovirus (hMPV)-positive patients; and 6 healthy controls. To evaluate RSV presence in these samples, clinical samples and RSV-infected cells were tested by Enzyme-Linked ImmunoSorbent Assay (ELISA), flow cytometry, immunofluorescence, and dot-blot assays. Specificity and sensitivity were determined for each mAb by using purified RSV antigens and antigens from different viruses. Infected cells and clinical samples tested with the three new mAbs resulted positive by immunofluorescence, ELISA, flow cytometry, and dot blot. No false positives were obtained in samples infected with other respiratory virus (hMPV) or from healthy controls. These results suggest that these new anti-RSV mAbs can be considered for the rapid and reliable detection of RSV on infected cells and clinical specimens by multiple immunological approaches.

Highly specific fiber optic immunosensor coupled with immunomagnetic separation for detection of low levels of Listeria monocytogenes and L. ivanovii

BACKGROUND

Immunomagnetic separation (IMS) and immunoassays are widely used for pathogen detection. However, novel technology platforms with highly selective antibodies are essential to improve detection sensitivity, specificity and performance. In this study, monoclonal antibodies (MAbs) against Internalin A (InlA) and p30 were generated and used on paramagnetic beads of varying diameters for concentration, as well as on fiber-optic sensor for detection.

RESULTS

Anti-InlA MAb-2D12 (IgG2a subclass) was specific for Listeria monocytogenes and L. ivanovii, and p30-specific MAb-3F8 (IgM) was specific for the genus Listeria. At all bacterial concentrations (10³-10⁸ CFU/mL) tested in the IMS assay; the 1-μm diameter MyOne beads had significantly higher capture efficiency (P < 0.05) than the 2.8-μm diameter M-280 beads with both antibodies. The highest capture efficiency for MyOne-2D12 (49.2% for 10⁵ CFU/mL) was significantly higher (P < 0.05) than that of MyOne-3F8 (16.6 %) and Dynabeads anti-Listeria antibody (9 %). Furthermore, capture efficiency for MyOne-2D12 was highly specific for L. monocytogenes and L. ivanovii. Subsequently, we captured L. monocytogenes by MyOne-2D12 and MyOne-3F8 from hotdogs inoculated with mono- or co-cultures of L. monocytogenes and L. innocua (10-40 CFU/g), enriched for 18 h and detected by fiber-optic sensor and confirmed by plating, light-scattering, and qPCR assays. The detection limit for L. monocytogenes and L. ivanovii by the fiber-optic immunosensor was 3 × 10² CFU/mL using MAb-2D12 as capture and reporter antibody. Selective media plating, light-scattering, and qPCR assays confirmed the IMS and fiber-optic results.

CONCLUSIONS

IMS coupled with a fiber-optic sensor using anti-InlA MAb is highly specific for L. monocytogenes and L. ivanovii and enabled detection of these pathogens at low levels from buffer or food.

Multiplex fiber optic biosensor for detection of Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella enterica from ready-to-eat meat samples

Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella enterica are the most common foodborne bacterial pathogens and are responsible for many outbreaks. Therefore, multiplex detection of these three using a single assay platform is highly desirable. The objective was to develop and optimize a fiber optic sensor for simultaneous detection of these three from food. The streptavidin coated optical waveguides were immobilized with biotinylated polyclonal antibodies and exposed to the bacterial suspensions or enriched food samples for 2 h. Pathogens were detected after reacting with Alexa-Fluor 647-labeled monoclonal antibodies. Ready-to-eat beef, chicken and turkey meats were inoculated with each pathogen (~100 cfu/25 g), enriched in SEL (Salmonella, E. coli, Listeria), a multipathogen selective enrichment broth for 18 h and tested with the biosensor. The biosensor was able to detect each pathogen, individually or in a mixture with very little cross-reactivity. The limit of detection for the sensor was ~10(3) cfu/ml for all three pathogens. Furthermore, the biosensor successfully detected each pathogen, grown in a mixture from enriched meat samples under 24 h. The pathogen presence was further verified by PCR and immunofluorescence assay. The multiplex fiber optic sensor shows promise for detection of the three pathogens if present in the same sample eliminating the use of multiple single pathogen detection platforms.

Antibody-aptamer functionalized fibre-optic biosensor for specific detection of Listeria monocytogenes from food

AIM

To develop antibody-aptamer functionalized fibre-optic biosensor for specific detection of Listeria monocytogenes from food products.

METHODS AND RESULTS

Aptamer, a single-stranded oligonucleotide ligand that displays affinity for the target molecule, was used in the assay to provide sensor specificity. Aptamer-A8, specific for internalin A, an invasive protein of L. monocytogenes, was used in the fibre-optic sensor together with antibody in a sandwich format for detection of L. monocytogenes from food. Biotinylated polyclonal anti-Listeria antibody, P66, was immobilized on streptavidin-coated optical waveguide surface for capturing bacteria, and Alexa Fluor 647-conjugated A8 was used as a reporter. The biosensor was able to selectively detect pathogenic Listeria in pure culture and in mixture with other bacteria at a concentration of approx. 10(3) CFU ml(-1). This sensor also successfully detected L. monocytogenes cells from artificially contaminated (initial inoculation of 10(2) CFU 25 g(-1) ) ready-to-eat meat products such as sliced beef, chicken and turkey after 18 h of enrichment.

CONCLUSION

Based on the data presented in this study, the antibody-aptamer functionalized fibre-optic biosensor could be used as a detection tool for sensitive and specific detection of L. monocytogenes from foods.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study demonstrates feasibility and novel application of aptamer on fibre-optic biosensor platform for the sensitive detection of L. monocytogenes from food products.

LAP, an alcohol acetaldehyde dehydrogenase enzyme in Listeria, promotes bacterial adhesion to enterocyte-like Caco-2 cells only in pathogenic species

Listeria adhesion protein (LAP), an alcohol acetaldehyde dehydrogenase (lmo1634), interacts with host-cell receptor Hsp60 to promote bacterial adhesion during the intestinal phase of Listeria monocytogenes infection. The LAP homologue is present in pathogens (L. monocytogenes, L. ivanovii) and non-pathogens (L. innocua, L. welshimeri, L. seeligeri); however, its role in non-pathogens is unknown. Sequence analysis revealed 98 % amino acid similarity in LAP from all Listeria species. The N-terminus contains acetaldehyde dehydrogenase (ALDH) and the C-terminus an alcohol dehydrogenase (ADH). Recombinant LAP from L. monocytogenes, L. ivanovii, L. innocua and L. welshimeri exhibited ALDH and ADH activities, and displayed strong binding affinity (K D 2–31 nM) towards Hsp60. Flow cytometry, ELISA and immunoelectron microscopy revealed more surface-associated LAP in pathogens than non-pathogens. Pathogens exhibited significantly higher adhesion (P<0.05) to Caco-2 cells than non-pathogens; however, pretreatment of bacteria with Hsp60 caused 47–92 % reduction in adhesion only in pathogens. These data suggest that biochemical properties of LAP from pathogenic Listeria are similar to those of the protein from non-pathogens in many respects, such as substrate specificity, immunogenicity, and binding affinity to Hsp60. However, protein fractionation analysis of extracts from pathogenic and non-pathogenic Listeria species revealed that LAP was greatly reduced in intracellular and cell-surface protein fractions, and undetectable in the extracellular milieu of non-pathogens even though the lap transcript levels were similar for both. Furthermore, a LAP preparation from L. monocytogenes restored adhesion in a lap mutant (KB208) of L. monocytogenes but not in L. innocua, indicating possible lack of surface reassociation of LAP molecules in this bacterium. Taken together, these data suggest that LAP expression level, cell-surface localization, secretion and reassociation are responsible for LAP-mediated pathogenicity and possibly evolved to adapt to a parasitic life cycle in the host.

Development of an immunological method for studying the incorporation of ripening enzymes in cheese curd

Polyclonal antibodies raised against partly purified aminopeptidase were specific to cell-free extracts ofLactobacillus caseisubsp.pseudoplantarumUL 137, and did not cross react with any other proteins in Cheddar cheese, at least during the first week of maturation. A sandwich enzyme-linked immunosorbent assay (ELISA) was developed in order to quantify cell-free extracts in the cheese curd, and was also used to determine the efficiency of encapsulation of cell-free extracts in liposomes. This method was very sensitive and exhibited a detection limit of ∼10 μg total protein/g cheese and ∼1 μg total protein/ml liposome suspension. The efficiency of encapsulation of cell-free extracts into liposomes was ∼55–60%. The retention of liposome-encapsulated cell-free extracts was ∼14 times that of non-encapsulated extracts.

Production and characterization of a monoclonal antibody to Campylobacter jejuni

Campylobacter species are a group of spiral-shaped bacteria that can cause disease in humans and animals. We developed a high-affinity monoclonal antibody (MAb) probe that recognizes Campylobacter jejuni cells. Cell suspensions grown under microaerobic conditions at 42 degrees C for 20 h on Bolton agar plates with lysed horse blood were used as live and heat-killed preparations, centrifuged at 8,000 x g for 20 min, and resuspended in carbonate buffer (pH 9.6) for coating on the enzyme-linked immunosorbent assay plates. BALB/c mice were immunized with C. jejuni sonicated cells at 10(7) CFU/ml to generate MAb-producing hybridoma clones. Of about 500 initial hybridoma clones, MAb 33D2, which reacted with C. jejuni and Campylobacter coli, was selected for further evaluation. MAb 33D2 is in the immunoglobulin subclass G2a and had relatively weaker reactivity with the C. coli strains tested. MAb 33D2 did not show any cross-reactions with the nine non-Campylobacter bacteria tested in the enzyme-linked immunosorbent assay and had a stronger affinity for C. jejuni as live versus heat-killed cells. In Western blot assays, MAb 33D2 recognized two major antigens of 62 and 43 kDa in extracts from C. jejuni cells but only one antigen of 62 kDa in extracts from C. coli cells.

Screening and characterization of monoclonal antibodies to the surface antigens of Listeria monocytogenes serotype 4b

AIMS

This study aims to develop and characterize monoclonal antibodies (Mabs) with high specificity and affinity for surface antigens of an epidemiologically important serotype 4b of Listeria monocytogenes.

METHODS AND RESULTS

Hybridoma clones were derived from B lymphocytes of mice immunized with L. monocytogenes serotype 4b and screened against this strain by an enzyme-linked immunosorbent assay. Twenty-nine clones secreting Mabs reactive with formalin-killed bacteria were obtained; 15, 8, 5 and 1 Mabs were immunoglobulin subclasses IgG2a, IgG2b, IgM and IgG1, respectively. Immunofluorescence or immunogold labelling demonstrated all except five IgM and one IgG2a Mabs bound to the surface of a live L. monocytogenes serotype 4b. The majority of the 23 surface-binding Mabs recognized linear epitopes on a 77-kDa protein. These surface-binding Mabs exhibited little or no cross-reactivity with non-4b serotypes (1/2a, 1/2b, 3a, etc.) of L. monocytogenes, five other Listeria species, Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium.

CONCLUSIONS

The Mabs recognizing a 77-kDa surface protein are novel antibodies with specificity and affinity for L. monocytogenes serotype 4b.

SIGNIFICANCE AND IMPACT OF THE STUDY

These anti-77 kDa surface protein Mabs may be explored as reagents for the development of Mabs-based diagnostic immunoassays for L. monocytogenes serotype 4b strains.

Effects of Dielectrophoresis on Growth, Viability and Immuno-reactivity of Listeria monocytogenes

Dielectrophoresis (DEP) has been regarded as a useful tool for manipulating biological cells prior to the detection of cells. Since DEP uses high AC electrical fields, it is important to examine whether these electrical fields in any way damage cells or affect their characteristics in subsequent analytical procedures. In this study, we investigated the effects of DEP manipulation on the characteristics of Listeria monocytogenes cells, including the immuno-reactivity to several Listeria-specific antibodies, the cell growth profile in liquid medium, and the cell viability on selective agar plates. It was found that a 1-h DEP treatment increased the cell immuno-reactivity to the commercial Listeria species-specific polyclonal antibodies (from KPL) by ~31.8% and to the C11E9 monoclonal antibodies by ~82.9%, whereas no significant changes were observed with either anti-InlB or anti-ActA antibodies. A 1-h DEP treatment did not cause any change in the growth profile of Listeria in the low conductive growth medium (LCGM); however, prolonged treatments (4 h or greater) caused significant delays in cell growth. The results of plating methods showed that a 4-h DEP treatment (5 MHz, 20 Vpp) reduced the viable cell numbers by 56.8–89.7 %. These results indicated that DEP manipulation may or may not affect the final detection signal in immuno-based detection depending on the type of antigen-antibody reaction involved. However, prolonged DEP treatment for manipulating bacterial cells could produce negative effects on the cell detection by growth-based methods. Careful selection of DEP operation conditions could avoid or minimize negative effects on subsequent cell detection performance.

Characterization of new hybridoma clones producing monoclonal antibodies reactive against both live and heat-killed Listeria monocytogenes

The objective of this study is to develop high affinity monoclonal antibody (MAb) probes recognizing all major serotypes of Listeria monocytogenes cells. From 500 candidate hybridoma clones, 2 new monoclonal antibody-producing hybridomas were selected and evaluated. MAbs 22D10 and 24F6 reacted strongly with live cells of most serotypes of L. monocytogenes except 4c and 4e and with some L. innocua strains; MAb 22D10 reacted strongly with both live and heat-killed cells (100 degrees C for 20 min) of Listeria. Both MAbs 22D10 and 24F6 did not show any cross-reactions with the other non-Listeria G(+) bacteria tested in ELISA. The mixture of EM-7G1 and 22D10 or 24F6 reacted with all 13 major serotypes of live L. monocytogenes except serotype 4c, while none of these 3 MAbs when tested alone did so. MAb 22D10 mixed with 7G1 reacted with all heat-killed L. monocytogenes serotypes except 4c and 4e. In Western blots, MAbs 22D10 and 24F6 reacted with 1 major protein band of 66 kDa in extracts from L. monocytogenes, but with 2 major protein bands of 66 kDa and 76 kDa in extracts from L. innocua. These results suggest that MAbs 22D10 and 24F6 have high affinity for 11 of 13 serotypes of L. monocytogenes, both live and heat-killed cells. MAbs 22D10 and 24F6--in combination with species-specific MAb EM-7G1--should be useful candidates for use in an ELISA sandwich assays for detecting L. monocytogenes in RTE meat and poultry products.

Differential expression of InlB and ActA in Listeria monocytogenes in selective and nonselective enrichment broths

AIM

To investigate the effect of selective and nonselective media on the expression of ActA and InlB proteins in Listeria monocytogenes.

METHODS AND RESULTS

Polyclonal antibodies to InlB and ActA were used in western blotting to determine the effect of selective (BLEB, UVM, and FB) or nonselective (BHI and LB) enrichment broths or hotdog exudates. Of the 13 L. monocytogenes serotypes tested, 11 and 12 serotypes showed a strong InlB expression in brain heart infusion (BHI) and Luria-Bertani (LB), respectively, while only seven and one serotypes showed a strong ActA expression in these two respective broths, and others showed a weaker or no expression. On the contrary, in selective broths, expression of InlB was either very weak or undetectable. However, ActA expression was stronger in 12 serotypes when grown in buffered Listeria enrichment broth (BLEB), 11 in University of Vermont medium (UVM), and 10 in Fraser broth (FB). When tested in hotdog exudates, InlB and ActA were detected in serotypes grown at 37 degrees C but not at 4 degrees C. Transmission electron microscopy, enzyme-linked immunosorbent assay, and mRNA analysis further supported these observations.

CONCLUSION

Overall, selective enrichment broths promote ActA while nonselective broths promote InlB expression.

SIGNIFICANCE AND IMPACT OF THE STUDY

As commonly recommended enrichment broths show differential InlB and ActA expression, proper media must be selected to avoid false results during antibody-based detection of L. monocytogenes.

Bacteria-mediated delivery of nanoparticles and cargo into cells

Nanoparticles and bacteria can be used, independently, to deliver genes and proteins into mammalian cells for monitoring or altering gene expression and protein production. Here, we show the simultaneous use of nanoparticles and bacteria to deliver DNA-based model drug molecules in vivo and in vitro. In our approach, cargo (in this case, a fluorescent or a bioluminescent gene) is loaded onto the nanoparticles, which are carried on the bacteria surface. When incubated with cells, the cargo-carrying bacteria ('microbots') were internalized by the cells, and the genes released from the nanoparticles were expressed in the cells. Mice injected with microbots also successfully expressed the genes as seen by the luminescence in different organs. This new approach may be used to deliver different types of cargo into live animals and a variety of cells in culture without the need for complicated genetic manipulations.

Development of Listeria monocytogenes-specific immunomagnetic beads using a single-chain antibody fragment

A method for coupling single-chain antibody fragments (scFvs) to immunomagnetic beads (IMBs) was developed and evaluated using scFvs specific for Listeria monocytogenes. A plasmid vector, pBAD380, was constructed that allowed the expression of histidine-tagged biotinylated scFvs in Escherichia coli. The gene encoding a scFv specific for L. monocytogenes was cloned into pBAD380 and the 6-histidine-tagged biotinylated anti-L. monocytogenes scFvs were coupled to streptavidin-coated IMBs. The ability of the anti-L. monocytogenes scFv-IMBs to capture L. monocytogenes and other Listeria species was evaluated in comparison to commercially available anti-Listeria IMBs. The anti-L. monocytogenes scFv-IMBs displayed higher efficiencies of capture (1.38-19.04%) for most strains of L. monocytogenes than were observed for the anti-Listeria IMBs (0.05-3.35%); also, the anti-L. monocytogenes scFv-IMBs exhibited improved specificity for L. monocytogenes as determined by cell capture efficiency in pure and mixed cultures.

Prevalence of Antibodies Reactive to Pathogenic and Nonpathogenic Bacteria in Preimmune Serum of New Zealand White Rabbits

Polyclonal antibodies are typically produced in rabbits. The rabbit's health plays an important role in the quality of antibodies produced. Therefore, recommendations have been made by organizations on which bacteria and how frequently to test rabbit colonies. Since it is well known that rabbits may contain cross-reactive antibodies in their preimmune serum, it is common to test the rabbits for reactivity prior to immunization. Here preimmune sera from 19 different rabbits were tested with ELISA against 27 pathogenic and nonpathogenic bacterial cultures. ELISA results showed that Salmonella enterica serovar Typhimurium and Bacillus cereus AS4-12 had the highest average absorbance values (0.60 and 0.54, respectively) and the most preimmune serum samples testing positive was 17. Pseudomonas putrefaciens and B. subtilis had the lowest absorbance values (< 0.1) and did not test positive in any of the preimmune serum samples. Fourteen of the 27 cultures showed positive reactions with 50% or more of the preimmune serum samples tested. Fifty-three percent of the rabbit preimmune sera showed positive reactions with 10 or more bacterial cultures. In Western blot analyses, selected serum samples showing the highest ELISA values reacted with bands in the 97, 36, and 29 kDa regions or with bands in the 63 kDa and 32 kDa regions. Data suggest that the presence of cross-reactive antibodies in the preimmune serum is a common problem amongst the samples tested. Extensive preimmune serum testing should be implemented when polyclonal antibodies are intended for diagnostic testing.

Adhesion characteristics of Listeria adhesion protein (LAP)-expressing Escherichia coli to Caco-2 cells and of recombinant LAP to eukaryotic receptor Hsp60 as examined in a surface plasmon resonance sensor

Listeria adhesion protein (LAP) is an important adhesion factor in Listeria monocytogenes and interacts with its cognate receptor, mammalian heat shock protein 60 (Hsp60). The genetic identity of LAP was determined to be alcohol acetaldehyde dehydrogenase (Aad). A recombinant Escherichia coli strain expressing aad confirmed the involvement of Aad in adhesion to Caco-2 cells. Binding kinetics (ka) of recombinant LAP (rLAP) to Hsp60 was examined in a surface plasmon resonance sensor and was determined to be 5.35 x 10(8) M(-1) s(-1) and it was equivalent to the binding of anti-Hsp60 antibody (ka = 2.15 x 10(9) M(-1) s(-1)) to Hsp60. In contrast, Internalin B, an adhesion/invasion protein from L. monocytogenes, used as a control, had binding kinetics (ka) of only 2.9 x 10(6) M(-1) s(-1). The KD value of rLAP was 1.68 x 10(-8) M, which was significantly lower than Internalin B (KD = 6.5 x 10(-4) M). These results suggest that Hsp60 has significantly higher avidity for anti-Hsp60 antibody and LAP than Internalin B. In summary, LAP is identified as an alcohol acetaldehyde dehydrogenase and binding of recombinant E. coli to Caco-2 cells or rLAP to Hsp60 protein was found to be highly specific.

Production, characterisation and potential application of a novel monoclonal antibody for rapid identification of virulent Listeria monocytogenes

A panel of hybridomas was produced using intact Listeria monocytogenes serotype 1/2a cells as the immunogen. An IgG2a monoclonal antibody (mAb) 'mAb2B3' was isolated that reacted with L. monocytogenes but not with a representative panel of related Listeria spp. and non-Listeria spp. Binding activity was greatest against L. monocytogenes serotype 1/2a and was significantly enhanced when cells were prepared in Listeria enrichment broth (LEB). The reactive epitope was deduced, by immunoblot analysis, to be a surface localised protein of approximately 80 kilodaltons (kDa), putatively assumed to be internalin A (InlA). Recombinant InlA protein was subsequently expressed in Escherischia coli. When crude E. coli cell lysates were subjected to immunoblot analysis, it was demonstrated that the mAb bound specifically to the heterologously expressed recombinant InlA protein, thus confirming the specificity of the mAb. The mAb was further evaluated in a series of enzyme-linked-immunosorbent assay (ELISA)-based formats and in a surface plasmon resonance (SPR)-based biosensor platform. Both configurations were capable of differential identification of virulent L. monocytogenes at concentrations greater than or equal to 1x10(7) cells/ml. Notwithstanding the apparent insensitivity, the results indicate that InlA could be exploited as a marker for highly specific confirmatory identification of pathogenic L. monocytogenes following primary enrichment of suspect food samples, using the anti-InlA antibody 'mAb2B3', described herein.

Selective enrichment media affect the antibody-based detection of stress-exposed Listeria monocytogenes due to differential expression of antibody-reactive antigens identified by protein sequencing

Selective enrichment broths are frequently used to recover stressed Listeria cells to detectable levels, but the ability of antibodies to detect these cells from various commonly used enrichment media is unknown. In this study, a polyclonal (PAb) and monoclonal (MAb) antibody were used to examine the variation in antigen expression on healthy or stress-recovered Listeria monocytogenes cells grown in brain heart infusion broth, buffered Listeria enrichment broth (BLEB), Listeria repair broth (LRB), University of Vermont medium (UVM), and Fraser broth (FB) for immunodetection. Indirect enzyme-linked immunosorbent assay (ELISA) data showed that L. monocytogenes subjected to stresses (acid, cold, heat, and salt) and then grown in BLEB gave the highest reaction with the anti-Listeria PAb while those grown in LRB gave the highest reaction with the MAb C11E9. Cells grown in UVM and FB gave poor ELISA values with both antibodies. Western blotting with PAb revealed differential expression of surface proteins of 62, 58, 50, 43, and 30 kDa on L. monocytogenes cells, with most proteins displaying elevated expression in BLEB and LRB but reduced or no expression in UVM or FB. Similar differential expressions were noticed for C11E9. PAb-reactive proteins were identified as putative LPXTG-motif cell-wall anchor-domain protein (62 kDa; lmo0610), flavocytochrome C fumarate reductase chain A homolog protein (58 kDa; lmo0355), enolase (50 kDa; lmo2455), glyceraldehyde 3-phosphate dehydrogenase (43 kDa; lmo2459), and hypothetical phospho-sugar binding protein (30 kDa; lmo0041), respectively, and the MAb-reactive 66-kDa protein was confirmed to be N-acetylmuramidase (lmo2691). In conclusion, BLEB and LRB favorably supported increased expression of antigens and proved to be superior to UVM and FB for immunodetection of stressed L. monocytogenes cells.

Single-chain Fv antibody with specificity for Listeria monocytogenes

Single chain antibodies (scFv) exhibiting specific binding to Listeria monocytogenes strains were isolated from a pool of random scFvs expressed on the surface of filamentous bacteriophages. Positive selection (panning) using L. monocytogenes was used to enrich for phage clones with the desired binding affinity, and negative selection using L. innocua and L. ivanovii was used to remove phages expressing cross-reactive antibody fragments. A single phage clone, P4:A8, was selected using two independent panning schemes. A rapid assay was devised to determine phage antibody binding specificity and was used to develop a selectivity profile for individual phage clones. The P4:A8 clone was screened against a panel of bacteria consisting of eight strains of L. monocytogenes, one each of the other six species of Listeria and nine other relevant bacterial species. A collection of individual clones from the penultimate panning was also screened against a subset of the panel of bacteria. The selectivity profiles indicate that multiple clones, including P4:A8, exhibit binding to one or more strains of L. monocytogenes without cross-reactivity toward any other species in the panel. This is the first report of a species-specific antibody for viable cells of L. monocytogenes (i.e., the ability to bind to L. monocytogenes without cross-reactivity toward any other species of Listeria).

A multifunctional micro-fluidic system for dielectrophoretic concentration coupled with immuno-capture of low numbers of Listeria monocytogenes

In this study, we demonstrated a micro-fluidic system with multiple functions, including concentration of bacteria using dielectrophoresis (DEP) and selective capture using antibody recognition, resulting in a high capture efficiency of bacterial cells. The device consisted of an array of oxide covered interdigitated electrodes on a flat silicon substrate and a approximately 16 microm high and approximately 260 microm wide micro-channel within a PDMS cover. For selective capture of Listeria monocytogenes from the samples, the channel surface was functionalized with a biotinylated BSA-streptavidin-biotinylated monoclonal antibody sandwich structure. Positive DEP (at 20 V(pp) and 1 MHz) was used to concentrate bacterial cells from the fluid flow. DEP could collect approximately 90% of the cells in a continuous flow at a flow rate of 0.2 microl min(-1) into the micro-channel with concentration factors between 10(2)-10(3), in sample volumes of 5-20 microl. A high flow rate of 0.6 microl min(-1) reduced the DEP capture efficiency to approximately 65%. Positive DEP attracts cells to the edges of the electrodes where the field gradient is the highest. Cells concentrated by DEP were captured by the antibodies immobilized on the channel surface with efficiencies of 18 to 27% with bacterial cell numbers ranging from 10(1) to 10(3) cells. It was found that DEP operation in our experiments did not cause any irreversible damage to bacterial cells in terms of cell viability. In addition, increased antigen expression (antigens to C11E9 monoclonal antibody) on cell membranes was observed following the exposure to DEP.

Performance evaluation of a low conductive growth medium (LCGM) for growth of healthy and stressed Listeria monocytogenes and other common bacterial species

The performance of a low conductive growth medium (LCGM) (conductivity of <1300 microS) was evaluated for its ability to support growth of food borne bacterial pathogens including Listeria monocytogenes and to determine the expression of the two key virulence proteins in L. monocytogenes for possible applications in an impedance-based microfluidic biochip detection platform. Growth of Listeria was monitored spectrophotometrically and the lag phase, generation time, growth rate and maximum population density were determined using the Gompertz equation. LCGM had a lag phase of 2.3 h and showed a higher cell density compared to Luria Bertini (LB) broth. Length of lag phase was highly dependent on initial inoculum concentrations. The changes in conductivity with respect to growth in the low conductive medium were monitored using a conductivity probe. L. monocytogenes growth could be detected within 2 h (0.1 mS) in LCGM and within 6 h in LB. The performance of the media was also evaluated for the recovery of Listeria cells exposed to various stresses as 42 degrees C for 1, 2 or 6 h, an osmotic stress in 10.5% NaCl, an acidic stress at pH 2, 3 or 5 and a combined stress of 10.5% NaCl, pH 5 and 1 h exposure at 42 degrees C. The recovery rate was comparable with that of Tryptic soy broth containing yeast extract (TSBYE). L. monocytogenes in LCGM supported the expression of two key virulence markers, actin polymerization protein (ActA) and internalin B (InlB), which could be detected using specific antibodies. In general LCGM also supported the growth of several other bacterial species suggesting its implication in microbial quality monitoring of products. In conclusion, LCGM is a sensitive low conductive medium that supports the growth as well as the expression of virulence markers for potential applications in sensitive detection of L. monocytogenes or other food borne pathogens in impedance-based sensor platform.

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.

Monoclonal antibodies binding to the cell surface of Listeria monocytogenes serotype 4b

Serotype 4b strains of the food-borne pathogen Listeria monocytogenes are responsible for a large portion of sporadic listeric infections and all major food-borne listeriosis outbreaks in humans. Hybridomas were produced from three fusions with lymphocytes of ND4 mice immunized either with the insoluble antigens of L. monocytogenes serotype 4b or with formalin-killed bacterial cells and screened for monoclonal antibodies (mAbs) reactive to L. monocytogenes serotype 4b. A set of 35 mAbs was identified by ELISA as having reactivity with both the insoluble antigen fraction and the whole-cell antigens. Thirteen of these mAbs belonged to immunoglobulin subclass G1 (IgG1), fifteen were IgG2a and seven mAbs were IgM. Only 20 out of the 35 mAbs were capable of detecting protein bands of various sizes ranging from 20 to 88 kDa in Western blots. Two of these mAbs, M2365 and M2367, were capable of binding to cell-surface antigens of live L. monocytogenes serotype 4b, as demonstrated by immunofluorescence microscopy and immunogold transmission electron microscopy. Immunofluorescence microscopy showed that M2365 and M2367 failed to bind to the cell surfaces of Escherichia coli O157 : H7, Salmonella enterica (serotype Typhimurium DT104) or Campylobacter jejuni. Evaluation of the cross-reactions of all 35 mAbs with whole-cell antigens of E. coli O157 : H7, S. Typhimurium, C. jejuni and Listeria innocua by ELISA indicated that the majority of the mAbs, including M2365 and M2367, did not cross-react with E. coli O157 : H7, S. Typhimurium or C. jejuni and showed no or a very weak reaction with L. innocua. Furthermore, M2365 and M2367 showed no reaction with whole-cell antigens derived from L. monocytogenes serotypes 1/2a, 1/2b and 3a, and from Listeria grayi, Listeria ivanovii and Listeria seeligeri, in an ELISA. Collectively, these data suggest that M2365 and M2367 have potential use in the development of immunological methods of laboratory diagnosis for L. monocytogenes serotype 4b in clinical or food samples.

Specific detection of cytopathogenic Listeria monocytogenes using a two-step method of immunoseparation and cytotoxicity analysis

The development of rapid methods for detection of viable Listeria monocytogenes is crucial to prevent listeriosis and product recalls. While immunomagnetic separation has been used for isolating Listeria spp., lack of specificity and pathogenicity determination render this method unsatisfactory. A two-step method using Protein A agarose beads (Immunobeads) coated with a more specific antibody, monoclonal antibody (MAb)-C11E9 for L. monocytogenes was developed. Immunobeads were allowed to capture Listeria cells from a variety of samples and tested for cytopathogenic action on a murine hybridoma B-lymphocyte, Ped-2E9 cell line by Trypan blue staining, and by an alkaline phosphatase (AP)-based cytotoxicity assay. The two-step method was used to test uninoculated hotdogs, bologna, and raw beef, chicken, and pork samples, following selective enrichment in half-Fraser broth. Pure culture studies proved the assay to be specific for L. monocytogenes, while a similar assay with Dynal Anti-Listeria immunomagnetic beads was positive for L. monocytogenes, L. ivanovii, and L. seeligeri. Detection and confirmation of cytopathogenicity of Listeria cells from food samples after 24-h selective enrichment were completed in 2-4 h. Isolates were further analyzed by the CAMP test for hemolytic activity and RiboPrinter for genomic patterns. Using immunoseparation and cytotoxicity as a two-step rapid method, viable L. monocytogenes could be isolated, detected, and confirmed as cytopathogenic in 28 h or less.

Detection of low levels of Listeria monocytogenes cells by using a fiber-optic immunosensor

Biosensor technology has a great potential to meet the need for sensitive and nearly real-time microbial detection from foods. An antibody-based fiber-optic biosensor to detect low levels of Listeria monocytogenes cells following an enrichment step was developed. The principle of the sensor is a sandwich immunoassay where a rabbit polyclonal antibody was first immobilized on polystyrene fiber waveguides through a biotin-streptavidin reaction to capture Listeria cells on the fiber. Capture of cells on the fibers was confirmed by scanning electron microscopy. A cyanine 5-labeled murine monoclonal antibody, C11E9, was used to generate a specific fluorescent signal, which was acquired by launching a 635-nm laser light from an Analyte 2000 and collected by a photodetector at 670 to 710 nm. This immunosensor was specific for L. monocytogenes and showed a significantly higher signal strength than for other Listeria species or other microorganisms, including Escherichia coli, Enterococcus faecalis, Salmonella enterica, Lactobacillus plantarum, Carnobacterium gallinarum, Hafnia alvei, Corynebacterium glutamicum, Enterobacter aerogenes, Pseudomonas aeruginosa, and Serratia marcescens, in pure or in mixed-culture setup. Fiber-optic results could be obtained within 2.5 h of sampling. The sensitivity threshold was about 4.3 x 10(3) CFU/ml for a pure culture of L. monocytogenes grown at 37 degrees C. When L. monocytogenes was mixed with lactic acid bacteria or grown at 10 degrees C with 3.5% NaCl, the detection threshold was 4.1 x 10(4) or 2.8 x 10(7) CFU/ml, respectively. In less than 24 h, this method could detect L. monocytogenes in hot dog or bologna naturally contaminated or artificially inoculated with 10 to 1,000 CFU/g after enrichment in buffered Listeria enrichment broth.

Heat shock protein 60 acts as a receptor for the Listeria adhesion protein in Caco-2 cells

The 104-kDa Listeria adhesion protein (LAP) in Listeria monocytogenes is involved in binding to various mammalian cell lines. However, the receptor that interacts with LAP in eukaryotic cells is unknown. In this study, scanning immunoelectron microscopy qualitatively demonstrated greater binding capacity of wild-type (WT) L. monocytogenes strain (F4244) than a LAP-deficient mutant strain (KB208) to Caco-2 cells. The goal of this study was identification of the host cell receptor for LAP. Using a Western blot ligand overlay assay, we identified a protein of 58 kDa to be the putative receptor for LAP from Caco-2 cells. N-terminal sequencing and subsequent database search identified this protein as heat shock protein 60 (Hsp60). Modified immunoseparation with protein A-Sepharose beads bound to the LAP-specific monoclonal antibody H7 (MAb-H7) and a sequential incubation with LAP preparation and Caco-2 lysate confirmed the receptor to be the same 58-kDa protein. Western blot analysis with anti-Hsp60 MAb of whole-cell adhesion between Caco-2 and WT also revealed the receptor protein to be a 58-kDa protein, thus corroborating the identification of Hsp60 as a host cell receptor for LAP. Furthermore, the anti-Hsp60 antibody also caused approximately 74% reduction in binding of L. monocytogenes WT to Caco-2 cells, whereas a control antibody, C11E9, had no effect on binding. The adhesion mechanism of L. monocytogenes to eukaryotic cells is a complex process, and identification of Hsp60 as a receptor for LAP adds to the list of previously discovered ligand-receptor modules that are essential to achieve successful adhesion.

A colony blot immunoassay for the rapid identification of Bacillus cereus

A colony blot immunoassay was developed for the rapid identification of Bacillus cereus using antibodies against the 28.5-kDa cell-surface antigen of B. cereus. Suspect colonies from plates were blotted onto a Whatman #541 membrane, dried, and fixed by UV irradiation. The membrane was then immersed in an anti-B. cereus antibody-horseradish peroxidase conjugate for 60 min. After washing and reacting with 4-chloro-1-naphthol and H2O2, the appearance of purple spots indicated the presence of B. cereus. This assay effectively identified 61 of 62 B. cereus strains tested. Among 38 non-B. cereus strains, which were other Bacillus spp. (19 genera), 36 gave true-negative results, and 2 showed false-positive results. The sensitivity and specificity for B. cereus were 98.4 and 94.7%, respectively. The present assay is easy to use, and the rapid identification of B. cereus can be completed in 2.5 h.

Expression of cellular antigens of Listeria monocytogenes that react with monoclonal antibodies C11E9 and EM-7G1 under acid-, salt- or temperature-induced stress environments

AIMS

To study the expression of cellular antigens of Listeria monocytogenes that react with monoclonal antibodies (MAbs) C11E9 and EM-7G1 under acid-, salt- or temperature-induced stress environments.

METHODS AND RESULTS

The reaction patterns of antibodies to L. monocytogenes held in stressful environments for a short duration (3 h) or grown for extended periods (16-72 h) were investigated. During both short or prolonged exposure to stress environments of high temperature (45 degrees C) and NaCl (>1.5%, w/v), reactions of whole cells of L. monocytogenes to antibodies were severely affected as determined by ELISA and by the reduced expression of the antibody-reactive 66 kDa antigen in the Western blot assay. Conversely, cold (4-15 degrees C) or acid (pH 2-3) stress environments had very little effect on antigen expression or antibody reaction. Additionally, heat-killed cells showed reduced reactions to these antibodies when compared with unheated cells. Artificially created stress environments in hotdog slurry also affected the antigen expression in L. monocytogenes. Immunoelectron microscopy revealed that the antibody-reactive antigens were uniformly present on the surface of the cells. Morphological characteristics following growth in stressed environments revealed that heat stress at 45 degrees C caused L. monocytogenes cells to be elongated and to form clumps; whereas, osmotic stress (5.5% NaCl, w/v) caused filamentous appearance with multiple septa along the length of the cell.

CONCLUSIONS

These results indicated that MAb C11E9 or EM-7G1 could detect L. monocytogenes from cold or acid-stress environments; however, they may show weaker reactions with heat or osmotically stressed cells or cells grown at 4 degrees C.

SIGNIFICANCE AND IMPACT OF THE STUDY

Bacteria in food are routinely subjected to various stresses, induced by cold, heat, salt or acid during processing and storage. Whether stresses would modify the expression of cellular antigens of L. monocytogenes is of a great concern for immunodetections in food products.

Characterization and application of a Listeria monocytogenes reactive monoclonal antibody C11E9 in a resonant mirror biosensor

Typical detection of Listeria monocytogenes involves selective enrichment, isolation and biochemical testing. Development of antibodies to Listeria species has improved detection; however, most antibodies detect all species of Listeria. A previously developed monoclonal antibody (MAb)-C11E9 was examined for its reaction to 13 L. innocua and 40 L. monocytogenes strains representing all 13 serotypes by ELISA. Absorbance values for L. monocytogenes strains were 0.44-3.58 and for L. innocua 0.22-1.44. ELISA reactions were divided into three arbitrary groups of high (Abs 1.0 or higher), intermediate (0.6-0.99) and low (0.18-0.59). Most L. monocytogenes strains (32/41, 78%) were in the high group while only 23% (3/13) of L. innocua were in the same group. In the Western blot assay, antibody reacted with phosphate-buffered saline (PBS) extracted protein preparations of 52, 66 and 97 kDa. Ribopattern of all strains was analyzed and no clear relationship was observed for antibody reaction and ribotype of a given strain. MAb C11E9 was used in a resonant mirror biosensor (IAsys sensor), but failed to detect any viable intact L. monocytogenes cells at levels as high as 10(8) cells/ml; however, it showed binding (85-150 arc/s) with the surface protein preparations containing the 97-, 66- and 52-kDa proteins at 208 mug/ml. Binding kinetics of L. monocytogenes and L. innocua surface protein extracts showed significantly (p<0.05) higher responses than the three other Listeria species (L. ivanovii, L. welshimeri and L. grayi), which could be detected in 10-20 min. These data corroborate with ELISA results. In summary, this study suggest that MAb-C11E9 is suitable for detection of all serotypes of L. monocytogenes despite cross-reaction with L. innocua and could be used for detection of soluble protein extracts in the resonant mirror (IAsys) biosensor.

Micro-assembly of functionalized particulate monolayer on C18-derivatized SiO2 surfaces

This work describes a simple approach to immobilize functionalized colloidal microstructures onto a C(18)-coated SiO(2) substrate via specific or non-specific bio-mediated interactions. Biotinylated bovine serum albumin pre-adsorbed onto a C(18) surface was used to mediate the surface assembly of streptavidin-coated microbeads (2.8 microm), while a bare C(18) surface was used to immobilize anti-Listeria antibody-coated microbeads (2.8 microm) through hydrophobic interactions. For a C(18) surface pre-adsorbed with bovine serum albumin, hydrophobic polystyrene microbeads (0.8 microm) and positively charged dimethylamino microbeads (0.8 microm) were allowed to self-assemble onto the surface. A monolayer with high surface coverage was observed for both polystyrene and dimethylamino microbeads. The adsorption characteristics of Escherichia coli and Listeria monocytogenes on these microbead-based surfaces were studied using fluorescence microscopy. Both streptavidin microbeads pre-adsorbed with biotinylated anti-Listeria antibody and anti-Listeria antibody-coated microbeads showed specific capture of L. monocytogenes, while polystyrene and dimethylamino microbeads captured both E. coli and L. monocytogenes non-specifically. The preparation of microbead-based surfaces for the construction of microfluidic devices for separation, detection, or analysis of specific biological species is discussed.

Composite surface for blocking bacterial adsorption on protein biochips

The design and fabrication of protein biochips requires characterization of blocking agents that minimize nonspecific binding of proteins or organisms. Nonspecific adsorption of Escherichia coli, Listeria innocua, and Listeria monocytogenes is prevented by bovine serum albumin (BSA) or biotinylated BSA adsorbed on SiO(2) surfaces of a biochip that had been modified with a C(18) coating. Biotinylated BSA forms a protein-based surface that in turn binds streptavidin. Because streptavidin has multiple binding sites for biotin, it in turn anchors other biotinylated proteins, including antibodies. Hence, biotinylated BSA simultaneously serves as a blocking agent and a foundation for binding an interfacing protein, avidin or streptavidin, which in turns anchors biotinylated antibody. In our case, the antibody is C11E9, an IgG-type antibody that binds Listeria spp. Nonspecific adsorption of another bacterium, Escherichia coli, is also minimized due to the blocking action of the BSA. The blocking characteristics of BSA adsorbed on C(18)-derivatized SiO(2) surfaces for construction of a protein biochip for electronic detection of pathogenic organisms is investigated.