Comparison of a cultural method with ListerScreen plus Rapid'L.mono or PCR-ELISA methods for the enumeration of L. monocytogenes in naturally contaminated sewage sludge

A streamlined procedure for advancing the detection and isolation of Listeria monocytogenes from artificially contaminated ground beef in a single working day

Listeria monocytogenes, a rod-shaped Gram-positive bacterium widely distributed in nature, can contaminate foods and represents a foodborne pathogen of public health significance causing a high mortality rate of 20%-30%. Rapid and reliable identification of foods and food-processing environments contaminated with L. monocytogenes is a crucial step in implementing effective intervention strategies to ensure food safety and limit the transmission of bacteria to humans. This study designed and refined a practical workflow to streamline and accelerate the detection of a low level of L. monocytogenes present in ground beef. The workflow coupled an abbreviated 5 h culture enrichment in PALCAM liquid medium with physical separation (filtration and centrifugation) to preprocess enrichment samples. Specific capture was achieved using magnetic separation with a bacteriophage endolysin-derived cell wall-binding domain in a Hyglos Listeria capture kit. Molecular detection was performed using a MicroSEQ L. monocytogenes RTi-PCR detection kit combined with a nested PCR strategy. Preprocessing of enrichment culture samples using a multi-stage filtration system constructed for the study or commercially available BagFilter Pull-up filter bags, in conjunction with centrifugation, enabled the recovery of ~30 colony-forming units (CFUs) from the enrichment culture of a 25 g ground beef sample artificially contaminated with 1 CFU of L. monocytogenes. Integration of magnetic separation into the workflow for capturing L. monocytogenes cells specifically from preprocessed samples and further cleaning up the samples yielded bacterial counts similar to those obtained by direct plating of preprocessed samples. The RTi-PCR-based molecular detection method integrated into the workflow was capable of detecting pure cultures of L. monocytogenes as low as 12.5 CFUs. Evaluation of the workflow using artificially ground beef demonstrated the consistent detection of L. monocytogenes within an 8 h workday in a 25 g sample unit containing the cell count as low as 2 CFU following a 5 h culture enrichment.

IMPORTANCE

Consuming foods contaminated with the bacterial pathogen Listeria monocytogenes can lead to the development of human listeriosis, a severe and life-threatening foodborne illness. Timely detection of L. monocytogenes present at a low level in foods and food processing environments is a necessary measure to prevent the spread of the Listeria-associated illness. This study designed and evaluated a multi-step workflow for testing L. monocytogenes in artificially contaminated food samples. The workflow was composed of a short 5 h culture enrichment, filtration-based sample preprocessing, magnetic separation, a single-tube nested RTi-PCR, and culture plating. It allowed L. monocytogenes to be detected within 8 h from a 25 g ground beef sample containing the target cells as low as 2 colony-forming units, significantly improving and streamlining the detection methods for this important foodborne pathogen.

Targeting Novel LPXTG Surface Proteins with Monoclonal Antibodies for Immunomagnetic Separation of Listeria monocytogenes

The Gram-positive bacterium Listeria monocytogenes causes a significantly high percentage of fatalities among human foodborne illnesses. Surface proteins, specifically expressed from a wide range of L. monocytogenes serotypes under selective enrichment culture conditions, can serve as targets for the isolation of this pathogen using antibody-based methods to facilitate molecular detection. In this study, monoclonal antibodies (MAbs), previously raised against the L. monocytogenes LPXTG surface proteins LMOf2365_0639 and LMOf2365_0148, were investigated for their ability to isolate L. monocytogenes from bacterial samples with immunomagnetic separation (IMS). Only 1 out of 35 MAbs against LMOf2365_0639, M3644, was capable of capturing L. monocytogenes. Among all the 24 MAbs examined against LMOf2365_0148, 4 MAbs, M3686, M3697, M3699, and M3700, were capable of capturing L. monocytogenes cells specifically from abbreviated primary selective enrichment cultures in either Palcam or LEB/UVM1 media or from mixed samples containing target and nontarget bacteria. MAb M3686 showed a unique specificity with the capability to capture strains of seven L. monocytogenes serotypes (1/2a, 1/2b, 1/2c, 3a, 4a, 4b, and 4d). These promising MAbs were subsequently characterized by quantitative measurements of antigen-binding affinity using surface plasmon resonance analysis and epitope mapping using overlapping recombinant polypeptides. The usefulness of these MAbs to LMOf2365_0148 in bacterial capture was consistent with their high affinities with K_D constants in the nanomolar range and can be explored further for the development of an automated IMS method suitable for routine isolation of L. monocytogenes from food and environmental samples.

Performance evaluation of waste stabilization ponds on removal of Listeria spp.: a case study of Isfahan, Iran

The aim of the present study was to assess performance of waste stabilization ponds (WSPs) on the removal of Listeria spp. in Isfahan, Iran. A total of 104 samples were taken from eight sampling locations from influent and effluent of a wastewater treatment plant (WWTP). Sewage samples were analyzed for the presence of Listeria spp. using selective enrichment protocol. Listeria isolates were also identified by biochemical and polymerase chain reaction (PCR) amplification. Listeria spp. was enumerated by a three tube most probable number (MPN) for total coliform counts (TC), fecal coliform counts (FC), total suspended solids (TSS), and total dissolved solids (TDS). In total, 54/104 (51.92%), 49/104 (47.11%), 36/104 (34.61%), and 27/104 (25.95%) samples were positive for Listeria spp., L. monocytogenes, L. innocua, and L. seeligeri, respectively. The mean MPN/100 mL enumeration of L. monocytogenes for influent, anaerobic, facultative ponds 1, 2, 3, 4 and maturation ponds 1 and 2 were 21.54, 10.61, 8, 5.77, 4, 2.54, 1.38, and 0.46, respectively. The removal percentage of Listeria spp. in the anaerobic, facultative, and maturation ponds were 44.71, 76.5, and 81.4%, respectively. Results showed that the WSPs were able to decrease the Listeria spp. levels significantly, although unable to remove them completely.

Recombinase Polymerase Amplification Combined with Lateral Flow Strip for Listeria monocytogenes Detection in Food

Listeria monocytogenes is an important food-borne pathogenic bacterium that causes human disease, resulting in economic losses worldwide. The current detection methods for L. monocytogenes are not well suited for direct field testing because they involve complicated, time-consuming operations. A simple, efficient method is vital for L. monocytogenes detection. In this study, we combined isothermal recombinase polymerase amplification (RPA) with a lateral flow (LF) strip to rapidly and reliably detect L. monocytogenes. In the presence of biotin- and digoxin-modified primers, RPA produced numerous digoxin- and biotin-attached duplex DNA products. These products were detected on an LF strip via dual immunoreactions (digoxin on the duplex DNA reacted with the anti-digoxin antibody on the gold nanoparticle (Au-NP) and the biotin on the duplex DNA captured by the streptavidin on the LF test zone). The accumulation of Au-NPs produced characteristic bands, enabling the visual detection of L. monocytogenes without instrumentation. This assay could be used to detect L. monocytogenes within 15 min, including DNA amplification with RPA for 10 min at 39 °C and visualization of the amplicons by LF strips for 5 min. Experiments confirmed a detection limit as low as 300 fg of DNA and 1.5 × 10¹ CFU in pure cultures. Furthermore, RPA-LF exhibited no cross-reactions with pathogens. Evaluation of the method with food samples indicated that the detection limit was substantially improved to 1.5 × 10° CFU for the original bacterial content in 25 g/mL samples after enrichment for 6 hr. RPA-LF can be used as a sensitive and rapid detection technique for L. monocytogenes.

PRACTICAL APPLICATION

Recombinase polymerase amplification (RPA) can amplify target DNA at 37 to 42 °C without a thermal cycler. Lateral flow (LF) strips are portable, cheap and easy to operate. RPA combined with LF strips to detect Listeria monocytogenes can be widely used in remote areas.

A novel method to detect Listeria monocytogenes via superparamagnetic lateral flow immunoassay

A novel strip test system combining immunomagnetic separation with lateral flow immunoassay (LFIA) was established for the accurate detection of Listeria monocytogenes. In this system, a pair of matched monoclonal antibodies was used to construct a sandwich immunoassay, in which superparamagnetic particles were coupled with one of the antibodies as a labeled antibody to capture the target bacteria, while the other antibody was immobilized on the detection zone. After a 20-min reaction, the strips were analyzed by a novel instrument which could detect the magnetic signal of the immunocomplex in a magnetic field. Sensitivity evaluation showed that the limit of detection (LOD) of the superparamagnetic LFIA system for L. monocytogenes was 10(4) CFU/mL, which was at least one log lower than conventional LFIA. No cross-reaction was observed when Salmonella, Escherichia coli O157:H7, or three types of harmless Listeria strains were tested. Further evaluation with actual food samples indicated that the superparamagnetic LFIA system showed 100 % concordance with real-time PCR. Therefore, this novel superparamagnetic LFIA system could be used as a rapid, sensitive, and specific method for the detection of L. monocytogenes.

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.

Survival of Listeria monocytogenes and Enterococcus faecium in sludge evaluated by real-time PCR and culture methods

AIMS

This study evaluates the behaviour in spiked sludge of a pathogenic bacteria, Listeria monocytogenes, by cultural and molecular techniques, and compares its survival with the one of a faecal indicator, Enterococcus faecium.

METHODS AND RESULTS

Listeria monocytogenes strain Scott A and E. faecium(T) were followed for 17 days after inoculation in sludge. Kinetics of survival depended on the bacteria and on the technique used [most probable number method, direct plate count or real-time quantitative PCR (qPCR)]. The concentration of L. monocytogenes decreased rapidly regardless of the technique, but the decrease was much more dramatic with culture techniques than with qPCR. On the contrary, the concentrations of culturable E. faecium(T) were stable.

CONCLUSIONS

The results suggest that the cells of L. monocytogenes strain Scott A might have entered a viable, but nonculturable (VBNC) status, whereas cells of the indicator bacteria, E. faecium(T), maintained themselves better and stayed culturable.

SIGNIFICANCE AND IMPACT OF THE STUDY

The difference of survival kinetics in the sludge of a faecal indicator (E. faecium) and a pathogenic bacterium (L. monocytogenes) may be linked to the fact that they either enter or do not enter into a VBNC status.

Development of solution phase hybridisation PCR-ELISA for the detection and quantification of Enterococcus faecalis and Pediococcus pentosaceus in Nurmi-type cultures

Nurmi-type cultures (NTCs), derived from the fermentation of caecal contents of specifically pathogen-free (SPF) birds, have been used successfully to control salmonella colonisation in chicks. These cultures are undefined in nature and, consequently, it is difficult to obtain approval from regulatory agencies for their use as direct fed microbials (DFMs) for poultry. Progress towards the generation of effective defined probiotics requires further knowledge of the composition of these cultures. As such, species-specific, culture-independent quantification methodologies need to be developed to elucidate the concentration of specific bacterial constituents of NTCs. Quantification of specific bacterial species in such ill-defined complex cultures using conventional culturing methods is inaccurate due to low levels of sensitivity and reproducibility, in addition to slow turnaround times. Furthermore, these methods lack selectivity due to the nature of the accompanying microflora. This study describes the development of a rapid, sensitive, reliable, reproducible, and species-specific culture-independent, solution phase hybridisation PCR-ELISA procedure for the detection and quantification of Enterococcus faecalis and Pediococcus pentosaceus in NTCs. In this technique, biotin-labelled primers were designed to amplify a species-specific fragment of a marker gene of known copy number, in both species. Resulting amplicons were hybridised with a dinitrophenol (DNP)-labelled oligonucleotide probe in solution and were subsequently captured on a streptavidin-coated microtitre plate. The degree of binding was determined by the addition of IgG (anti-DNP)-horseradish peroxidase conjugate, which was subsequently visualised using a chromogenic substrate, tetramethylbenzidine. This novel quantitative method was capable of detecting E. faecalis and P. pentosaceus at levels as low as 5 CFU per PCR reaction.

Methods for the isolation and identification of Listeria spp. and Listeria monocytogenes: a review

Listeria monocytogenes is an important food-borne pathogen and is widely tested for in food, environmental and clinical samples. Identification traditionally involved culture methods based on selective enrichment and plating followed by the characterization of Listeria spp. based on colony morphology, sugar fermentation and haemolytic properties. These methods are the gold standard; but they are lengthy and may not be suitable for testing of foods with short shelf lives. As a result more rapid tests were developed based on antibodies (ELISA) or molecular techniques (PCR or DNA hybridization). While these tests possess equal sensitivity, they are rapid and allow testing to be completed within 48 h. More recently, molecular methods were developed that target RNA rather than DNA, such as RT-PCR, real time PCR or nucleic acid based sequence amplification (NASBA). These tests not only provide a measure of cell viability but they can also be used for quantitative analysis. In addition, a variety of tests are available for sub-species characterization, which are particularly useful in epidemiological investigations. Early typing methods differentiated isolates based on phenotypic markers, such as multilocus enzyme electrophoresis, phage typing and serotyping. These phenotypic typing methods are being replaced by molecular tests, which reflect genetic relationships between isolates and are more accurate. These new methods are currently mainly used in research but their considerable potential for routine testing in the future cannot be overlooked.