Listeria monocytogenes in foods-From culture identification to whole-genome characteristics

Listeria monocytogenes is an important foodborne pathogen, which is able to persist in the food production environments. The presence of these bacteria in different niches makes them a potential threat for public health. In the present review, the current information on the classical and alternative methods used for isolation and identification of L. monocytogenes in food have been described. Although these techniques are usually simple, standardized, inexpensive, and are routinely used in many food testing laboratories, several alternative molecular-based approaches for the bacteria detection in food and food production environments have been developed. They are characterized by the high sample throughput, a short time of analysis, and cost-effectiveness. However, these methods are important for the routine testing toward the presence and number of L. monocytogenes, but are not suitable for characteristics and typing of the bacterial isolates, which are crucial in the study of listeriosis infections. For these purposes, novel approaches, with a high discriminatory power to genetically distinguish the strains during epidemiological studies, have been developed, e.g., whole-genome sequence-based techniques such as NGS which provide an opportunity to perform comparison between strains of the same species. In the present review, we have shown a short description of the principles of microbiological, alternative, and modern methods of detection of L. monocytogenes in foods and characterization of the isolates for epidemiological purposes. According to our knowledge, similar comprehensive papers on such subject have not been recently published, and we hope that the current review may be interesting for research communities.

Development of loop-mediated isothermal amplification-lateral flow dipstick as a rapid screening test for detecting Listeria monocytogenes in frozen food products using a specific region on the ferrous iron transport protein B gene

Background and Aim: Listeria monocytogenes is a critical foodborne pathogen that infects pregnant females and their newborns and older adults and individuals with comorbidities. It contaminates fresh vegetables, fruits, ready-to-eat foods, and frozen food products consumed by individuals. The culture conventional detection methods for L. monocytogenes are time-consuming, taking 4 days. This study aimed to describe the development and comparison of loop-mediated isothermal amplification (LAMP)- lateral flow dipstick (LFD), LAMP assay to PCR, and conventional culture for detecting L. monocytogenes in frozen food products. Materials and Methods: Five LAMP primer sets, including F3, B3, forward inner primer, and backward inner primer, were designed from a specific region on ferrous iron transport protein B gene (feoB gene) to amplify LAMP products. The DNA probe was created, and the detection limit was determined in pure culture and purified DNA, as well as the detection in 20 frozen food product samples. Results: The LMfeoB4 LAMP primer sets and DNA probe were LAMP products amplified at 60°C for 50 min. The specificity of the assay revealed no cross-reactivity with other pathogenic bacteria. The limit of detection (LOD) of the LAMP-LFD and LAMP assays using purified genomic DNA was 219 fg/μL both in LAMP and LAMP-LFD assays. The LOD of LAMP and LAMP-LFD assays in pure culture was 4.3×102 colony-forming unit (CFU)/mL and 43 CFU/mL, respectively. The LOD of the LAMP-LFD assay using artificially inoculated chicken in frozen food samples with pre-enrichment was 3.2×102 CFU/mL. The LAMP-LFD was also more sensitive than the LAMP assay and polymerase chain reaction. Finally, LAMP-LFD revealed no false positives in any of the 20 frozen food product samples. Conclusion: LAMP-LFD assay using a specific region on the feoB gene to detect L. monocytogenes was highly specific, sensitive, faster, and convenient, making it a valuable tool for the monitoring and rapid screening of L. monocytogenes in frozen food products. This technique is applicable to the development of detection technologies for other pathogens in food products.

Cas12aFDet: A CRISPR/Cas12a-based fluorescence platform for sensitive and specific detection of Listeria monocytogenes serotype 4c

The CRISPR/Cas12a system has displayed remarkable potential in the development of new methods for nucleic acid detection owing to the trans-cleavage activity of Cas12a. Despite the tremendous development in recent years, existing CRISPR/Cas12a-based methods have several limitations such as the time-consuming process, which takes up to 2 h, and the risk of aerosol contamination during DNA amplicon transfer. Herein, we propose a CRISPR/Cas12a-based fluorescence detection platform named "Cas12aFDet" for rapid nucleic acid detection that overcomes these limitations. By integrating PCR or recombinase-aided amplification (RAA) methods with Cas12a-mediated cleavage in a sealed reaction tube, Cas12aFDet-based detection of amplified products could be accomplished within 15 min, while avoiding amplicon contamination. The detection limits of PCR-based Cas12aFDet and RAA-based Cas12aFDet were determined to be 3.37 × 10¹ cfu/mL and 1.35 × 10² cfu/mL of Listeria monocytogenes serotype 4c in pure culture, respectively. Most importantly, RAA-based Cas12aFDet exhibited 0.64 aM sensitivity for DNA detection, and showed high specificity for detection of other serotypes of Listeria and non-Listeria strains. Furthermore, the feasibility of the RAA-based Cas12aFDet method was evaluated in spiked and natural samples, enabling the quantitative detection of 1.35 × 10⁸-1.35 × 10³ cfu/g fresh grass carp of the target L. monocytogenes serotype 4c, and the results obtained for 22 natural aquatic samples were highly consistent with those of the culture-based serotyping method. The established Cas12aFDet platform is expected to provide a new paradigm for the sensitive and specific detection of pathogens in food safety and clinical diagnosis.

Recombinase polymerase amplification with polymer flocculation sedimentation for rapid detection of Staphylococcus aureus in food samples

Currently, rapid, sensitive, and convenient visual detection methods for Staphylococcus aureus (S. aureus) are scarce. In this study, a novel detection method based on recombinase polymerase amplification (RPA) and polymer flocculation sedimentation (PFS) was developed. Twelve effective primer combinations derived from four forward primers F1, F2, F3, F4, and three reverse primers R1, R2, R3 targeting the nuc gene of S. aureus were designed and screened by a polymerase chain reaction and RPA methods. RPA reaction conditions, including temperature, time, and volume as well as PEG8000 and NaCl concentrations range, were optimized. Moreover, the specificity and sensitivity of the RPA-PFS assay were further analyzed. Finally, the potential use of the RPA-PFS assay was evaluated using artificially S. aureus contaminated food samples, including pork, beef, shrimp, fish, cheese, cabbage, leftover rice, egg, milk, and orange juice. Results showed that the SA5 (F2/R2) combination was the optimal primer candidate. The optimal temperature range, the shortest time and the minimal volume of RPA reaction were 40-42 °C, 10 min and 10 μL, respectively and the optimal PEG8000/NaCl concentrations were 0.2 g/mL and 2.5 M, respectively, for the adsorption between magnetic beads and RPA products. The RPA-PFS method could detect as little as 13 fg genomic DNA of S. aureus and was also specific for five target S. aureus as well as twenty-seven non-target foodborne bacteria. The limit of detection of RPA-PFS for S. aureus in artificially contaminated food samples was 38 CFU/mL (g). Besides, RPA-PFS has directly been judged by the naked eye and has totally taken less than 20 min. In short, the assay RPA-PFS developed in this study is a rapid, sensitive, and specific visual detection method for S. aureus.

Development of a duplex lateral flow dipstick test for the detection and differentiation of Listeria spp. and Listeria monocytogenes in meat products based on loop-mediated isothermal amplification

Listeria spp. are a group of gram-positive bacteria consisting of 20 species. Among them, Listeria monocytogenes is one of the major species that infects humans since it contaminates raw fruits, vegetables, and many others food products. The conventional methods for the detection of Listeria spp. and L. monocytogenes are time-consuming, taking 5-7 days. Herein, a duplex lateral flow dipstick (DLFD) test combined with loop-mediated isothermal amplification (LAMP) was developed for the identification of Listeria spp. and L. monocytogenes within approximately 45 min with the optimized LAMP reaction times at 63 °C. Under the optimized conditions, the method detection limits (MDL) with reference to genomic DNA and pure culture were 900 femtograms (fg) and 20 cfu/mL, respectively. The LAMP-DLFD showed no cross-reactivity with eighteen - other pathogenic bacteria such as Salmonella spp., Staphylococcus aureus, Escherichia coli, Campylobacter coli, C. jejuni, Enterococcus faecalis, Vibrio cholerae, V. parahaemolyticus, Pseudomonas aeruginosa, Shigella dysenteriae, S. flexneri, Bacillus cereus, Lactobacillus acidophilus, L. casei and Pediococcus pentosaceus. Among 100 samples of food products, LAMP-DLFD demonstrated 100% accuracy when compared to other standard detection methods, such as ISO11290-1, enzyme-linked fluorescent assay (ELFA) technology (VIDAS) and PCR. In conclusion, LAMP-DLFD proved to be highly specific and sensitive assays for screening detection of Listeria spp. and L. monocytogenes.

Development of a loop-mediated isothermal amplification assay targeting lmo0753 gene for detection of Listeria monocytogenes in wastewater

Contaminated wastewater plays an important role in the transmission of Listeria monocytogenes in the environment. In this study, a loop-mediated isothermal amplification (LAMP) assay for sensitive detection of L.  monocytogenes in wastewater from treatment plants was developed, validated and compared to conventional PCR. The lmo0753 gene which codes for a Crp/Fnr family transcription factor, was targeted to design four specific primers to detect L.  monocytogenes in 60 min at 63°C in a water bath. Amplification products were visualized by agarose gel electrophoresis. The detection limit of the LAMP assay was 65 fg µl^-1 of DNA and 38 CFU per ml. Conventional PCR was 10 times less sensitive than LAMP assay with primers targeting the HlyA gene. A total of 70 crude wastewater samples collected at different treatment stages (aeration tank, pre chlorination and post chlorination), were tested directly by LAMP and PCR. Samples from aeration and pre-chlorination stages tested positive with LAMP and culture method but not with conventional PCR. LAMP assay was tolerant to inhibitors present in wastewater and circumvented the need for isolation of pure DNA for detection. Both LAMP assay and culture method failed to detect L.  monocytogenes in post-chlorinated wastewater, confirming the efficiency of the treatment process in the removal of L.  monocytogenes. SIGNIFICANCE AND IMPACT OF THE STUDY: Treated wastewater effluent contains Listeria monocytogenes which survives conventional wastewater treatment processes and can re-enter human food chain, thus it is imperative to detect L.  monocytogenes using a rapid and an inexpensive method. To the best of our knowledge, this is the first report of a loop-mediated isothermal amplification (LAMP) assay, targeting the lmo0753 gene for detection of L.  monocytogenes in wastewater from treatment plants. The LAMP assay detects L.  monocytogenes in 60 min at 63°C in a water bath. LAMP does not require isolation of pure genomic DNA hence it is a user friendly method for L.  monocytogenes detection.