Foodborne Listeria monocytogenes: A Real Challenge in Quality Control

Diagnostic challenges with accurate identification of Listeria monocytogenes isolates from food and environmental samples in South Africa

Background: The 2017–2018 listeriosis outbreak in South Africa warranted testing for Listeria monocytogenes in food products and processing environments. Diagnostic tests are needed to accurately differentiate L. monocytogenes from other Listeria species.Objective: The study assessed the performance of the commonly used tests in our setting to accurately identify L. monocytogenes.Methods: The study was conducted in a public health laboratory in South Africa. Cultured isolates from food and environmental samples were tested both prospectively and retrospectively between August 2018 and December 2018. Isolates were phenotypically identified using tests for detecting β-haemolysis, Christie-Atkins-Munch-Peterson, alanine arylamidase (AlaA), mannosidase, and xylose fermentation. Listeria monocytogenes isolates were identified using automated systems, Microscan Walkaway Plus 96, Vitek® MS, Vitek® 2 and Surefast Listeria monocytogenes PLUS PCR. All results were compared to whole-genome sequencing results.Results: β-haemolysis and Christie-Atkins-Munch-Peterson tests gave delayed positivity or were negative for L. monocytogenes and falsely positive for one strain of Listeria innocua. The AlaA enzyme and Colorex Listeria agar lacked specificity for L. monocytogenes identification. Based on a few phenotypic test results, an aberrant L. monocytogenes strain and Listeria seeligeri strain were reported. All automated platforms overcalled L. monocytogenes in place of other Listeria species.Conclusion: No test was ideal in differentiating Listeria species. This is an issue in resource-limited settings where these tests are currently used. Newer technologies based on enzyme-linked immunosorbent assay and other molecular techniques specific to L. monocytogenes detection need to be investigated. 

Application of Melting Temperature in Melting Curve of qPCR to Determine Listeria monocytogenes Presence in Golden Needle Mushroom

This study developed a method to determine Listeria monocytogenes presence in golden needle mushrooms by melting temperature (Tm) in a melting curve of qPCR. For identical samples (n = 35), the results for L. monocytogenes presence determined by Tm values were compared with the results from a conventional detection method (culture-based procedures). The samples that showed the negative result in the conventional method were subsequently examined with the Tm value of qPCR. Tm values for Escherichia coli (87.5 ± 0.4°C), Salmonella (87.6 ± 0.1°C), Staphylococcus aureus (79.2 ± 0.0°C), Listeria innocua (80.5 ± 0.0°C), Listeria ivanovii (79.0 ± 0.4°C), Listeria welshimeri (78.8 ± 0.4°C), and Listeria monocytogenes (83.7 ± 0.2°C) were different, and thus, no similar Tm values of L. monocytogenes were observed with other bacteria. From 35 golden needle mushrooms, 26 samples (74.3%) were L. monocytogenes positive with Tm value of qPCR, but only 13 samples (37.1%) of 35 samples were L. monocytogenes positive using the conventional detection method. Of the samples that were positive with the Tm value of qPCR, but negative with the conventional detection method, 4 samples were selected randomly, and typical L. monocytogenes colonies were detected in CHROMagar. These results indicate that the Tm value in the melting curve of qPCR can be used to detect L. monocytogenes in golden needle mushrooms.

Occurrence of Listeria monocytogenes in Artisanal Cheeses from Poland and Its Identification by MALDI-TOF MS

Listeria monocytogenes is a foodborne pathogen. A source of infection can be artisanal cheeses. Identification of the Listeria species is important for the protection of public health and the food industry. This study aimed to examine artisanal cheeses for the presence of L. monocytogenes and the effectiveness of the MALDI-TOF MS method in the identification of the L. monocytogenes isolates. A total of 370 samples of artisanal cheeses were examined. L. monocytogenes was found in 23 cheese samples (6.2%). The reliability of L. monocytogenes identification achieved by MALDI-TOF MS was varied, and the vast majority of the isolates (27/32) were identified only to the secure genus, probable species level. This study showed that (i) the occurrence of L. monocytogenes in the artisanal cheeses was at a higher level than that in the other EU countries, (ii) the standard of species identification of L. monocytogenes isolates from artisanal cheeses achieved by MALDI-TOF MS was not satisfactory and (iii) the presence of L. monocytogenes in artisanal cheeses remains a problem with regard to the food safety criterion and a potential public health risk.

Predominance of Distinct Listeria Innocua and Listeria Monocytogenes in Recurrent Contamination Events at Dairy Processing Facilities

: The genus Listeria now comprises up to now 21 recognized species and six subspecies, with L. monocytogenes and L. innocua as the most prevalent sensu stricto associated species. Reports focusing on the challenges in Listeria detection and confirmation are available, especially from food-associated environmental samples. L. innocua is more prevalent in the food processing environment (FPE) than L. monocytogenes and has been shown to have a growth advantage in selective enrichment and agar media. Until now, the adaptive nature of L. innocua in FPEs has not been fully elucidated and potential persistence in the FPE has not been observed. Therefore, the aim of this study is to characterize L. innocua (n = 139) and L. monocytogenes (n = 81) isolated from FPEs and cheese products collected at five dairy processing facilities (A-E) at geno- and phenotypic levels. Biochemical profiling was conducted for all L. monocytogenes and the majority of L. innocua (n = 124) isolates and included a rhamnose positive reaction. L. monocytogenes isolates were most frequently confirmed as PCR-serogroups 1/2a, 3a (95%). Pulsed-field gel electrophoresis (PFGE)-typing, applying the restriction enzymes AscI, revealed 33 distinct Listeria PFGE profiles with a Simpson's Index of Diversity of 0.75. Multi-locus sequence typing (MLST) resulted in 27 STs with seven new L. innocua local STs (ST1595 to ST1601). L. innocua ST1597 and ST603 and L. monocytogenes ST121 and ST14 were the most abundant genotypes in dairy processing facilities A-E over time. Either SSI-1 (ST14) or SSI-2 (ST121, all L. innocua) were present in successfully FPE-adapted strains. We identified housekeeping genes common in Listeria isolates and L. monocytogenes genetic lineage III. Wherever there are long-term contamination events of L. monocytogenes and other Listeria species, subtyping methods are helpful tools to identify niches of high risk.

A Comparison of Four Molecular Methods for Detection of Aflatoxin-Producing Aspergillus in Peanut and Dried Shrimp Samples Collected from Local Markets around Pathum Thani Province, Thailand

Aspergillus flavus is an aflatoxin-producing fungus which is poisonous to humans and animals when consumed. Detecting the fungus can help to prevent this danger. The four molecular methods, namely, conventional isothermal amplification (LAMP), PCR, quantitative LAMP (qLAMP), and qPCR, were compared to determine their efficiency for A. flavus detection. Thirty samples of peanut and dried shrimp were collected from 15 markets around Pathum Thani Province in Thailand. The samples were artificially infected with 10⁸ conidia/ml of A. flavus for 1 hr and enriched for one day to represent real contamination. The results show that the sensitivity detection for A. flavus in PCR, LAMP, qPCR, and qLAMP was 50 ng, 5 ng, 5 pg, and 5 pg, respectively. Aspergillus in 30 peanut and dried shrimp from the market was detected by all four methods. The detection rate was about 20%, 60%, 100%, and 100% with PCR, LAMP, qPCR, and qLAMP, respectively. The molecular detection technique, especially LAMP, qPCR, and qLAMP, can detect this pathogenic fungi very rapidly with high sensitivity and reliability in comparison to conventional PCR.

MIR spectroscopy as alternative method for further confirmation of foodborne pathogens Salmonella spp. and Listeria monocytogenes

Listeriosis and Salmonellosis are two of the most common foodborne diseases. Consequently, an early and accurate detection of Listeria monocytogenes and Salmonella spp. in food products is a critical concern of public health policies. Therefore, it is of great interest to develop rapid, simple, and inexpensive alternatives for pathogen detection in food products. In this study, mid-infrared spectroscopy has been successfully used to confirm Listeria species and the presence of Salmonella isolated from food samples. This methodology showed to be very sensitive and could be a rapid alternative to detect these important pathogens, allowing to obtain results in a few minutes after previous growth in selective media, avoiding the confirmation procedures that delay the achievement of the results for up to 2 days.