Evaluation of chromogenic media for the detection of Listeria species in food

An Alternative Rapid Confirmation Method for Identifying Listeria monocytogenes from a Variety of 125 g Food Samples Within Two Days of a PCR Presumptive Positive

Cultural confirmation following detection of a Listeria monocytogenespresumptive positive can take 3-7 days to finalize; this uncertainty is a point of frustration for food producers needing to make time-sensitive disposition decisions. To address the demand for shortened time-to-results, an alternative L. monocytogenes confirmation method consisting of two components, (i) a secondary screen using a different rapid method, and (ii) concurrent cultural isolation followed by next-day colony identification was evaluated. For the study, four food matrices (hot dogs, peanut butter, frozen vegetables, and multicomponent frozen meals) were inoculated with low levels (0.36-1.39 MPN/125 g) of L. monocytogenes per the AOAC guidelines for a matrix study. Analyses were performed on 125 g test portions and started with a PCR primary screen (Bio-Rad iQ-Check Listeria monocytogenes II). Next, all enriched food samples underwent a secondary screen by bioMérieux's GENE-UP LMO2 Real-Time PCR and VIDAS LMX ELFA along with streaking onto RAPID'L.mono Agar. Presumptive positive L. monocytogenes colonies were identified utilizing a high throughput rapid identification method (Hygiena's BAX System L. monocytogenes Real-Time PCR assay, Neogen's ANSR isothermal nucleic acid amplification assay, and Bruker's MALDI Biotyper). Importantly, this study evaluated multiple commercially available options for the secondary screen (n = 2) and rapid identification (n = 3) to allow for easy adoption by testing laboratories. Overall, there was no statistically significant difference (p ≤ 0.05) between the number of L. monocytogenes-positive 125 g samples obtained by the cultural reference method and the alternative confirmation methods (regardless of which method combinations were evaluated). Additionally, this study supports that, when both the primary and secondary screen methods yield a positive result, the sample could be considered a confirmed positive for L. monocytogenes.

Comparison between qPCR, VIDAS immunoassays and agar streaking for the detection of Listeria monocytogenes from food and environmental surfaces containing and not containing Listeria innocua

Comparisons among a qPCR assay, VIDAS® assays and a conventional agar streaking method following the same enrichment for the detection of Listeria monocytogenes were performed under two challenging conditions. In the first comparison, L. innocua and L. monocytogenes were coinoculated into sausages at ratios (L. innocua-to-L. monocytogenes) of 10, 100, 1000, and 10 000. qPCR provided the most sensitive detection at all ratios after both 24-h and 48-h enrichments. A modified VIDAS® LMO2 assay (i.e., replacement of the kit-specified enrichment scheme with the enrichment scheme used in this study) and agar streaking yielded equivalent results when the ratio was 10 and 100; agar streaking was more sensitive when the ratio was 1000; neither method could detect L. monocytogenes at the ratio of 10 000. Enrichment duration of 48 h was needed for modified VIDAS® to detect L. monocytogenes when the ratio was 1000. Agar streaking after 24-h enrichment isolated L. monocytogenes better than after 48-h enrichment when the ratio was 100 and 1000. In the second comparison, we followed the validation guidelines of AOAC International and inoculated L. monocytogenes, without any L. innocua, onto lettuce and stainless-steel surfaces at low levels. The numbers of positive samples detected by qPCR, VIDAS® LIS assay, modified VIDAS® LMO2 assay, and agar streaking after 48-h enrichment were not statistically different. Our data showed that qPCR was the most sensitive method, while agar streaking and VIDAS® performed reasonably well. Streaking after 24-h enrichment was needed when background flora could overgrow L. monocytogenes during prolonged enrichment, and this is critical for confirming rapid screening assays. Appropriate selection of enrichment duration and rapid assays will enhance the testing of L. monocytogenes in food and environmental samples.

An Exploration of Listeria monocytogenes, Its Influence on the UK Food Industry and Future Public Health Strategies

Listeria monocytogenes is a Gram-positive intracellular pathogen that can cause listeriosis, an invasive disease affecting pregnant women, neonates, the elderly, and immunocompromised individuals. Principally foodborne, the pathogen is transmitted typically through contaminated foods. As a result, food manufacturers exert considerable efforts to eliminate L. monocytogenes from foodstuffs and the environment through food processing and disinfection. However, L. monocytogenes demonstrates a range of environmental stress tolerances, resulting in persistent colonies that act as reservoirs for the reintroduction of L. monocytogenes to food contact surfaces and food. Novel technologies for the rapid detection of L. monocytogenes and disinfection of food manufacturing industries have been developed to overcome these obstacles to minimise the risk of outbreaks and sporadic cases of listeriosis. This review is aimed at exploring L. monocytogenes in the UK, providing a summary of outbreaks, current routine microbiological testing and the increasing awareness of biocide tolerances. Recommendations for future research in the UK are made, pertaining to expanding the understanding of L. monocytogenes dissemination in the UK food industry and the continuation of novel technological developments for disinfection of food and the food manufacturing environment.

Assessment of Reference Method Selective Broth and Plating Media with 19 Listeria Species Highlights the Importance of Including Diverse Species in Listeria Method Evaluations

ABSTRACT

Reference methods developed for detection of Listeria monocytogenes are commonly used for detection of Listeria at the genus level. Improved method performance data are needed because this genus has expanded from 6 to 26 species and now includes several Listeria sensu lato species, which can have phenotypes distinct from those of Listeria sensu stricto. We evaluated growth of 19 Listeria species, including 12 recently described Listeria sensu lato species, using the media specified by (i) the U.S. Food and Drug Administration (FDA) Bacteriological Analytical Manual, (ii) the U.S. Department of Agriculture (USDA) Microbiology Laboratory Guidebook, and (iii) the International Organization for Standardization (ISO). The FDA broth enrichment procedure allowed all species to grow to detectable levels (≥4 log CFU/mL), yielded the highest mean growth (7.58 log CFU/mL), and was the only procedure with which no Listeria sensu lato species yielded significantly higher growth than did a comparison Listeria sensu stricto species. With the USDA and ISO broth enrichment procedures, several Listeria sensu lato species yielded significantly higher growth than did either Listeria seeligeri or Listeria ivanovii, suggesting that these two Listeria sensu stricto species could be outgrown by Listeria sensu lato species. On selective and differential agar media, L. seeligeri, L. ivanovii, and Listeria grayi produced colonies with atypical morphology and/or growth of these species was inhibited (which may lead to incorrect classification of a sample as negative), whereas several newly described Listeria sensu lato species grew to high levels and produced colonies with typical morphology. Overall, our study results indicate that the ability to detect various Listeria species can be impacted by the specific broth and selective and differential agar used. Our data can help guide selection of appropriate media and detection methods for environmental Listeria monitoring programs and methods that are most likely to detect the targeted Listeria groups (e.g., Listeria sensu stricto, which appear to be the most appropriate index organisms for the pathogen L. monocytogenes).

HIGHLIGHTS

ON-rep-seq as a rapid and cost-effective alternative to whole-genome sequencing for species-level identification and strain-level discrimination of Listeria monocytogenes contamination in a salmon processing plant

Identification, source tracking, and surveillance of food pathogens are crucial factors for the food-producing industry. Over the last decade, the techniques used for this have moved from conventional enrichment methods, through species-specific detection by PCR to sequencing-based methods, whole-genome sequencing (WGS) being the ultimate method. However, using WGS requires the right infrastructure, high computational power, and bioinformatics expertise. Therefore, there is a need for faster, more cost-effective, and more user-friendly methods. A newly developed method, ON-rep-seq, combines the classical rep-PCR method with nanopore sequencing, resulting in a highly discriminating set of sequences that can be used for species identification and also strain discrimination. This study is essentially a real industry case from a salmon processing plant. Twenty Listeria monocytogenes isolates were analyzed both by ON-rep-seq and WGS to identify and differentiate putative L. monocytogenes from a routine sampling of processing equipment and products, and finally, compare the strain-level discriminatory power of ON-rep-seq to different analyzing levels delivered from the WGS data. The analyses revealed that among the isolates tested there were three different strains. The isolates of the most frequently detected strain (n = 15) were all detected in the problematic area in the processing plant. The strain level discrimination done by ON-rep-seq was in full accordance with the interpretation of WGS data. Our findings also demonstrate that ON-rep-seq may serve as a primary screening method alternative to WGS for identification and strain-level differentiation for surveillance of potential pathogens in a food-producing environment.

Growth of Listeria monocytogenes Inoculated on Packaged Fresh-Cut Turnips Stored at 4 and 10°C

ABSTRACT

The ecology of Listeria monocytogenes has been previously investigated in various whole and minimally processed raw vegetables, but not in turnips. A 2018 national Canadian recall for packaged fresh-cut turnips contaminated with L. monocytogenes raised concerns about turnips being able to support the growth of this microorganism. Thus, this study examined the growth potential of L. monocytogenes in fresh-cut turnips stored at 4 and 10°C. The bacterial microbiota of each brand of purchased turnips was also partially determined to evaluate the diversity of bacteria present on the product. Turnips were mist inoculated at an initial level of 3.0 log CFU/g using a five-strain L. monocytogenes cocktail. Samples were then stored at either 4 or 10°C for 10 days, with enumeration occurring at 0, 5, and 10 days. L. monocytogenes did grow on turnips stored at 10°C, with increases ranging from 0.87 to 1.84 log CFU/g over the 10-day storage period (P < 0.05). In contrast, L. monocytogenes was able to survive but not grow on turnips stored at 4°C for 10 days. This study reinforces the importance of strict temperature control within processing, retail, and household consumer settings. Avoiding temperature abuse conditions and storing packaged fresh-cut turnips under refrigerator conditions (≤4°C) can serve as an important hurdle to prevent and/or limit the growth of L. monocytogenes on these products.

HIGHLIGHTS

Listeria monocytogenes incidence changes and diversity in some Brazilian dairy industries and retail products

Listeria monocytogenes can cause listeriosis, a severe foodborne disease. In Brazil, despite very few reported cases of listeriosis, the pathogen has been repeatedly isolated from dairies. This has led the government to implement specific legislation to reduce the hazard. Here, we determined the incidence of L. monocytogenes in five dairies and retail products in the Southeast and Midwest regions of Brazil over eight months. Of 437 samples, three samples (0.7%) from retail and only one sample (0.2%) from the dairies were positive for L. monocytogenes. Thus, the contamination rate was significantly reduced as compared to previous studies. MultiLocus Sequence Typing (MLST) was used to determine if contamination was caused by new or persistent clones leading to the first MLST profile of L. monocytogenes from the Brazilian dairy industry. The processing environment isolate is of concern being a sequence-type (ST) 2, belonging to the lineage I responsible for the majority of listeriosis outbreaks. Also, ST3 and ST8 found in commercialized cheese have previously been reported in outbreaks. Despite the lower incidence, dairy products still pose a potential health risk and the occurrence of L. monocytogenes in dairies and retail products emphasize the need for continuous surveillance of this pathogen in the Brazilian dairy industry.

Differentiation of different mixed Listeria strains and also acid-injured, heat-injured, and repaired cells of Listeria monocytogenes using fourier transform infrared spectroscopy

Fourier transform infrared (FT-IR) spectroscopy was used to differentiate mixed strains of Listeria monocytogenes and mixed strains of L. monocytogenes and Listeria innocua. FT-IR spectroscopy was also applied to investigate the hypothesis that heat-injured and acid-injured cells would return to their original physiological integrity following repair. Thin smears of cells on infrared slides were prepared from cultures for mixed strains of L. monocytogenes, mixed strains of L. monocytogenes and L. innocua, and each individual strain. Heat-injured and acid-injured cells were prepared by exposing harvested cells of L. monocytogenes strain R2-764 to a temperature of 56 ± 0.2°C for 10 min or lactic acid at pH 3 for 60 min, respectively. Cellular repair involved incubating aliquots of acid-injured and heat-injured cells separately in Trypticase soy broth supplemented with 0.6% yeast extract for 22 to 24 h; bacterial thin smears on infrared slides were prepared for each treatment. Spectral collection was done using 250 scans at a resolution of 4 cm(-1) in the mid-infrared wavelength region. Application of multivariate discriminant analysis to the wavelength region from 1,800 to 900 cm(-1) separated the individual L. monocytogenes strains. Mixed strains of L. monocytogenes and L. monocytogenes cocultured with L. innocua were successfully differentiated from the individual strains when the discriminant analysis was applied. Different mixed strains of L. monocytogenes were also successfully separated when the discriminant analysis was applied. A data set for injury and repair analysis resulted in the separation of acid-injured, heat-injured, and intact cells; repaired cells clustered closer to intact cells when the discriminant analysis (1,800 to 600 cm(-1)) was applied. FT-IR spectroscopy can be used for the rapid source tracking of L. monocytogenes strains because it can differentiate between different mixed strains and individual strains of the pathogen.

Identification of non-Listeria spp. bacterial isolates yielding a β-D-glucosidase-positive phenotype on Agar Listeria according to Ottaviani and Agosti (ALOA)

Agar Listeria according to Ottaviani and Agosti (ALOA) is the mandatory medium used for the detection and enumeration of Listeria monocytogenes in foods according to the official International Organization for Standardization (ISO) methods. On ALOA, Listeria spp. appear as bluish-green colonies due to the production of β-D-glucosidase, an enzyme that cleaves 5-bromo-4-chloro-3-indolyl-β-D-glucopyranoside, a chromogenic substrate included in the formulation of the medium. The present work reports on bacterial isolates (n=64) from ready-to-eat soft cheeses, which are able to grow on ALOA, forming bluish-green colonies and therefore phenotypically resemble Listeria spp. All isolates were also capable of growing on the selective media PALCAM and RAPID L'mono. The isolates were characterised with biochemical tests including those specified in the ISO standards for the confirmation of Listeria spp. and identified via partial sequencing of their 16S rRNA gene. According to sequencing results the isolates represented 12 different bacterial species or species-groups belonging to seven different genera: Bacillus spp. (B. circulans, B. clausii, B. licheniformis and B. oleronius), Cellulosimicrobium spp. (C. funkei), Enterococcus spp. (E. faecalis, E. faecium/durans), Kocuria spp. (K. kristinae), Marinilactibacillus spp. (M. psychrotolerans), Rothia spp. (R. terrae) and Staphylococcus spp. (S. sciuri and S. saprophyticus subsp. saprophyticus/xylosus). Cellulosimicrobium spp. have never been previously isolated from foods. These results significantly extend the list of bacteria previously known as capable of growing on ALOA as bluish-green colonies and suggest that there may be room for further improvement in the medium's inhibitory properties towards non-Listeria spp., Gram-positive bacteria present in foods.

Prevalence and contamination patterns of Listeria monocytogenes in Flammulina velutipes plants

Four mushroom (Flammulina velutipes) production plants were sampled to investigate the prevalence and contamination source of Listeria monocytogenes. Among 295 samples, the prevalence of L. monocytogenes was 18.6%; the contamination appeared to originate from the mycelium-scraping machinery, contaminating both the product and upstream packaging equipment. Of 55 L. monocytogenes isolates, lineages I.1 (1/2a-3a) and II.2 (1/2b-3b-7) accounted for 65.5% and 34.5%, respectively. In addition, lineage I.1 formed significantly thicker biofilms than those within lineage II.2, as determined by crystal violet staining and scanning electron microscopy. Genotype analyses of L. monocytogenes isolates using enterobacteria repetitive intergenic consensus-polymerase chain reaction, and random amplified polymorphic DNA revealed that the surfaces of mycelium-scraping machinery may serve as the main source of L. monocytogenes contamination in three of the four plants. This study was the first report to explore the potential contamination sources of L. monocytogenes in the mushroom production chain, thereby providing baseline information for adopting prophylactic measures for critical control points during production in mushroom plants to avoid L. monocytogenes contamination.

Development of a novel selective and differential medium for the isolation of Listeria monocytogenes

A new medium (lecithin and levofloxacin [LL] medium) is described for the isolation of Listeria monocytogenes from food samples. LL medium includes lecithin from soybeans for the detection of phosphatidylinositol-specific phospholipase C (PI-PLC) and phosphatidylcholine-specific phospholipase C (PC-PLC) produced by L. monocytogenes. Levofloxacin is incorporated to inhibit the growth of microorganisms other than L. monocytogenes, especially Bacillus cereus, shown to possess PI-PLC and PC-PLC activities. L. monocyogenes produced white colonies with a halo on LL medium, whereas Listeria innocua appeared as white colonies without a halo. Levofloxacin at 0.20 mg/liter completely inhibited the growth of B. cereus, while the growth of L. monocytogenes was unaffected. In the second phase of the study, the sensitivity and the specificity of LL medium were compared to those of modified Oxford agar (MOX) and two chromogenic media (Brilliance Listeria agar and CHROMagar Listeria), using a total of 250 food samples. From 200 unspiked food samples, the specificity of LL medium (96.0%) was superior to that of MOX (72.0%) and similar to the specificities of Brilliance Listeria agar (96.5%) and CHROMagar Listeria (94.5%). From 50 spiked food samples, LL medium and CHROMagar Listeria represented the highest sensitivities (96.0%), followed by Brilliance Listeria agar (92.0%) and MOX (54.0%). Also, LL medium showed the highest confirmation rate (98.8%), followed by Brilliance Listeria agar (98.7%), CHROMagar Listeria (98.3%), and MOX (52.0%). On the basis of its good specificity and cost effectiveness, LL medium is useful for the isolation of L. monocytogenes from food samples.

Reduced detectability of Listeria monocytogenes in the presence of Listeria innocua

Recent foodborne crises have demonstrated the importance of monitoring food safety. In terms of microbiological criteria, food safety requires the reliable detection of pathogens such as Listeria monocytogenes along the food chain by appropriate analytical methods. However, indications exist that accompanying Listeria innocua strains suppress the growth of L. monocytogenes during selective enrichment, which may cause reduced or even inhibited detection. To study these effects, the limit of detection of L. monocytogenes was investigated in the presence of L. innocua using the International Organization for Standardization standard method ISO 11290-1 and the VIDAS LDUO system, an automated method based on enzyme-linked fluorescence technology. The challenge was to provide low initial Listeria concentrations at sufficient precision to quantify the influence on the probability of detection of L. monocytogenes. The application of reference materials appropriate for quantitative test methods and a standardized dilution procedure were necessary to ensure accurate CFU levels of defined proportions of mixtures of both Listeria species. During selective enrichment, overgrowth of L. monocytogenes by L. innocua could be confirmed, leading to high rates of false-negative results. Moreover, with both methods, a significant decrease in the detectability of L. monocytogenes could be quantified at ratios of 2:1 at very low concentrations representative of natural contamination levels often found in foods and environments. It is concluded that there is a need to improve existing procedures with respect to selective enrichment, as well as the detection techniques.

Limitation in the detection of Listeria monocytogenes in food in the presence of competing Listeria innocua

AIMS

Detectability of Listeria monocytogenes at 10(0) CFU per food sample in the presence of Listeria innocua using standard microbiological detection was evaluated and compared with the real-time PCR-based method.

METHODS AND RESULTS

Enrichment in half-Fraser broth followed by subculture in Fraser broth according to EN ISO 11290-1 was used. False-negative detection of 10(0) CFU L. monocytogenes was obtained in the presence of 10(1) CFU L. innocua per sample using the standard detection method in contrast to more than 10(5) CFU L. innocua per sample using real-time PCR. Identification of L. monocytogenes on the chromogenic medium by the standard procedure was impossible if L. innocua was able to overgrow L. monocytogenes by more than three orders of magnitude after the enrichment in model samples. These results were confirmed using naturally contaminated food samples.

CONCLUSIONS

Standard microbiological method was insufficient for the reliable detection of 10(0) CFU L. monocytogenes in the presence of more than 10(0) CFU of L. innocua per sample. On the other hand, if the growth of L. monocytogenes was sufficient to reach the concentration equal to the detection limit of PCR, the amount of the other microflora present in the food sample including L. innocua was not relevant for success of the PCR detection of L. monocytogenes.

SIGNIFICANCE AND IMPACT OF THE STUDY

After the enrichment, the PCR detection is more convenient than the standard one as PCR detection is not compromised by other present microflora.

Evaluation of the international reference methods NF EN ISO 11290-1 and 11290-2 and an in-house method for the isolation of Listeria monocytogenes from retail seafood products in france

Retail seafood products were analyzed on their use-by date using the international reference methods NF EN ISO 11290-1 and 11290-2 (collectively method R) or an in-house method (method B) for the isolation of Listeria monocytogenes. The sensitivity of the methods was about 78%. Method R detected more positive samples of smoked salmon and herb-flavored slices of smoked salmon than did method B, whereas the reverse was true for samples of carpaccio-like salmon, herb-flavored slices of raw salmon, and smoked trout. Most products produced a positive result after the first of two enrichments, and little difference was observed after changing the isolation medium (Listeria selective agar, L. monocytogenes blood agar, agar for Listeria according to Ottaviani and Agosti, Oxford agar, and Palcam agar). L. monocytogenes was isolated from 151 (27.8%) of the 543 samples, with concentrations mostly below 100 CFU/g. The pathogen prevalence and concentration in these seafood products varied greatly depending on the producer and the nature of the product. In certain cases, these differences could be explained by problems in cleaning and disinfection operations in the food-processing environment. The identities of L. monocytogenes isolates were confirmed by PCR, and isolates were characterized by random amplification of polymorphic DNA and pulsed-field gel electrophoresis (PFGE). PFGE patterns obtained with the enzymes Apal and AscI produced 26 different pulsotypes. In general, different pulsotypes were present in the different categories of seafood products and were not specific to one producer. The genetic diversity observed in the products was not related to the prevalence found at the manufacturing site. It is therefore important for producers to determine the source(s) of contamination of their product so the risks linked to the presence of L. monocytogenes can be reduced.

Evaluation of a new chromogenic agar for the detection of Listeria in food

AIM

Rapid identification of Listeria in food is important in protecting consumers from infection. The development of chromogenic media such as agar Listeria according to Ottaviani and Agosti (ALOA) has allowed more rapid detection of Listeria monocytogenes, with presumptive identification of this pathogenic species after only 24 h of incubation. The aim of this study was to evaluate Oxoid chromogenic Listeria agar (OCLA) in comparison with ALOA and a traditional, nonchromogenic medium, Oxford agar.

METHODS AND RESULTS

Media were compared using pure cultures, spiked food samples and naturally contaminated samples. Whilst development of typical colony morphology took 48 h on Oxford agar, Listeria spp. were frequently detected after 24 h of incubation on OCLA and ALOA. There was no significant difference in recovery between the two chromogenic media.

CONCLUSIONS

Results indicate that OCLA gives equivalent recovery of Listeria spp. compared with ALOA. Whilst L. monocytogenes was frequently detected after 24 h of incubation, a 48-h incubation time was necessary to ensure detection of both L. monocytogenes and other Listeria spp.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study has shown that a commercially available chromogenic medium other than ALOA is appropriate for use in the international standard method. The commercial availability of more than one medium will facilitate the more widespread use of the method, thus increasing confidence in the ability to detect L. monocytogenes in food in the presence of other Listeria spp.