Comparison of Listeria monocytogenes recoveries from spiked mung bean sprouts by the enrichment methods of three regulatory agencies

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.

The Use of Sprouts to Improve the Nutritional Value of Food Products: A Brief Review

Sprouts are vegetable foods rich in phytonutrients, such as glucosinolates, phenolics, and isoflavones. Many studies have shown that sprouts also have high concentrations of vitamins and minerals. In addition to the high concentration of nutrients, sprouts can present a reduction of anti-nutritional factors such as phytates, tannins, and oxalates, which increases the bioaccessibility of minerals. However, their nutritional composition depends on several factors, such as the type of sprout and the germination conditions. In recent years, these foods have been highly demanded because they are associated to many health benefits. Moreover, germination is an easy and fast process, and does not depend on specific climatic conditions (potentially more sustainable to growth). The use of sprouts for the elaboration of food products can be a good strategy to increase the nutritional value of certain products that are widely consumed worldwide. In this sense, studies that evaluated the impact of adding sprouted grains on the nutritional value of some products, as well as the effect on their sensory properties were searched in the scientific literature. Most of them used germinated grain flours to replace wheat flour in food products. The satisfactory results of these products were associated with the type of sprout used and with the level of replacement of the wheat flour. This review briefly explored the nutritional benefits and the sensory acceptance of food products made with added sprouts.

Evaluation of Methods of Enrichment and Compositing of Environmental Samples for Detection of Listeria monocytogenes

ABSTRACT

Various methods exist for the enrichment and detection of Listeria spp. and Listeria monocytogenes from environmental samples. Procedures for the compositing of environmental samples are not as well defined. In this study, different enrichment procedures involving buffered Listeria enrichment broth (BLEB), University of Vermont medium (UVM), and Fraser broth (FB) were evaluated to determine the limits of detection (LODs) for L. monocytogenes from culture and from swabs of stainless steel and to assess the efficacy of composite sampling by wet (pooling of primary enrichments) and dry (pooling of swabs) procedures. For detection of cells in pure culture, the computed values for the LOD at 95% probability (LOD95) using a single-step BLEB or two-step UVM-FB enrichment were 0.33 and 0.49 CFU/225 mL enrichment, respectively. No significant differences in detection were observed for procedures using either two-step BLEB-FB or UVM-FB enrichments for swabs of stainless steel when L. monocytogenes was inoculated at 2 to 6 log CFU; the LOD95 values were 3.82 and 3.62 log CFU per 4-in2 area, respectively. Wet compositing of L. monocytogenes from culture with and without romaine lettuce wash resident microbiota was conducted using BLEB-FB and UVM-FB enrichment methods; both allowed detection of the pathogen at ratios of 1:1, 1:2, 1:4, and 1:7 (1 positive sample to x negative samples) with no loss in sensitivity. From swabs of stainless steel, L. monocytogenes was detected similarly for both wet and dry composites of up to eight samples (1:7) with romaine lettuce wash. However, the BLEB-FB method allowed significantly faster detection (after 24 h of FB incubation) in composites of 1:4 and 1:7 samples compared with the UVM-FB method under the conditions tested. The results of this study provide data to evaluate the efficacies of the different enrichment procedures and aid in assessing the use of wet and dry compositing of environmental samples for use as part of a Listeria control plan in food production and processing facilities.

HIGHLIGHTS

Variability in lag duration of Listeria monocytogenes strains in half Fraser enrichment broth after stress affects the detection efficacy using the ISO 11290-1 method

A collection of 23 Listeria monocytogenes strains of clinical and food origin was tested for their ability to recover and grow out in half Fraser enrichment broth following the ISO 11290-1:2017 protocol. Recovery of sub-lethally heat-injured cells in half Fraser broth was compared to reference cells with no stress pre-treatment. The enrichments were followed over time by plate counts and the growth parameters were estimated with the 3-phase model which described the data best. The reference cells without stress pre-treatment showed a short lag duration, which ranged from 1.4 to 2.7 h. However, significant variation in the ability to recover after 60 °C heat stress was observed among the tested strains and resulted in a lag duration from 4.7 to 15.8 h. A subset of strains was also exposed to low-temperature acid stress, and the lag duration showed to be also stress dependent. Scenario analyses and Monte Carlo simulations were carried out using the growth parameters obtained in the enrichments. This demonstrated that when starting with one cell, the detection threshold for efficient transfer of at least one cell to the secondary enrichment step, i.e. 2 log₁₀ CFU/ml, was not reached by 11 of 23 strains tested (48%) after exposure to 60 °C heat stress. Increasing the incubation time from 24 to 26 h and the transfer volume from 0.1 to 1.0 ml can increase the average probability to transfer at least one cell to the secondary enrichment step from 79.9% to 99.0%. When optimizing enrichment procedures, it is crucial to take strain variability into account as this can have a significant impact on the detection efficacy.