Short pre-enrichment and modified matrix lysis. A comparative study towards same-day detection of Listeria monocytogenes

Surface monitoring of L. monocytogenes by real-time fluorescence and colorimetric LAMP

Listeria monocytogenes is a major foodborne pathogen affecting developing, and developed countries. The analysis of food contact surfaces in food industries is key for better controlling this pathogen. The current study focused on the development, optimization, and evaluation of a rapid and simple method for the detection of L. monocytogenes on stainless steel surfaces, suitable for decentralized setups, taking advantage of Loop-mediated isothermal amplification (LAMP). This was accomplished using a general pre-enrichment broth (TSB), with a simple DNA extraction based on a chelating resin, and final isothermal amplification. Two different detection strategies were tested, real-time fluorescence and naked-eye colorimetric, which were evaluated after 5, 7, and 24 h of pre-enrichment. Regardless the detection chemistry selected, after 5-7 h of pre-enrichment, 103-104 CFU/cm2 were needed to obtain a positive result, while after 24 h, it was possible to detect concentrations below 10 CFU/cm2. Within each given time, all the performance parameters calculated, relative sensitivity, specificity, and accuracy, reached values higher than 80-90%; likewise, a Cohen's k of concordance with a culture-based approach higher than 0.8. Overall, the most sensitive assay can be performed in roughly 25 h. This time-to-result outperforms commercial kits with the added value of specifically detecting L. monocytogenes instead of Listeria spp. KEY POINTS: • Real-time fluorescence and naked-eye colorimetric, were compared for the novel assay. • An LOD50 of 3.4 CFU/cm2 and 4.2 CFU/cm2 was calculated for the two assays. • Three pre-enrichment times were compared providing 24 h better results.

Evaluation of Covalent Organic Frameworks for the low-cost, rapid detection of Shiga Toxin-producing Escherichia coli in ready-to-eat salads

BACKGROUND

Ready-to-eat products, such as leafy greens, must be carefully controlled as they are directly consumed without any treatment to reduce the presence of potential pathogens. Food industries, especially those that process products with short shelf-life, demand rapid detection of foodborne pathogens such as Shiga Toxin-producing Escherichia coli (STEC). In this sense, molecular methods can fulfill both requirements of turnaround time and consumer safety. The most popular rapid methods are those based on real-time PCR (qPCR) however, vegetables contain inhibitory compounds that may inhibit the amplification reaction thus, there is a need for novel sample preparation protocols.

RESULTS

In the current study, a low-cost sample treatment based on sequential filtration steps was developed. This protocol was combined with covalent organic frameworks (COFs), and compared against a chelating resin, to evaluate their performance by multiplex qPCR targeting the major virulence genes of STEC, namely stx1, stx2, and eae, along with the rfbE for the specific identification of serogroup O157 due to its particularly high incidence, and an Internal Amplification Control to assess reaction inhibition. The optimized sample treatment effectively removed vegetable qPCR inhibitory compounds, and it was possible to detect STEC in spiked ready-to-eat salad samples in one working day, roughly 5 h, with an LOD₅₀ of 8.7 CFU/25 g with high diagnostic sensitivity and specificity. The method was also assessed in samples with cold-stressed bacteria with good results, further demonstrating its applicability.

SIGNIFICANCE

It was demonstrated for the first time that COFs are suitable for DNA extraction and purification. In addition to this, due to the tunable nature of these materials, it is envisioned that future modifications in terms of pore size or combination with magnetic materials, will allow to further improve their performance. In addition to this, the rapid and low-cost sample treatment protocol developed demonstrated suitable for the rapid screening of STEC vegetable samples.