Optimization of a Viability PCR Method for the Detection of Listeria monocytogenes in Food Samples

Establishment of a Protocol for Viability qPCR in Dental Hard Tissues

The aim of the study was to establish a live/dead qPCR with propidium monoazide (PMA) that can quantitatively differentiate between viable/non-viable microorganisms in dental hard tissues. Human premolars (n = 88) were prepared with nickel-titanium instruments and incubated with E. faecalis (21 d). Subsequently, the bacteria in half of the teeth were devitalized by heat inactivation (100 °C, 2 h). The following parameters were tested: PMA concentrations at 0 µmol (control), 50 µmol, 100 µmol, and 200 µmol; PMA incubation times of 30 min and 60 min, and blue light treatment for 30 min and 60 min. The teeth were ground using a cryomill and the bacterial DNA was quantified using qPCR, ANOVA, and p = 0.05. The qPCR of the control group detected a similar number of avital 9.94 × 106 and vital 1.61 × 107 bacterial cells. The use of PMA inhibited the amplification of DNA from non-viable cells during qPCR. As a result, the best detection of avital bacteria was achieved with the following PMA parameters: (concentration, incubation time, blue light treatment) 200-30-30; 5.53 × 104 (avital) and 1.21 × 100.7 (vital). The live/dead qPCR method using PMA treatment is suitable for the differentiation and quantification of viable/non-viable microorganisms in dentin, as well as to evaluate the effectiveness of different preparation procedures and antimicrobial irrigants in other biological hard substances.

A Review of Modern Methods for the Detection of Foodborne Pathogens

Despite the recent advances in food preservation techniques and food safety, significant disease outbreaks linked to foodborne pathogens such as bacteria, fungi, and viruses still occur worldwide indicating that these pathogens still constitute significant risks to public health. Although extensive reviews of methods for foodborne pathogens detection exist, most are skewed towards bacteria despite the increasing relevance of other pathogens such as viruses. Therefore, this review of foodborne pathogen detection methods is holistic, focusing on pathogenic bacteria, fungi, and viruses. This review has shown that culture-based methods allied with new approaches are beneficial for the detection of foodborne pathogens. The current application of immunoassay methods, especially for bacterial and fungal toxins detection in foods, are reviewed. The use and benefits of nucleic acid-based PCR methods and next-generation sequencing-based methods for bacterial, fungal, and viral pathogens' detection and their toxins in foods are also reviewed. This review has, therefore, shown that different modern methods exist for the detection of current and emerging foodborne bacterial, fungal, and viral pathogens. It provides further evidence that the full utilization of these tools can lead to early detection and control of foodborne diseases, enhancing public health and reducing the frequency of disease outbreaks.

The microbial diversity and flavour metabolism of Chinese strong flavour Baijiu: a review

Strong flavour Baijiu is widely consumed in China and is produced by the fermentation of grains using microbial starters. However, a comprehensive understanding of the diversity and metabolic characteristics of microbial communities involved in the solid-state fermentation of Baijiu is important for determining the relationship between microbial composition, flavour metabolism and understanding Baijiu fermentation conditions. Although studies have examined the metabolic pathways and impact of major processes on flavour compounds in strong flavour Baijiu, aspects of the fermentation process remain unexplored. In this review, methods are discussed for the optimisation of microbial diversity in strong flavour Baijiu and associated effects on the flavour of Baijiu. Recent studies are reviewed on starters (Daqu), fermented grains (Jiupei), and pit mud together with the effects of microbial composition on the quality of strong flavour Baijiu. The challenges of Baijiu research and production are discussed, including the role of the microbial diversity of Daqu and Jiupei in the flavour composition of strong flavour Baijiu. This review contributes to the current understanding of processing strong flavour Baijiu and serves as a reference for screening flavour related microorganisms, which is valuable for improving the quality of strong flavour Baijiu.

Surveilling cellular vital signs: toward label-free biosensors and real-time viability assays for bioprocessing

Cell viability is an essential facet of mammalian and microbial bioprocessing. While robust methods of monitoring cellular health remain critically important to biomanufacturing and biofabrication, the complexity of advanced cell culture platforms often poses challenges for conventional viability assays. This review surveys novel approaches to discern the metabolic, morphological, and mechanistic hallmarks of living systems - spanning subcellular and multicellular scales. While fluorescent probes coupled with 3D image analysis generate rapid results with spatiotemporal detail, molecular techniques like viability PCR can distinguish live cells with genetic specificity. Notably, label-free biosensors can detect nuanced attributes of cellular vital signs with single-cell resolution via optical, acoustic, and electrical signals. Ultimately, efforts to integrate these modalities with automation, machine learning, and high-throughput workflows will lead to exciting new vistas across the cell viability landscape.

Listeria monocytogenes Assessment in a Ready-to-Eat Salad Shelf-Life Study Using Conventional Culture-Based Methods, Genetic Profiling, and Propidium Monoazide Quantitative PCR

Listeriosis is almost entirely transmitted through foods contaminated with Listeria monocytogenes. Ready-to-eat foods present a particular challenge due to their long refrigerated shelf-life, not requiring any heat treatment before consumption. In this work, a shelf-life assessment of an industrially produced ready-to-eat salad was performed using conventional culture-based and molecular methods. L. monocytogenes isolates were confirmed and serogrouped using multiplex PCR, and genetic subtyping was performed by pulsed-field gel electrophoresis (PFGE). PMAxx-qPCR was used as an alternative method for L. monocytogenes quantification in foods. Salad samples were kept at 4 °C, 12 °C, and 16 °C for eight days and analysed. At 4 °C, acceptable results were obtained considering hygiene indicators, i.e., Enterobacteriaceae (ranging from 3.55 ± 0.15 log cfu/g to 5.39 ± 0.21 log cfu/g) and aerobic mesophilic colony counts (5.91 ± 0.90 log cfu/g to 9.41 ± 0.58 log cfu/g) throughout the study, but the same did not happen at 12 °C and 16 °C. L. monocytogenes culture-based quantification exhibited low numbers (<1 log cfu/g) for all temperatures. From 30 presumptive isolates, 10 (33.3%) were confirmed as L. monocytogenes with the majority belonging to serogroup IVb. PFGE subtyping showed that 7 of the 10 L. monocytogenes isolates had 100% of pulsotype similarity, suggesting a possible common contamination source. PMAxx-qPCR revealed a statistically higher L. monocytogenes quantification (>3 log cfu/g) when compared to the conventional culture-based method, suggesting viable but non-culturable forms. Taken together, results underline the need to combine conventional methods with more sensitive, specific, and rapid ones for L. monocytogenes assessment in ready-to-eat foods shelf-life studies to reduce the potential risk for consumers.

Survival and Virulence of Listeria monocytogenes during Storage on Chocolate Liquor, Corn Flakes, and Dry-Roasted Shelled Pistachios at 4 and 23°C

ABSTRACT

The survival and virulence of Listeria monocytogenes was assessed during storage on three low-moisture foods (LMFs): chocolate liquor, corn flakes, and shelled, dry-roasted pistachios (water activity [aw] of 0.18, 0.27, and 0.20, respectively). The LMFs were inoculated with a four-strain cocktail of L. monocytogenes at 8 log CFU/g, dried, held until the aw stabilized, and then stored at 4°C and 25 to 81% relative humidity (RH) and at 23°C and 30 to 35% RH for at least 336 days. At 4°C, L. monocytogenes remained stable on the LMFs for at least 336 days. At 23°C, L. monocytogenes levels declined on the chocolate liquor, corn flakes, and pistachios at initial rates of 0.84, 0.88, and 0.32 log CFU/g/month, respectively. After 8 months at 23°C, L. monocytogenes levels on the chocolate liquor and corn flakes decreased to below the limit of detection (i.e., 0.48 log CFU/g). Relative populations of each strain were assessed before storage (i.e., day 0) and after 6 and 12 months of storage at 23 and 4°C, respectively. Generally, a decline in the relative level of the serotype 1/2a strain was observed during storage, coupled with the relative increase in other strains, depending on the LMF and storage temperature. The total viable populations of L. monocytogenes determined by the PMAxx quantitative PCR method after >12 months of storage at 4°C were significantly (1.8- to 3.7-log) higher than those obtained by plating on tryptic soy agar with yeast extract. Decreases in the culturable population of L. monocytogenes during storage on the LMFs were the result of both cellular inactivation and transition to a viable-but-nonculturable state. The surviving cells, specifically after long-term storage at 4°C on the chocolate liquor and pistachios, remained infectious and capable of intracellular replication in Caco-2 enterocytes. These results are relevant for predictive modeling used in microbial health risk assessments and support the addition of LMFs to food safety questionnaires conducted during listeriosis outbreaks.

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