Propidium Monoazide Quantitative Real-Time Polymerase Chain Reaction for Enumeration of Some Viable but Nonculturable Foodborne Bacteria in Meat and Meat Products

Quantitative Detection of Viable but Nonculturable Vibrio parahaemolyticus in Frozen Bivalve Molluscs

Vibrio parahaemolyticus is a foodborne pathogen diffusely distributed in the marine environment and often isolated from raw seafood belonging to different species, mostly shellfish. Ingestion of under- or uncooked seafood contaminated by V. parahaemolyticus can cause severe gastrointestinal symptoms in humans. Due to its ability to withstand low temperatures, Vibrio spp. could survive in frozen seafoods for long periods by entering the viable but nonculturable state (VBNC) and may constitute an unrecognized source of food contamination and infection. In the present study, seventy-seven frozen bivalve molluscs (35 mussels; 42 clams) were subjected to the detection and enumeration of viable V. parahaemolyticus using standard culture methods. VBNC forms were detected and quantified by applying an optimized protocol based on Propidium Monoazide (PMA) and Quantitative PCR (qPCR). All samples were negative for both the detection and enumeration of V. parahaemolyticus by the standard culture methods. VBNC forms were detected in 11.7% of the samples (9/77), with values ranging from 1.67 to 2.29 Log CFU/g. Only clam samples were positive for the detection of VBNC forms. The results of this study highlighted that VBNC V. parahaemolyticus may be present in frozen bivalve molluscs. Further data on the prevalence of VBNC V. parahaemolyticus in frozen seafood are needed in order to perform a robust risk assessment.

Current Perspectives on Viable but Non-Culturable Foodborne Pathogenic Bacteria: A Review

Foodborne diseases caused by foodborne pathogens pose risks to food safety. Effective detection and efficient inactivation of pathogenic bacteria has always been a research hotspot in the field of food safety. Complicating these goals, bacteria can be induced to adopt a viable but non-culturable (VBNC) state under adverse external environmental stresses. When in the VBNC state, pathogens cannot form visible colonies during traditional culture but remain metabolically active and toxic. The resulting false negative results in growth-related assays can jeopardize food safety. This review summarizes the latest research on VBNC foodborne pathogens, including induction conditions, detection methods, mechanism of VBNC formation, and possible control strategies. It is hoped that this review can provide ideas and methods for future research on VBNC foodborne pathogenic bacteria.

Antibacterial activity of bioactive compounds extracted from red kidney bean (Phaseolus vulgaris L.) seeds against multidrug-resistant Enterobacterales

In the present study, biologically active compounds such as phenolic-rich extract (PRE), 7S globulin (vicilin), and 11S globulin (legumin) from red kidney bean (Phaseolus vulgaris L.) seeds were extracted and evaluated as antibacterial agents against multidrug-resistant (MDR) Enterobacterales isolated from both animal and human sources. The overall occurrence rate of Enterobacterales was 43.6%, which significantly differed between animal (38.75%) and human (56.67%) sources. Antimicrobial susceptibility testing revealed that Enterobacterales isolates exhibited full resistance (100%) to amoxicillin-clavulanic acid, followed by ampicillin (75.44%), erythromycin (71.93%), cefoxitin (70.18%), amoxicillin (66.66%), ceftriaxone (64.91%), and trimethoprim/sulfamethoxazole (56.14%). Worthy of note, 97.92% of Enterobacterales isolates were MDR. The total phenolic contents (TPC; 53 ± 2 mg GAE g-1) and total flavonoid contents (TFC; 26 ± 1 mg QE g-1) were recorded. The major phenolic and flavonoid components were catechol (17.63 μg/mL) and hesperidin (11.37 μg/mL), respectively. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was performed to detect the 7S and 11S globulin‘s molecular mass. The data revealed that red kidney bean protein isolate (KPI) includes two major portions: 7S and 11S globulins. The bioactive compounds of Phaseolus vulgaris were investigated for their antibacterial activities against Enterobacterales for the first time. The protein component (MIC = 0.125 – 2 μg/mL; 53.85%) and its 7S and 11S globulin subunits (MIC = 0.5 – 2 μg/mL; 30.77% each) were the most potent extracts, whereas the methanolic extract was the least effective one (MIC = 2 μg/mL; 15.38%). The results displayed the potential of protein bioactive compounds as a hopeful candidate for enhancing future medication plans for the treatment of Enterobacterales originating from animal and human sources.

Point-of-Care Lateral Flow Detection of Viable Escherichia coli O157:H7 Using an Improved Propidium Monoazide-Recombinase Polymerase Amplification Method

The detection of both viable and viable but non-culturable (VBNC) Escherichia coli O157:H7 is a crucial part of food safety. Traditional culture-dependent methods are lengthy, expensive, laborious, and unable to detect VBNC. Hence, there is a need to develop a rapid, simple, and cost-effective detection method to differentiate between viable/dead E. coli O157:H7 and detect VBNC cells. In this work, recombinase polymerase amplification (RPA) was developed for the detection of viable E. coli O157:H7 through integration with propidium monoazide (PMAxx). Initially, two primer sets, targeting two different genes (rfbE and stx) were selected, and DNA amplification by RPA combined with PMAxx treatment and the lateral flow assay (LFA) was carried out. Subsequently, the rfbE gene target was found to be more effective in inhibiting the amplification from dead cells and detecting only viable E. coli O157:H7. The assay’s detection limit was found to be 10² CFU/mL for VBNC E. coli O157:H7 when applied to spiked commercial beverages including milk, apple juice, and drinking water. pH values from 3 to 11 showed no significant effect on the efficacy of the assay. The PMAxx-RPA-LFA was completed at 39 °C within 40 min. This study introduces a rapid, robust, reliable, and reproducible method for detecting viable bacterial counts. In conclusion, the optimised assay has the potential to be used by the food and beverage industry in quality assurance related to E. coli O157:H7.

Method-Dependent Implications in Foodborne Pathogen Quantification: The Case of Campylobacter coli Survival on Meat as Comparatively Assessed by Colony Count and Viability PCR

The aim of the present study was to address method-dependent implications during the quantification of viable Campylobacter coli cells on meat over time. Traditional colony counting on selective and non-selective culture media along with an optimized viability real-time PCR utilizing propidium monoazide-quantitative PCR (PMA-qPCR), spheroplast formation and an internal sample process control (ISPC), were comparatively evaluated for monitoring the survival of C. coli on fresh lamb meat during refrigeration storage under normal atmospheric conditions. On day zero of three independent experiments, lamb meat pieces were artificially inoculated with C. coli and then stored under refrigeration for up to 8 days. Three meat samples were tested on different days and the mean counts were determined per quantification method. An overall reduction of the viable C. coli on lamb meat was observed regardless of the applied quantification scheme, but the rate of reduction followed a method-dependent pattern, the highest being observed for colony counting on modified charcoal cefoperazone deoxycholate agar (mCCDA). Univariate ANOVA indicated that the mean counts of viable C. coli using PMA-qPCR were significantly higher compared to Columbia blood agar (CBA) plating (0.32 log₁₀ cell equivalents, p = 0.015) and significantly lower when mCCDA was compared to CBA plating (0.88 log₁₀ CFU, p < 0.001), indicating that selective culture on mCCDA largely underestimated the number of culturable cells during the course of meat storage. PMA-qPCR outperformed the classical colony counting in terms of quantifying both the culturable and viable but non-culturable (VBNC) C. coli cells, which were generated over time on meat and are potentially infectious and equally important from a public health perspective as their culturable counterparts.

How to Evaluate Non-Growing Cells-Current Strategies for Determining Antimicrobial Resistance of VBNC Bacteria

Thanks to the achievements in sanitation, hygiene practices, and antibiotics, we have considerably improved in our ongoing battle against pathogenic bacteria. However, with our increasing knowledge about the complex bacterial lifestyles and cycles and their plethora of defense mechanisms, it is clear that the fight is far from over. One of these resistance mechanisms that has received increasing attention is the ability to enter a dormancy state termed viable but non-culturable (VBNC). Bacteria that enter the VBNC state, either through unfavorable environmental conditions or through potentially lethal stress, lose their ability to grow on standard enrichment media, but show a drastically increased tolerance against antimicrobials including antibiotics. The inability to utilize traditional culture-based methods represents a considerable experimental hurdle to investigate their increased antimicrobial resistance and impedes the development and evaluation of effective treatments or interventions against bacteria in the VBNC state. Although experimental approaches were developed to detect and quantify VBNCs, only a few have been utilized for antimicrobial resistance screening and this review aims to provide an overview of possible methodological approaches.

The diagnostic tools for viable but nonculturable pathogens in the food industry: Current status and future prospects

Viable but nonculturable (VBNC) microorganisms have been recognized as pathogenic contaminants in foods and environments. The failure of VBNC cells to form the visible colonies hinders the ability to use conventional media for their detection. Efficient and rapid detection of pathogens in the VBNC state is a prerequisite to ensure the food safety and public health. Despite their nonculturability, VBNC cells have distinct characteristics, such as morphology, metabolism, chemical composition, and gene and protein expression, that have been used as the basis for the development of abundant diagnostic tools. This review covers the current status and advances in various approaches for examining microorganisms in the VBNC state, including but not limited to the methodological aspects, advantages, and drawbacks of each technique. Existing methods, such as direct viable count, SYTO/PI dual staining, and propidium monoazide quantitative polymerase chain reaction (PCR), as well as some techniques with potential to be applied in the future, such as digital PCR, enhanced-surface Raman spectroscopy, and impedance-based techniques, are summarized in depth. Finally, future prospects for the one-step detection of VBNC bacteria are proposed and discussed. We believe that this review can provide more optional methods for researchers and promote the development of rapid, accurate detecting methods, and for inspectors, the diagnostic tools can provide data to undertake risk analysis of VBNC cells.

In vitro discrimination and classification of Microbial Flora of Poultry using two dispersive Raman spectrometers (microscope and Portable Fiber-Optic systems) in tandem with chemometric analysis

Discrimination and classification of eight strains related to meat spoilage and pathogenic microorganisms commonly found in poultry meat were successfully carried out using two dispersive Raman spectrometers (Microscope and Portable Fiber-Optic systems) in combination with chemometric methods. Principal components analysis (PCA) and multi-class support vector machines (MC-SVM) were applied to develop discrimination and classification models. These models were certified using validation data sets which were successfully assigned to the correct bacterial species and even to the right strain. The discrimination of bacteria down to the strain level was performed for the pre-processed spectral data using a 3-stage model based on PCA. The spectral features and differences among the species on which the discrimination was based were clarified through PCA loadings. In MC-SVM the pre-processed spectral data was subjected to PCA and utilized to build a classification model. When using the first two components, the accuracy of the MC-SVM model was 97.64% and 93.23% for the validation data collected by the Raman Microscope and the Portable Fiber-Optic Raman system, respectively. The accuracy reached 100% for the validation data by using the first eight and ten PC's from the data collected by Raman Microscope and by Portable Fiber-Optic Raman system, respectively. The results reflect the strong discriminative power and the high performance of the developed models, the suitability of the pre-processing method used in this study and that the low accuracy of the Portable Fiber-Optic Raman system does not adversely affect the discriminative power of the developed models.

Quantitative Polymerase Chain Reaction Coupled With Sodium Dodecyl Sulfate and Propidium Monoazide for Detection of Viable Streptococcus agalactiae in Milk

Streptococcus agalactiae is an important pathogen causing bovine mastitis. The aim of this study was to develop a simple and specific method for direct detection of S. agalactiae from milk products. Propidium monoazide (PMA) and sodium dodecyl sulfate (SDS) were utilized to eliminate the interference of dead and injured cells in qPCR. Lysozyme (LYZ) was adopted to increase the extraction efficiency of target bacteria DNA in milk matrix. The specific primers were designed based on cfb gene of S. agalactiae for qPCR. The inclusivity and exclusivity of the assay were evaluated using 30 strains. The method was further determined by the detection of S. agalactiae in spiked milk. Results showed significant differences between the SDS–PMA–qPCR, PMA–qPCR and qPCR when a final concentration of 10 mg/ml (R² = 0.9996, E = 95%) of LYZ was added in DNA extraction. Viable S. agalactiae was effectively detected when SDS and PMA concentrations were 20 μg/ml and 10 μM, respectively, and it was specific and more sensitive than qPCR and PMA–qPCR. Moreover, the SDS–PMA–qPCR assay coupled with LYZ was used to detect viable S. agalactiae in spiked milk, with a limit of detection of 3 × 10³ cfu/ml. Therefore, the SDS–PMA–qPCR assay had excellent sensitivity and specificity for detection of viable S. agalactiae in milk.

Mapping the dominant microbial species diversity at expiration date of raw meat and processed meats from equine origin, an underexplored meat ecosystem, in the Belgian retail

Although equine meats and their derived smoked or fermented products are popular in some regions of the world, they only form a minor fraction of the global meat consumption. The latter may explain why their associated bacterial communities have not received much attention. In the present study, 69 different samples of equine meats and meat products were investigated. The samples consisted of raw meat from horses (17 samples) and zebra (7), as well as non-fermented but smoked (24) and fermented (21) horse meat products. After purchase, all samples were stored at 4 °C and analysed at expiration date. Besides an estimation of the total microbial counts, specific attention was paid to the identification of lactic acid bacteria (LAB) and catalase-positive cocci, in particular the group of coagulase-negative staphylococci (CNS), involved, due to their technological relevance in view of the elaboration of meat products. Samples that were loosely wrapped in butcher paper instead of vacuum- or modified-atmosphere packages were also screened for pseudomonads and enterobacterial species. In total, 1567 bacterial isolates were collected, subjected to (GTG)₅-PCR fingerprinting of genomic DNA, and identified by multiple gene sequencing (based on the 16S rRNA, pheS, rpoA, rpoB, and/or tuf genes). Overall, the bacterial species diversity consisted mostly of LAB but was contingent on the type of product. Raw meat was dominated by Carnobacterium divergens, Lactobacillus sakei, Lactococcus piscium, and Leuconostoc gelidum, with zebra meat being particularly rich in lactococci. Smoked and fermented horse meat products contained mostly Lb. sakei and, to a lesser degree, Lactobacillus curvatus. In addition, several catalase-positive cocci (mostly Staphylococcus equorum), Anoxybacillus sp., Brevibacterium sp., Brochothrix thermosphacta, and the enterobacterial species Hafnia alvei were found.