Quantitative Detection of Viable but Nonculturable Cronobacter sakazakii Using Photosensitive Nucleic Acid Dye PMA Combined with Isothermal Amplification LAMP in Raw Milk

Assessing probiotic viability in mixed species yogurt using a novel propidium monoazide (PMAxx)-quantitative PCR method

Viability is a prerequisite for any therapeutic benefits associated with the ingestion of probiotic bacteria. Current culture-based techniques are inadequate for the enumeration of probiotics in mixed-species food products. This study utilized a quantitative PCR (qPCR) method coupled with propidium monoazide (PMAxx), and novel species-specific tuf gene primers to selectively enumerate Lacticaseibacillus rhamnosus, Bifidobacterium spp., and yogurt starter cultures in mixed-species probiotic yogurt. The method was optimized for PMAxx concentration and specificity and evaluated for efficiency and applicability. PMAxx-qPCR showed high specificity to the target organisms in mixed-species yogurt, quantifying only viable cells. The linear dynamic ranges were established over five to seven orders of magnitude. The assay was reliable with an efficiency of 91-99%, R2 values > 0.99, and a good correlation to the plate count method (r = 0.882). The results of this study demonstrate the high selectivity, improved lead time, and reliability of PMAxx-qPCR over the culture-dependent method, making it a valuable tool for inline viability verification during processing and improving probiotic quality assurance for processors and consumers.

Establishment of real-time fluorescence and visual LAMP for rapid detection of Escherichia coli O157:H7 and kits construction

Escherichia coli O157:H7 is a common pathogenic bacterium in food and water that can pose a threat to human health. The aim of this study was to develop loop-mediated isothermal amplification (LAMP) method for the detection of E. coli O157:H7 in food based on the specific gene Ecs_2840 and to construct rapid detection kits based on the established methods. Specifically, we established two methods of real-time fluorescent LAMP (RT-LAMP) and visual LAMP with calcein as an indicator. In pure bacterial culture, the cell sensitivity and genomic sensitivity of the RT-LAMP kit were 8.8 × 100 CFU ml-1 and 4.61 fg µl-1, respectively. The sensitivity of the visual LAMP kit was 2.35 × 100 CFU ml-1 and 4.61 fg µl-1. Both kits had excellent specificity and anti-interference performance. In addition, milk inoculated with 2.26 × 100 CFU ml-1E. coli O157:H7 could be detected within the reaction time after enrichment for 3 h. The results showed that the LAMP kits were rapid, sensitive, and specific for the detection of E. coli O157:H7 in food and had good application prospects in food safety surveillance.

Development of a real-time loop-mediated isothermal amplification method for monitoring Pseudomonas lurida in raw milk throughout the year of pasture

Introduction

The psychrophilic bacterium Pseudomonas lurida (P. lurida) and its thermostable alkaline proteases can seriously damage raw milk quality.

Methods

In this study, specific primers were designed for P. lurida's gyrB and aprX genes, and a real-time loop-mediated isothermal amplification (RealAmp) rapid detection method was developed for the early monitoring of P. lurida and its proteases in raw milk. A phylogenetic tree of the gyrB and aprX genes of P. lurida was constructed to analyze the homology of the design sequence of the RealAmp primer. The DNA of 2 strains of P. lurida and 44 strains of non-P. lurida were detected via RealAmp to analyze the specificity of the primer.

Results

It was found that aprX-positive proteases were produced by P. lurida-positive strains only when Pseudomonas fluorescens was negative. The dissociation temperatures of gyrB and aprX in the RealAmp-amplified products were approximately 85.0°C and 90.0°C, respectively. Moreover, DNA was detected through a 10-fold dilution of P. lurida in a pure bacterial solution and artificially contaminated skimmed milk. The limit of detection of P. lurida DNA copy number in the pure bacterial solution was 8.6 copies/μL and that in the 10% skimmed milk was 5.5 copies/μL. Further, 144 raw milk samples throughout the year from three farms in Hebei province were analyzed using RealAmp. The highest detection rate of P. lurida was 56% in the first and third quarters, and that of proteases was 36% in the second quarter. The detection rates of P. lurida and its proteases were the highest in samples collected from pasture 2 (52 and 46%, respectively), and the ability of P. lurida to produce proteases reached 88%.

Discussion

In conclusion, RealAmp established an early and rapid method for the detection of P. lurida and its proteases in raw milk samples, allowing the identification and control of contamination sources in a timely manner to ensure the quality of milk and dairy products.