Escherichia coli O157:H7 is a major foodborne pathogen causing food poisoning. Standard detection method for foodborne E. coli is plate counting, which is time consuming and omits VBNC cells. PSR is an isothermal amplification technique identifying trace amount DNA. However, its effectiveness is influenced by residual DNA from dead cells. This study aimed at designing a PMA-PSR system for accurate determination of viable cells of E. coli O157:H7. Firstly, PSR was optimized in fluorescence indicator, betaine concentration and reaction time. The optimized system could successfully identify rfbE, stx1 and stx2 genes within 45 min at 65 °C. Secondly, sensitivity and specificity were tested in pure culture and different food samples. With 100% specificity, PSR could determine DNA amount as low as 1.12 pg/μL. Thirdly, a PMA-PSR system was designed for accurate determination of viable cells. Cells were treated with 5 μg/mL PMA to remove residual DNA from dead cells. Pretreated cells were adapted to PSR, thus viable cells were specifically detected. Fourthly, the system was applied in food samples with VBNC cells. Viable cells could be accurately determined in accordance with regular assay. In conclusion, the PMA-PSR system was an efficient tool for accurate determination of E. coli O157:H7 viable cells.
Vibrio parahaemolyticus is a predominate cause of microbiological food safety. This study aimed to develop a polymerase spiral reaction (PSR) based assay for rapid and visual detection on V. parahaemolyticus . Firstly, the specificity and conservativity of species-specific target tlh gene were determined. Secondly, targeting tlh and virulence factors tdh and trh , PSR assay was developed and optimized using constructed positive plasmid controls and standard strain. Visualized result determination using fluorescent dye was applied to avoid false positive issues. Thirdly, 44 strains were used for specificity evaluation. Fourthly, pure and mixed species artificial contamination systems were included to test its applicability. In addition, a propidium monoazide (PMA)-PSR assay was set up to determine viable but non-culturable (VBNC) cells. Target tlh gene acquired high specificity and PSR assay was optimized to carry out at 65ᵒC for 1 h. Detection limit of PSR assay in purified genomic DNA, plasmid and artificially contaminated food samples were 5.31–53.1 pg/μL, 100 copies, and 10 3 CFU/mL, respectively, demonstrating the sensitivity as 100-fold higher than regular PCR. With PMA-PSR assay, VBNC cells were accurately determined without the disruption of dead cells and food compositions. Thus, PSR is applicable in detection of V. parahaemolyticus viable cells and virulence factors.
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