Rapid and accurate detection of viable Escherichia coli O157:H7 in milk using a combined IMS, sodium deoxycholate, PMA and real-time quantitative PCR process

To improve the accuracy of current methods for the detection of Escherichia coli O157:H7 foodborne disease outbreaks, two reagents, propidium monoazide (PMA) and sodium deoxycholate (SD), were utilized to eliminate the interference of dead and injured cells for qPCR that was combined with immunomagnetic separation (IMS) for cell enrichment in an IMS-SD-PMA-qPCR assay. The optimal SD concentration and the optimal incubation time with SD were recorded at 0.1% and 20 min, respectively. The number of bacteria survivors was compared using plate counts, qPCR, PMA-qPCR, and SD-PMA-qPCR assays after the cell suspensions were heat treated at 63 °C or freeze stored at −20 °C. Cell suspensions treated or not treated with PMA and treated with SD resulted in significantly lower number of bacteria survivors than those analyzed without SD treatment which indicated that dead cell DNA was eliminated with SD. More importantly, the number of bacteria survivors from those analyzed with SD-PMA-qPCR showed a direct correlation with those analyzed using the plate counts methods. In addition, in order to improve the limit of detection (LOD) and shorten the detection time, immunomagnetic separation (IMS) was adopted to capture and enrich the target bacteria. Using spiked milk as a matrix, the IMS-SD-PMA-qPCR showed a detection limit of 102 CFU/mL. Significantly, even in the presence of 106 CFU/mL of non-target bacteria, the LOD for the SD-PMA-qPCR with IMS separation for E. coli O157:H7 in spiked milk matrix was recorded at 102 CFU/mL. This combination assay holds promise for the detection of foodborne E. coli O157:H7.