Growth delay analysis of high-salt injured Escherichia coli O157:H7 in fermented soybean paste by real-time PCR and comparison of this method with other estimation methods

Abstract The aim of this study was to develop an estimation method for bacterial injury levels of Escherichia coli O157:H7 in food products. Bacterial growth was monitored in recovery medium and quantified by real-time PCR to quantify the growth delay phenomenon as growth delay time (GDT). The relationship between GDT and stress levels (salt-concentration: 9, 13, 15, and 17%; stress exposure times:0, 6, 12, 18, and 24 h) was determined. The GDT increased with stress exposure time, and the higher the salt concentration was, the faster the GDT increased. Moreover, bacterial injury levels in 15% NaCl-containing media were estimated by a conventional culture-based method, bacterial membrane destruction assay, NADPH content assay, and intercellular ATP content assay. In these estimations, the result suggested a decrease in bacterial activity and increase in bacterial membrane destruction from 6 to 12 h of exposure, which was consistent with the results of the GDT monitoring method. Furthermore, the estimation of bacterial injury levels in high-salt food (fermented soybean paste) was investigated, and GDT extension was successfully observed until the bacteria completely reached sublethal conditions. Therefore, GDT monitoring by real-time PCR showed the ability to estimate bacterial injury levels along with other existing methods.