Growth delay analysis of heat-injured Salmonella Enteritidis in ground beef by real-time PCR

We aimed to estimate the bacterial injury level from ground beef samples that underwent various heat exposure treatments. The growth delay time (GDT) in recovery medium was estimated by real-time PCR monitoring assay. Samples of Salmonella Enteritidis in PBS and ground beef were exposed to heat stress in water bath at 52.5-62.5 °C for 0-60 min. Heat-treated samples were transferred to fresh trypticase soy broth, and S. Enteritidis growth recovery was monitored by real-time PCR. Sampling was conducted every 2 h, and total DNA was extracted. The S. Enteritidis cell number was estimated by real-time PCR, and growth recovery curve was constructed from the DNA copy number of the Salmonella invA gene. Growth recovery curve was used for kinetic analysis of GDT. Injured bacteria level in ground beef samples after heat exposure shown differences compared to PBS, where ground beef samples had lower variations in GDT than did PBS samples. Relationship between GDT and heat exposure time was observed where the slope of GDT increased as heat exposure time extended. Recovery of heat-treated S. Enteritidis in near sub-lethal conditions in PBS and ground beef samples, which could not be evaluated by traditional culture methods, was successfully monitored by real-time PCR.