Estimation of viable Salmonella cell numbers in meat and meat product using real-time PCR

Recently we developed a novel method for estimating viable Salmonella cell numbers by means of a 5'-nuclease real-time PCR [Fujikawa et al., J. Food Hyg. Japan, 47, 151-156 (2006)]. The method was based on the increase kinetics of the target DNA region (inv A) of the microorganism growing in a culture medium during incubation. The index for the PCR was the threshold cycle. In this study, we validated the method for application in food. Namely, Salmonella cells spiked into ground chicken, pork, and beef and raw hamburger patty at various cell concentrations were cleaned up using buoyant density centrifugation and the Salmonella cell numbers were estimated with our method. Linear decreases in the threshold cycle value were observed during incubation of the samples. The standard curves for the cell number in all food samples were almost identical. With a standard curve using the mean parameter values, we successfully estimated viable Salmonella cell concentrations of the foods. The results indicate that our method is applicable for viable cell number estimation of the target microorganism in foods. Further, we used this method to study Salmonella growth in ground chicken stored at a constant temperature.