A rapid sample preparation method for PCR detection of food pathogens based on buoyant density centrifugation

The use of buoyant density centrifugation (BDC) to prepare samples for PCR analysis of food pathogens is described. Blue cheese and milk homogenates were inoculated with Shigella flexneri and layered on top of Percoll media. After BDC, the food homogenates remained in the upper part of the centrifuge tube, separated from the bacteria, which retained viability and were concentrated below the lighter Percoll layer. PCR inhibitors stayed in the homogenate and PCR analyses of treated samples consistently detected 10(4) cfu g-1 of blue cheese and 500 cfu ml-1 of milk, respectively. Differences in the density of live and killed Sh. flexneri and Yersinia enterocolitica were detected by BDC but were dependent on the mechanism of killing.