Development of a technique to quantify the effectiveness of enrichment regimes in recovering "stressed" Listeria cells

Abstract

A rapid, reliable microwell plate method based on the most probable number (MPN) technique was used to determine the effectiveness of five enrichment regimes in the recovery and enumeration of Listeria spp. cells from five seafood products. The products tested were chosen to reflect conditions under which cells were exposed to the "stresses" associated with a variety of food-processing techniques, such as treatments involving an ethanol-based marinade, lowered pH (acetic acid), heat, sugar and salt brine (Gravilax), or frozen storage. Either Listeria monocytogenes and Listeria innocua were present in food samples as natural contaminants or L. monocytogenes was added in the laboratory. Listeria repair broth (LRB), buffered Listeria enrichment broth, Listeria enrichment broth (LEB), Fraser broth, and University of Vermont modified Listeria enrichment broth were used to recover Listeria cells. The effectiveness of these enrichment regimes was found to be dependent on the type of stresses the cells had been exposed to. After exposure to ethanol, recovery of L. monocytogenes cells was inhibited in enrichment regimes involving a nonselective period of resuscitation. On exposure to acetic acid, there were no significant differences (P < 0.05) between any of the regimes used. With heat-stressed cells, LRB recovered significantly fewer (P < 0.05) cells than did any other medium. On exposure to osmotic stress (elevated sugar and salt concentrations), LEB recovered the fewest cells. The largest number of cells was recovered from frozen fish (Hoki [Macruronus novazelandiae]) fillets with LRB. No single enrichment regime was consistently the most effective.