Efficacy of Enrichment Media for Recovery of Heat-Injured Listeria monocytogenes

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Abstract
The efficacy of University of Vermont (UVM) and Listeria enrichment broth (LEB) enrichment broths for the recovery of heat-injured Listeria monocytogenes was determined in pure culture and in the presence of competitive microflora from chicken and Brie cheese. Both UVM and LEB were found to be effective for recovery of viable L. monocytogenes. Only UVM consistently allowed recovery of heat-injured L. monocytogenes. The different concentrations of NaCl used in these media did not account for these differences. The pH of LEB and UVM broth containing mixed microflora from chicken and Brie cheese was 5.2 to 5.7 and 6.7 to 6.8, respectively. The presence of glucose and the lack of an adequate buffering system in LEB accounted for these differences in recovery. Recovery of heat-injured cells of pure culture L. monocytogenes was reduced at pH 5.5 to 5.8, and no recovery was detected below pH 5.5 when pH adjusted LEB was used. UVM is recommended as a primary enrichment broth for recovery of heat-injured L. monocytogenes.