RECOVERY LIMITS OF HEAT-INJURED LISTERIA-MONOCYTOGENES FROM ENRICHMENT BROTH AND ENRICHED CULTURES OF INOCULATED FOODS

Recovery of heat-injured Listeria monocytogenes strain LM82 was evalua ted quantitatively in Listeria enrichment broth (LEB) and in enriched cultures of cooked shrimp and Brie cheese. LM82 cells [10(8) colony fo rming units (CFU)/ml] were heated for 60 min at 52C in phosphate-buffe red saline. After 24 and 48 h enrichment, injured LM82 (6 replicates a t each of 5 inoculation levels) were isolated on 3 selective media: li thium chloride-phenylethanol-moxalactam agar (LPMA), modified McBride agar (MMA) and Oxford agar (OXA). The recovery limit was expressed as a 50% end point value (RL50), which is the calculated inoculation valu e necessary to recover LM82 on half of the replicates of each type of isolation agar plate after streaking from the enrichment of measured i noculum. The RL50 values for injured cells were comparable to those of uninjured cells after 48 h enrichment in LEB without food. The type o f isolation agar did not affect the RL50 value, although with food, MM A gave consistently but not significantly higher values, i.e., recover y inferior to that of LPMA and OXA. RL50 values were higher in Brie an d cooked shrimp, presumably because of the competitive microflora in t hose foods. Addition of lactose or pyruvate to LEB improved recovery b ut had little or no effect when foods were present.