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

Abstract

ABSTRACT Recovery of heat‐injured Listeria monocytogenes strain LM82 was evaluated quantitatively in Listeria enrichment broth (LEB) and in enriched cultures of cooked shrimp and Brie cheese. LM82 cells [108 colony forming units (CFU)/ml] were heated for 60 min at 52C in phosphate‐buffered saline. After 24 and 48 h enrichment, injured LM82 (6 replicates at each of 5 inoculation levels) were isolated on 3 selective media: lithium 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 value necessary to recover LM82 on half of the replicates of each type of isolation agar plate after streaking from the enrichment of measured inoculum. The RL50 values for injured cells were comparable to those of uninjured cells after 48 h enrichment in LEB without food. The type of isolation agar did not affect the RL50 value, although with food, MMA gave consistently but not significantly higher values, i.e., recovery inferior to that of LPMA and OXA. RL50 values were higher in Brie and cooked shrimp, presumably because of the competitive microflora in those foods. Addition of lactose or pyruvate to LEB improved recovery but had little or no effect when foods were present.