Purpose: We have determined the heat-stress adaptation and heat tolerance response of 37 diverse strains of Lm representing all 13 serotypes and of 7 other Listeria spp.
Methods: A diverse set of 37 Lm strains were evaluated for heat tolerance at 60°C with 107 CFU/ml in TSBYE after pre-exposure to mild heat of 48°C for 0, 5, 15, 30, 60 and 90 min. Survival curves were plotted and D-values were calculated. Lag phase of mild heated cells was observed by measuring the optical density at 630 nm at regular intervals. The stability of heat-stress adaptation after pre-exposure to 48°C was determined by cooling Lm cells to 25°C or down to 4°C before 60°C lethal stress. Survivals were enumerated on TSA plates enriched with esculin and ferric ammonium citrate.
Results: The 37 strains of Lm were classified into three groups based on their survival at 60°C/10 min: low (0-2 log CFU/ml), medium (2-4 log) and high (4-6 log) heat tolerant strains. Heat-stress adaptation response of Lm was dependent on the time of pre-exposure to mild heat-stress. About 30-60 min pre-exposure to 48°C led to a maximum heat-stress adaptation in Lm strains where cells of some strains exhibited 5 log CFU/ml greater survival than non-stressed cells when subjected to 60°C lethal challenge. The heat-stress adaptation was stable in Lm after cooling cells from 48°C to 25°C for 2 h or for up to 24 h if cooled from 48°C to 4°C prior to challenging at 60°C.
Significance: Heat-stress adaptation of Lm differs with different strains. The stability of heat-stress adaptation in Lm after cooling cells to 4°C may create a risk for ready-to-eat foods.