Thermal inactivation of Listeria monocytogenes within bovine milk phagocytes

Abstract

Thermal resistance of intracellular and freely suspended Listeria monocytogenes that was associated with a milkborne outbreak of listeriosis was studied by using the sealed tube and slug flow heat exchanger methods. Test temperatures for the former method were 57.8, 62.8, 66.1, and 68.9°C (136, 145, 151, and 156°F, respectively); whereas those for the latter method were 66.1, 68.9, 71.7, and 74.4°C (151, 156, 161, and 166°F, respectively). The heating menstruum was sterile, whole milk. The intracellular inoculum was generated from an in vitro phagocytosis reaction by using endotoxin-induced bovine milk phagocytes. The phagocyte population consisted of 88% neutrophils, 8% macrophages, and 4% lymphocytes. Neutrophils harbored the majority of intracellular L. monocytogenes. The mean level of infectivity in the phagocyte population was 43%, and there were 26.1 ± 19.3 bacteria per cell (104 viable cells per ml of test milk). Initial bacterial counts for the freely suspended and intracellular experiments (the latter was based on a sonically disrupted sample) were 106 L. monocytogenes cells per ml. Heat-stressed bacteria were recovered by direct plating in parallel with recovery from an enrichment broth; both methods gave comparable results. The predicted D62.8(°C) (145°F) value for intracellular sealed tube studies was 53.8 s (Z(D) = 5.6°C [10.0°F]), indicating a safe 33.4 D margin of inactivation for vat pasteurization (62.8°C for 30 min). The predicted D71.7(°C) (161°F) value for intracellular slug flow heat exchanger studies was 4.1 s (Z(D) = 8.0°C [14.4°F]), indicating a potentially unsafe 3.7 D margin of inactivation for the high-temperature, short-time pasteurization minimum (71.7°C for 15 s). Results of extracellular and intracellular experiments were similar for both test systems. These data indicate that, under the defined parameters of this study, the intracellular position of L. monocytogenes does not significantly augment heat resistance.