Resistance of Listeria monocytogenes to Heat after Adaptation to Environmental Stresses

Abstract

Changes in the thermotolerance of Listeria monocytogenes at different stages of growth and in the presence of stress factors were studied. Stresses investigated include starvation and treatment with hydrogen peroxide, ethanol, and low pH. Stressed cells were suspended in phosphate buffer (0.1 M, pH 7.0) and their thermotolerance was determined by measuring D_56°C-value using the capillary tube method. D_56°C remained constant (1.0 min) during the exponential phase of growth and then increased to a maximum of 8.6 min during the late exponential and early stationary phases. Cells of L. monocytogenes were harvested when in the exponential phase and suspended in phosphate buffer at pH 7.0. The cells were then starved by incubating the suspension for up to 163 h at 30°C. D_56°-values increased during starvation and reached a maximum of 13.6 min. Ethanol (0.5 to 12%, vol/vol), acid (pH 4.0 to 7.0), or H₂O₂ (27 to 500 ppm) was added to L. monocytogenes cultures in the exponential phase and the cells were allowed to grow for 1 h (one to two doublings of the control treatment). All treatments caused increases in the thermotolerance of the pathogen. Maximum tolerance was observed in cells exposed to 4 to 8% ethanol, pH 4.5, and 500 ppm H₂O₂; the corresponding averages for D_56°C-values were 4.1, 8.8, and 2.9 min, respectively.

Keywords: D-values; Stress; heat resistance.