Single-Cell Growth Probability of Listeria monocytogenes at Suboptimal Temperature, pH, and Water Activity

The single-cell growth probability of Listeria monocytogenes was characterized in tryptone soy broth supplemented with 0.6% yeast extract at temperature ranging from 5 to 25°C, pH ranging from 4.4 to 6.5, and water activity ranging from 0.919 to 0.989. Growth was monitored visually in 96-microwell plates and the growth probability was deduced from concentrations estimated with the MPN calculation. Models were proposed to describe the increase of the probability from 0 to 1 with increasing values of environmental factors. An exponential model was used for the temperature and the individual minimal temperature for growth ranged from -3.6°C [-4.5°C, -2.7°C] to 17.3°C [16.0°C, 18.6°C]. An inverse exponential model was convenient for the pH and the minimal pH for growth of individual cells ranged from 4.34 [3.93, 4.75] to 5.93 [4.85, 7.01]. A linear model was used for the water activity and the minimal value for growth of individual cells ranged from 0.917 [0.909, 0.925] to 0.988 [0.966, 1]. In spite of the great between-experiment variability, the minimal values estimated for the growth limits of individual cells were in accordance with values usually proposed for L. monocytogenes populations. This study provides models allowing the prediction of the growth probability of a few cells contaminating chilled foods with suboptimal pH and water activity improving thus the assessment of the behavior of L. monocytogenes cells naturally contaminating foods.