Temperature, pH, and Strain of Pathogen as Factors Affecting Inactivation of Listeria monocytogenes by Chlorine.

Listeria monocytogenes strain Scott-A was treated with 1 ppm available chlorine at different temperatures and pH values. Different strains of L. monocytogenes (California, Scott-A and V7) were also exposed to 1 ppm available chlorine at pH 7 and 25°C. The initial population of L. monocytogenes was 1 × 108 to 3.2 × 108 CFU/ml of sodium hypochlorite solution. Survival of L. monocytogenes was measured by surface-plating (on tryptose agar) samples taken at intervals of 30 s to 1 h of exposure to hypochlorite solution. Larger numbers of L. monocytogenes strain Scott-A survived at 25 than at 35°C. The smallest number was observed when cells were exposed to the hypochlorite solution at 5°C. The higher the pH values, in the range of 5 to 9, the greater were the numbers of survivors of L. monocytogenes strain Scott-A. Of the strains studied, California was the most resistant, while V7 was the least resistant to the hypochlorite solution.