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

Abstract

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 x 10 8 to 3.2 X 10 8 CFU/ml of sodium hypochlorite solution. Survival of L. monocytogenes was measured by surf ace-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 so-lution 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. Chlorine in small concentrations (0.15-0.25 ppm) is effective against many vegetative organisms, including common pathogens of both intestinal and respiratory ori-gin (exclusive of the tubercle bacillus) (21). Inactivation of microorganisms using chlorine is controlled by its ef-fectiveness as a disinfectant and by the susceptibility of a given organism to chlorine. The influence of pH and temperature on the effectiveness of chlorine against many organisms is well known. Different strains of a species may vary in susceptibility to chlorine (13,14). To inacti-vate L. monocytogenes using chlorine it is important to consider the temperature and pH of the chlorine solution, and the sensitivity of different strains of L. monocytogenes to chlorine. This report deals with these factors and is a continuation of the work on inactivation of L. monocyto-genes which was described earlier (8).