Inactivation of Listeria monocytogenes by chlorine

Cells of Listeria monocytogenes strain Scott-A were harvested from cultures, washed, and then treated with a solution of sodium hypochlorite at 25°C and pH 7. The cells were more resistant to chlorine when they were (a) harvested from a 24-rather than 48-h-old culture, (b) grown in tryptose broth rather than on a slant of tryptose agar, and (c) washed and suspended using a 20 rather than 0.312 mM phosphate buffer solution. Cells of L. monocytogenes were exposed for 30 s-4 h to sodium hypochlorite solutions that contained 0.5-10 ppm available chlorine. Generally, the number of survivors decreased rapidly during the first 30 s followed by a slower decrease during the rest of the exposure time. The initial count of L. monocytogenes in the suspension (1 × 10 -3.2 × 10 /ml) decreased 0.49 to 6.4 orders of magnitude during the first 30 s of exposure to the chlorine solutions. The effect of the presence of organic substances on the strength of hypochlorite solutions was studied. Presence of 0.05 or 0.1% peptone caused a large and rapid loss of available chlorine. Glucose or lactose (up to 1%) had almost no effect on the concentration of available chlorine.