Antimicrobial Effect of Chlorine on Listeria monocytogenes

The antimicrobial effect of reagent-grade sodium hypochlorite (SH) and household bleach (HB) on 2 strains of Listeria monoctiytogenes (Scott A and LCDC 81-861, both serotype 4a) was determined. After 24 h of growth in tryptic soy broth, cells were centrifuged, and pellets resuspended in potassium phosphate buffer (pH 7.0). Three-milliliter portions of the cell suspensions were then added to 27 ml of phosphate buffer containing about 0, 5, 10, 50, 100, or 200 ppm free residual chlorine. Cells were exposed to the chlorine for 20, 60, 120 and 300 s, at which time the chlorine was neutralized with 0.01 M sodium thiosulfate. Populations of surviving cells were determined by plating samples of the neutralized solution on tryptic soy agar and incubating the plates for 48 h at 30°C before counting. Chlorine concentrations less than about 50 ppm showed no antimicrobial effect but exposure to 50 ppm or greater chlorine resulted in no viable cells being recovered. Results for both SH and HB were similar. Dipping Brussels sprouts containing about 6 log₁₀ colony forming units (CFU) L. monocytogenes /g into a 200-ppm chlorine solution for 10 s reduced viable cells recovered on McBrides agar by about 2 log₁₀ CFU/g. Dipping Brussels sprouts in water alone reduced populations by about 1 log₁₀ CFU/g.