Inactivation of Surface-adherent Listeria monocytogenes Hypochlorite and Heat

Abstract

Inactivation by hypochlorite of Listeria monocytogenes cells adherent to stainless steel was determined. Adherent cell populations were prepared by incubating stainless steel slides with a 24 h culture of L. monocytogenes for 4 h at 21°C. Adherent microcolonies were prepared by growing L. monocytogenes on stainless steel slides submerged in a 1:15 dilution of tryptic soy broth at 21°C. The slides were then rinsed and transferred to fresh sterile broth every 2 d with a total incubation time of 8 d. Although the 4 h and 8 d adherent populations were at similar levels, 8 d adherent cells were over 100 times more resistant than the 4 h adherent cell population when exposed to 200 ppm hypochlorite for 30 s. When stainless steel slides containing adherent cells were heated at 72°C both adherent cell populations were inactivated after 1 min. Detectable numbers of L. monocytogenes remained on stainless steel slides after treatment at 65°C for 3 min when adherent 8 d cells were tested but not when adherent 4 h cells were used.