Survival and heat resistance of Listeria monocytogenes after exposure to alkali and chlorine

A strain of Listeria monocytogenes isolated from a drain in a food-processi ng plant was demonstrated, by determination of D values, to be more resista nt to the lethal effect of heat at 56 or 59 degreesC following incubation f or 45 min in tryptose phosphate broth (TPB) at pH 12.0 than to that of incu bation for the same time in TPB at pH 7.3, Cells survived For at least 6 da ys when they were suspended in TPB at pHs 9.0, 10.0, and 11.0 and stored at 4 or 21 degreesC, Cells of L. monocytogenes incubated at 37 degreesC for 4 5 min and then stored for 48 or 144 h in TPB at pH 10.0 were more resistant to heat treatment at 56 degreesC than were cells stored in TPB at pH 7.3. The alkaline-stress response in L. monocytogenes may induce resistance to o therwise lethal thermal-processing conditions. Treatment of cells in 0.05 M potassium phosphate buffer (pH 7.00 +/- 0.05) containing 2.0 or 2.4 mg of free chlorine per liter reduced populations by as much as 1.3 log(10) CFU/m l, while treatment with 6.0 mg of free chlorine per liter reduced populatio ns by as much as 4.02 log(10) CFU/ml, Remaining subpopulations of chlorine- treated cells exhibited some injury, and cells treated with chlorine for 10 min were more sensitive to heating at 56 degreesC than cells treated for 5 min. Contamination of foods by L. monocytogenes cells that have survived e xposure to processing environments ineffectively cleaned or sanitized with alkaline detergents or disinfectants may have more severe implications than previously recognized. Alkaline-pH-induced cross-protection oft. monocytog enes against heat has the potential to enhance survival in minimally proces sed as well as in heat-and-serve foods and in foods on holding tables, in f ood service facilities, and in the home, Cells surviving exposure to chlori ne, in contrast, are more sensitive to heat; thus, the effectiveness of the rmal processing in achieving desired log(10)-unit reductions is not comprom ised in these cells.