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-processing plant was demonstrated, by determination of D values, to be more resistant to the lethal effect of heat at 56 or 59°C following incubation for 45 min in tryptose phosphate broth (TPB) at pH 12.0 than to that of incubation for the same time in TPB at pH 7.3. Cells survived for at least 6 days when they were suspended in TPB at pHs 9.0, 10.0, and 11.0 and stored at 4 or 21°C. Cells of L. monocytogenes incubated at 37°C for 45 min and then stored for 48 or 144 h in TPB at pH 10.0 were more resistant to heat treatment at 56°C than were cells stored in TPB at pH 7.3. The alkaline-stress response in L. monocytogenes may induce resistance to otherwise 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/ml, while treatment with 6.0 mg of free chlorine per liter reduced populations 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°C than cells treated for 5 min. Contamination of foods by L. monocytogenes cells that have survived exposure 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 of L. monocytogenes against heat has the potential to enhance survival in minimally processed as well as in heat-and-serve foods and in foods on holding tables, in food service facilities, and in the home. Cells surviving exposure to chlorine, in contrast, are more sensitive to heat; thus, the effectiveness of thermal processing in achieving desired log(10)-unit reductions is not compromised in these cells.

从食品加工厂排水口分离得到的一株单核细胞增生李斯特菌（Listeria monocytogenes），经D值（D value）测定证实：在pH 12.0的胰蛋白胨磷酸盐肉汤（tryptose phosphate broth, TPB）中孵育45分钟后，其对56℃或59℃热致死效应的抵抗力，显著强于在pH 7.3的TPB中孵育相同时长的菌体。当该菌悬浮于pH 9.0、10.0及11.0的TPB中并于4℃或21℃储存时，至少可存活6天。将单核细胞增生李斯特菌于37℃孵育45分钟后，置于pH 10.0的TPB中储存48或144小时的菌体，对56℃热处理的抵抗力，显著高于在pH 7.3的TPB中储存的菌体。单核细胞增生李斯特菌的碱性胁迫应答可诱导其对原本具有致死性的热加工条件产生抗性。将菌体置于含2.0或2.4 mg/L游离氯的0.05 M磷酸钾缓冲液（pH 7.00±0.05）中处理，可使菌群数量降低最多1.3 log₁₀ CFU/mL；而使用6.0 mg/L游离氯处理时，菌群数量可降低最多4.02 log₁₀ CFU/mL。经氯处理的残留菌群存在一定损伤，且处理10分钟的菌体对56℃加热的敏感性高于处理5分钟的菌体。经未充分清洁或使用碱性洗涤剂、消毒剂消毒的加工环境暴露后存活的单核细胞增生李斯特菌，其对食品的污染可能带来比此前认知更为严重的危害。碱性pH诱导的单核细胞增生李斯特菌耐热交叉保护特性，可增强其在轻度加工食品、即热食品、餐饮服务设施保温台以及家庭环境中的存活能力。与之相反，经氯处理存活的菌体对热的敏感性更高，因此针对这类菌体实施热处理时，仍可达到预期的log₁₀单位菌落数降幅。