Survival and Heat Resistance of Listeria monocytogenes after Exposure to Alkali and Chlorine

Abstract

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 crossprotection 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.

Sensitivity of Escherichia coli O157:H7 to Commercially Available Alkaline Cleaners and Subsequent Resistance to Heat and Sanitizers

“…Results also support observations made for L. monocytogenes, which exhibited higher D 56°C and D 59°C values after exposure to tryptose phosphate broth at pH 12.0 than did cells treated at pH 7.3 (31). Cells of Vibrio parahaemolyticus that were adapted to an environment at pH 9.0 for 2 h showed increased resistance to heat, crystal violet, deoxycholic acid, and hydrogen peroxide (14).…”

Habituation of Foodborne Pathogens Exposed to Extreme pH Conditions: Genetic Basis and Implications in Foods and Food Processing Environments

“…A strain of L. monocytogenes freshly isolated from a food processing environment was shown to be more resistant to heating at 56 or 59˚C following incubation for 45 min in tryptose phosphate broth (TPB) at pH 12.0 compared to treatment for the same time in TPB at pH 7.3 (Taormina & Beuchat, 2001). Cells of L. monocytogenes survived at least 6 days in TPB at pH 9.0, 10.0, and 11.0 stored at 4 or 21˚C, and cells stored in TPB at pH 10.0 for 48 h were more resistant to heating at 56˚C than were cells stored in TPB at pH 7.3 for the same time.…”

Role of Branched-Chain Fatty Acids in pH Stress Tolerance in Listeria monocytogenes

“…However, little is known about pH stress-associated modulation of FAs in L. monocytogenes (21), the wider role of FA modulation in its responses to nonthermal stresses, or the cross-protection mechanisms which operate in this hardy pathogen (19,27,35,44).…”

“…Reduced growth of pH-sensitive mutants at adverse pH (5.0/9.0) was alleviated by the addition of 2-methylbutyrate (an anteiso-FA precursor), suggesting that anteiso-FAs are important in adaptation to adverse pH. The balance between anteiso-and iso-FAs may be more important than changes in the amounts and/or degrees of saturation of FAs in pH adaptation.Listeria monocytogenes can grow under a wide range of pH stress, i.e., 4.1 to 9.0 (38, 46), increasing its abilities to persist during food processing and attempts to decontaminate food-processing environments (15,(42)(43)(44)(45). It also has particularly impressive capacities to modulate its membrane lipids to maintain membrane fluidity and transport functions (10,40,41) in response to temperature (1), salt (7), and CO 2 /anaerobiotic (21) stress.…”