Oxidative-stress Resistance Response of Listeria monocytogenes and Its Cross Functionality against Lethal Processing Treatments

Introduction: Listeria monocytogenes (Lm) is capable of oxidative stress response upon pre-exposure to a sublethal H₂O₂ which allows it to survive subsequent lethal H₂O₂ stress. There is no knowledge on the effect of series of other sublethal stresses in combination with oxidative stress on the survival of Lm under lethal inactivation treatments.

Purpose: Determine the oxidative stress adaptive response in Lm serotypes 1/2a and 4b when cells were pre-exposed to a single or a series of sublethal stresses and their subsequent resistance to lethal H₂O₂, acid, alkaline and antimicrobials.

Methods: Concentrations of H₂O₂ and pre-exposure times were determined for the formation of oxidative-stress resistant phenotype of Lm which was detected by its survival in lethal 1000 ppm H₂O₂ at 37°C or 25°C. Lm cells pre-exposed to a sequential sublethal stresses (alkali pH of 9 followed by 50 ppm H₂O₂ for 30 min in TSBYE) were compared with that cells pre-exposed to a single sublethal stress for their subsequent cross resistance to lethal stresses (pH 3.5, pH 11.75, 60 °C,  and QAC 2.5 or 5ppm).

Results: The oxidative stress resistant phenotype of Lm was induced when Lm cells were exposed to 50 ppm H₂O₂ for 30 min at 25°C and 37°C in TSBYE. When Lm cells were pre-exposed to sublethal alkali (pH 9) followed by sublethal H₂O₂ (50 ppm), it resulted in a phenotype that exhibited a greater survival in 1000 ppm of H₂O₂ by more than 4 log CFU/ml than the cells that were pre-exposed to sublethal H₂O₂ (50 ppm) alone.  Also, sequential alkali- oxidative sublethally stressed cells of Lm exhibited greater resistance against both lethal alkali (pH 11.75) and QAC (5 ppm).

Significance: Highly stable oxidative-stress and alkali-stress resistant phenotype of Lm is formed when cells were pre-exposed to mild alkaline stress which was not known previously.