Listeria monocytogenes Sigb-Induced Hypersensitivity Against Oxidative Stress Is Mediated By a Down Regulation of Catalase Expression

Introduction:  Listeria monocytogenes is a foodborne pathogen that causes listeriosis, a life-threatening disease. Listeria monocytogenes needs to cope with hostile conditions, such as oxidizing environments, in order to survive and cause disease. Oxidative stress (OS) occurs during aerobic respiration, disinfection process (oxidative disinfectants), and during the infectious process. SigB is a well-known transcriptional regulator involved in regulating the expression of numerous stress- and virulence-related genes, whose role in OS is still controversial.

Purpose: This research was conducted to clarify the role of sigB against OS during different stages of the cell cycle.

Methods:   Listeria monocytogenes 10403S, wild type (WT) and sigB mutant (ΔsigB), were challenged with H₂O₂ in mid-exponential and stationary phases of growth. Bacterial viability was assessed by quantifying the CFU during the 60 min of challenge. In paralle,l the catalase activity was determined, using a novel visual method. Cell proliferation assays were performed by infecting Caco-2 (MOI=50) cells and following intracellular growth for 12 hours. Transcription of the catalase gene (kat) in different stages of growth was determine by RT-qPCR, using LightCycler 480 software to calculate the relative expression in comparison to the 16S rRNA gene.

Results: During stationary phase, the sigB mutant was significantly more resistant to H₂O₂ than the WT strain (~5 logs difference, p <0.05). However, during mid-exponential phase both strains showed similar resistance to OS (~4 logs reduction). During mid-exponential phase, the catalase activity was very low, both in WT and ΔsigB strains; however, after 12/14 hours of growth, the ΔsigB presented stronger catalase activity than the WT. The catalase activity assays showed a good correspondence with kat transcription, which was constantly up-regulated in ΔsigB (p <0.05), and demonstrated peak expression at ~12 hours of growth. The proliferation assays showed that both strains have similar intracellular growth.

Significance: These findings will help us understand, in depth, the OS resistance mechanisms of this pathogen and reduce the occurrence of disease.