Acid or salt adaptation of Listeria monocytogenes 10403S grown until exponential phase aerobically, enhances sensitivity to oxidative stress.

Aims: The work aimed at investigating a possible role of sigB in catalase transcription and activity in Listeria monocytogenes. Furthermore, we also aimed to investigate whether sigB upregulation during the exponential phase, due to acid or salt adaptation, could result in hypersensitivity to oxidative stress. Finally, we investigated how this discovery could be used in the wider concept of Hurdle Technology through combination of different stresses.

Methods and results: Listeria monocytogenes 10403S WT and ΔsigB strains were grown aerobically, and catalase transcription and activity were assessed at different growth stages. Catalase transcription peaked at 6 h of growth in both strains, with ΔsigB showing higher levels. Subsequently, from 8 to 10 h, a major drop to similarly low levels occurred for both strains. However, catalase activity peaked 2 h later (at 8 h of growth) than transcription and remained higher in ΔsigB beyond this point. To evaluate stress adaptation, exponential-phase cells were exposed to sub-lethal acidic conditions (pH 4.5; HCl) or salt (0.5 mol l-1 NaCl) and later subjected to H2O2 or sonication (tested only with acid). Adaptation increased sensitivity in the wild type (WT) but not in ΔsigB, underpinning the negative role of sigB upregulation. Acid adaptation reduced catalase activity in both strains, explaining the reduced oxidative stress resistance, although salt adaptation did not affect catalase activity. After adaptation to acid or salt, application of oxidative stress without removing the initial adaptation stresses resulted in a higher synergistic effect in both WT and ΔsigB.

Conclusion: The above synergistic effects are important for our understanding of listerial oxidative stress resistance and optimization of relevant oxidative stress decontamination processes (e.g. oxidative compounds, ultrasound, and plasma treatments) but also virulence.