Figure 4. Role of AsrR in oxidative stress response.

(A) Survival after a 30-min stress with 2 mM H₂O₂ for the parental (HM1070, black bar), mutant (ΔasrR, light grey bar) and complemented (ΔasrR::asrR, dark grey bar) strains. Cells were collected in stationnary phase (Stat), growth exponential phase (Expo), and exponential growth phase in the presence of deferoxamine (DFX) or tiron. Values are expressed as mean percentages (± standard deviation) of survival cells after oxidative stress compared to unstressed conditions from at least three independent experiments. (B) Menadione susceptibility of HM1070, ΔasrR and ΔasrR::asrR strains. E. faecium strains were 4-times serially diluted from a standardized (MacFarland = 1) cell suspension and spotted on BHI agar plates supplemented without or with (0.5 mM) menadione. Experiments were repeated at least three times and similar results were obtained. (C) Time course of intracellular survival of E. faecium parental (HM1070, diamonds), mutant (ΔasrR, triangles) and trans-complemented (ΔasrR/pOri23ΩasrR, squares) strains within murine peritoneal macrophages. Data are the mean numbers (± standard deviations) of viable intracellular bacteria per 10⁵ macrophages from three independent experiments in triplicate. The ΔasrR strain found to be more susceptible in vitro to H₂O₂ and menadione oxidative stress showed impaired survival in mouse macrophages compared to the parental and complemented strains.