Fig 3. SirRH controls the oxidative stress response of S. pneumoniae in both culture media and pneumocytes.

(A) The ΔsirR mutant is sensitive to H₂O₂. The ΔsirR, wr sirR⁺, ΔsodA and wt cells were grown in BHI and then exposed at BHI medium containing 20 mM H₂O₂ for 2 h. After that, viable cells were determined by spreading dilutions in BHI-blood-agar plates and incubating these at 37°C for 16 h. (B) Inhibition of ROS production in A549 cells does not affect the intracellular survival of the ΔsirR mutant. A549 cells were infected with the ΔsirR, ΔsodA and wt cells and intracellular survival was determined as described in the Fig 1 legend. White bars correspond to non-virus infected cells, black bars to IAV-infected cells and gray bars to NAC-treated cells. For all panels, data are representative of at least three independent experiments and statistically significant differences are indicated as p<0.05 (*) or p<0.001 (***).