Fig. 2. S. sanguinis ΔspxB protects against H₂O₂ mediated killing.

a A representative H₂O₂ protection assay is shown. SK36 was grown on BHI agar plates, overlaid with soft agar containing S. epidermidis F (indicator strain), and challenged with different concentrations of H₂O₂ delivered by previously soaked filter discs. The left panel shows the respective control plate without S. sanguinis. b S. sanguinis ΔspxB was grown on BHI agar. Subsequently, the indicator strain was inoculated in close proximity to SK36 ΔspxB and challenged by an H₂O₂-soaked filter strip. The concentrations of H₂O₂ are indicated. c The appearance of the protective effect was tested with SK36 ΔspxB/ΔccpA under planktonic and high-cell-density conditions over a time course. Upper panel: 200 μl of supernatant derived from a liquid culture was sterile filtered at the indicated time points and added to BHI plates. Lower panel: bacteria were applied to sterile filters and then placed onto BHI plates. At the indicated time points, the filters containing the bacteria were removed from the plates. Afterward, the plates were overlaid with soft agar containing S. epidermidis F and then challenged with H₂O₂-soaked filter discs. Presented is also the planktonic growth monitored by optical density. SK36 wt, spxB, and ccpA single mutants serve as reference.