Fig. 3. S. epidermidis Sph does not harm the host.

A, Lysis of keratinocytes by the indicated bacterial strains. Bacteria were seeded at a ratio of 2:1 to keratinocytes. After 1-day incubation, the final ratio of bacteria to keratinocytes was about 700:1. Cytotoxicity is expressed as percent relative to complete lysis with triton X100 (100% lysis). n=3/group. B, Lysis of human erythrocytes by the indicated bacteria as measured by heme release. The bacteria to erythrocyte ratio was 5000:1. n=8/group. See Fig. S5 for results with sheep erythrocytes and bacterial culture filtrates. C, Biofilm formation in TSB/0.125% glucose. Biofilms were grown for 24 h in microtiter plates, stained with crystal violet, and absorption at 560 nm was measured. n=8/group. See Fig. S5 for results with different glucose concentrations. D, Comparison of enzymatic activities (sphingomyelin cleavage) of purified Sph and S. aureus β-toxin at equal concentrations (1.5 μg/ml). n=3/group. E, Binding of S. epidermidis Sph and S. aureus β-toxin to liposomes and keratinocytes. Binding was determined and measured via Western Blots and densitometry of positive bands. n=6/group. Statistical analysis is by two-tailed, unpaired t-tests. F, Structure of S. epidermidis Sph modeled using SWISS MODEL (https://swissmodel.expasy.org) based on the closest related sequence available (PDB 2uyr.1.A; B. cereus sphingomyelinase N57A). The three non-conserved amino acids of the edge metal binding site are shown with side chains (K91, V132, S133; see Fig. S1). The solvent-exposed loop is highlighted in yellow. Illustrations of the model obtained by SWISS MODEL were performed using Protean 3D (Lasergene 17). G, Structure comparison of S. epidermidis Sph with B. cereus Sph (PDB 2uyr.1.A) and S. aureus β-toxin (PDB 3i5v.1.A). Blue color designates conserved, red color divergent parts. A-E, Error bars show the mean ± SD. S. e., S. epid., S. epidermidis; S. c., S. carnosus.