Briefly, overnight cultures of S. epidermidis were diluted 1:200 and inoculated into wells of polystyrene microtiter plates (200 μL per well) at 37°C for 24 h. At different time points (0, 6, 12, and 24 h), DNase I (Takara Bio, Kyoto, Japan) was added at 28 U/200 μL. After incubation, the wells were gently washed three times with 200 μL PBS and stained with 2% crystal violet for 5 min. Absorbance was determined at 570 nm. To determine whether saeRS affects cell death in biofilms, S. epidermidis cells were cultivated in FluoroDish (FD35-100, WPI, USA) as previously described [7]. Briefly, overnight cultures of S. epidermidis grown

in TSB medium were diluted 1:200, inoculated into dishes (2 mL per dish), and then incubated at 37°C for 24 h. The dishes were then carefully washed with PBS and stained with a LIVE/DEAD kit (containing SYTO9 and PI, Invitrogen Molecular Probes, USA) following the manufacturer’s instructions. SYTO9 stains BMN 673 cell line viable bacteria green while PI stains dead bacteria red. Biofilms of S. epidermidis 1457

and SE1457ΔsaeRS were observed under a Leica TCS SP5 confocal laser scanning microscope (CLSM) using a 63 ×(zoom ×3) objective lens and the Z-stack composite confocal photomicrographs of viable cells, dead cells, and both cells (viable & dead) were generated by Leica LAS AF softwear (version 1.8.1). The fluorescence quantity of each stack was determained using ImageJ software. Electron microscopy For scanning electron microscopy (SEM), biofilms

HIF-1�� pathway were grown in TSB for 24 h at 37°C with fragments of an introvenous catheter, rinsed with PBS three times, fixed with a 2% (w/v) solution of glutaraldehyde prepared in phosphate-buffered saline, and then observed under a TECNAI- 12 field emission source instrument (Philips, Eindhoven, The Netherlands). For transmission electron microscopy (TEM), bacteria grown for 24 h were stained by mixing with a 1% (w/v) solution of uranyl acetate on an electron microscope grid covered with a carbon-coated Formvar film. S. epidermidis cells were observed using a Hitachi S-520 electron microscope (Hitachi, Tokyo, Japan). RNA extraction and microarray analysis Overnight cultures of S. epidermidis 1457 and 1457 ΔsaeSR were diluted 1:200 into fresh TSB and grown at 37°C to an OD600 of 3.0 (mid-exponential growth). Eight millilitres cAMP of bacterial cultures were pelleted, washed with ice-cold saline, and then homogenized using 0.1 mm Ziconia-silica beads in Mini-Beadbeater (Biospec) at a speed of 4800 rpm. The bacterial RNA was isolated using a QIAGEN RNeasy kit according to the standard QIAGEN RNeasy protocol. The microarray was manufactured by in situ synthesis of 14,527, 60-mer long oligonucleotide probes (Agilent, Palo Alto, CA, USA), selected as previously described [21]. It covers > 95% of all ORFs annotated in strains ATCC12228 (GeneBank accession number NC_004461), ATCC35984 (GeneBank accession number NC_002976), SE1457 (unpublished sequence).