AraC-Type Regulator Rbf Controls the Staphylococcus epidermidis Biofilm Phenotype by Negatively Regulating the icaADBC Repressor SarR

Regulation oficaADBC -encoded polysaccharide intercellular adhesin (PIA)/poly-N -acetylglucosasmine (PNAG) production in staphylococci plays an important role in biofilm-associated medical-device-related infections. Here, we report that the AraC-type transcriptional regulator Rbf activatesicaADBC operon transcription and PIA production inStaphylococcus epidermidis . Purified recombinant Rbf did not bind to theica operon promoter region in electrophoretic mobility shift assays (EMSAs), indicating that Rbf regulatesica transcription indirectly. To identify the putative transcription factor(s) involved in Rbf-mediatedicaADBC regulation, the ability of recombinant Rbf to interact with the promoter sequences of knownicaADBC regulators was investigated. Recombinant Rbf bound to thesarR promoter and not thesarX ,sarA ,sarZ ,spx , andsrrA promoters. Reverse transcription (RT)-PCR demonstrated that Rbf acts as a repressor ofsarR transcription. PIA expression and biofilm production were restored to wild-type levels in anrbf sarR double mutant grown in brain heart infusion (BHI) medium supplemented with NaCl, which is known to activate theica locus, but not in BHI medium alone. RT-PCR further demonstrated that although Rbf does not bind thesarX promoter, it nevertheless exerted a negative effect onsarX expression. Apparently, direct downregulation of the SarR repressor by Rbf has a dominant effect over indirect repression of the SarX activator by Rbf in the control ofS. epidermidis PIA production and biofilm formation.IMPORTANCE The importance ofStaphylococcus epidermidis as an opportunistic pathogen in hospital patients with implanted medical devices derives largely from its capacity to form biofilm. Expression of theicaADBC -encoded extracellular polysaccharide is the predominant biofilm mechanism inS. epidermidis clinical isolates and is tightly regulated. Here, we report that the transcriptional regulator Rbf promotesicaADBC expression by negatively regulating expression ofsarR , which encodes anica operon repressor. Furthermore, Rbf indirectly represses theica operon activator, SarX. The data reveal complicated interplay between Rbf and two Sar family proteins in fine-tuning regulation of the biofilm phenotype and indicate that in the hierarchy of biofilm regulators, IcaR is dominant over the Rbf-SarR-SarX axis.