RsbU-dependent regulation of Staphylococcus epidermidis biofilm formation is mediated via the alternative sigma factor σB by repression of the negative regulator gene icaR

Transposon mutagenesis of rsbU leads to a biofilm-negative phenotype in Staphylococcus epidermidis. However, the pathway of this regulatory mechanism was unknown. To investigate the role of RsbU in the regulation of the alternative sigma factor sigma(B) and biofilm formation, we generated different mutants of the sigma(B) operon in S. epidermidis strains 1457 and 8400. The genes rsbU, rsbV, rsbW, and sigB, as well as the regulatory cascade rsbUVW and the entire sigma(B) operon, were deleted. Transcriptional analysis of sarA and the sigma(B)-dependent gene asp23 revealed the functions of RsbU and RsbV as positive regulators and of RsbW as a negative regulator of sigma(B) activity, indicating regulation of sigma(B) activity similar to that characterized for Bacillus subtilis and Staphylococcus aureus. Phenotypic characterization of the mutants revealed that the dramatic decrease of biofilm formation in rsbU mutants is mediated via sigma(B), indicating a crucial role for sigma(B) in S. epidermidis pathogenesis. However, biofilm formation in mutants defective in sigma(B) or its function could be restored in the presence of subinhibitory ethanol concentrations. Transcriptional analysis revealed that icaR is up-regulated in mutants lacking sigma(B) function but that icaA transcription is down-regulated in these mutants, indicating a sigma(B)-dependent regulatory intermediate negatively regulating IcaR. Supplementation of growth media with ethanol decreased icaR transcription, leading to increased icaA transcription and a biofilm-positive phenotype, indicating that the ethanol-dependent induction of biofilm formation is mediated by IcaR. This icaR-dependent regulation under ethanol induction is mediated in a sigma(B)-independent manner, suggesting at least one additional regulatory intermediate in the biofilm formation of S. epidermidis.