Introduction: Biofilm formation by Staphylococcus aureus  is an important virulence determinant mediated by the polysaccharide intercellular adhesin (PIA) encoded by the ica operon or mediated by surface and extracellular proteins. SarX is a 250-residue two-domain SarA homolog that activates spa  transcription. Previous studies demonstrated that Staphylococcus epidermidis  SarX protein regulated the transcriptional activity of the agr  and ica  loci and controlled the biofilm phenotype, primarily by regulating icaADBC  transcription and PIA production.
Results: In this study, biofilm formation and detachment of the clinical isolate S. aureus  SA75 were significantly decreased in the sarX  mutant strain. The effect of sarX  mutation on S. aureus  biofilm formation was related to the production of PIA and not to that of eDNA. Deletion of sarX  was associated with a 1.8-fold reduction in spa  transcription as determined by RT-PCR analysis, and this reduction could be restored by chromosomal complementation of sarX . Expression of Spa protein was also decreased in the S. aureus sarX  mutant.
Conclusion: sarX  promoted biofilm production of S. aureus  that may primarily be mediated through increasing ica  operon expression and PIA production. Furthermore, deletion of sarX  reduced S. aureus  biofilm formation by downregulating spa .
Keywords: Staphylococcus aureus , ica , biofilm formation, sarX , spa