A novel SigB(Q225P) mutation in Staphylococcus aureus retains virulence but promotes biofilm formation

Staphylococcus aureus is an important pathogen that produces abundant virulence factors, which cause various diseases that burden human health worldwide. The stress response regulon called sigma factor B (SigB) is a well-characterized global regulator that is involved in the regulation of S. aureus virulence, pigmentation, and biofilm formation. However, the regulatory network upon SigB in S. aureus is incompletely described. Here, we identified a novel substitution mutation, SigB(Q225P), which contributed the nonpigmented phenotype of S. aureus. The S. aureus mutant carrying SigB(Q225P) substitution lacks staphyloxanthin, a key virulence factor in protecting bacteria from host-oxidant killing, but retains bacterial pathogenicity with pleiotropic alterations in virulence factors, resulting in similar lethality and abscess formation ability in animal models. We also reported the SigB(Q225P) promotion of biofilm formation in S. aureus. Real-time quantitative polymerase chain reaction (RT-qPCR) revealed that the expression of nuc gene, which encodes thermonuclease, was significantly downregulated, resulting in accumulation of eDNA in the biofilm of SigB(Q225P) mutant strain. LacZ reporter assay showed that SigB(Q225P) influenced the activity of nuc promoter. Furthermore, electrophoretic mobility shift assay (EMSA) and Bio-layer interferometry (BLI) assay revealed that both SigB and SigB(Q225P) proteins could directly bind to nuc gene promoter; however, the binding activity decreased for SigB(Q225P). Our data renewed the understanding of the relationship between S. aureus golden pigment and its virulence and suggested that a single substitution mutation in SigB might enhance the biofilm formation of S. aureus by directly downregulating nuc expression.