Deletion of the alternative sigma factor σB in Staphylococcus aureus reveals its function as a global regulator of virulence genes

A deletion of the sigB operon was constructed in three genetically distinct Staphylococcus aureus strains, and the phenotypes of the resulting mutants were analyzed. Compared to the corresponding mild-type strains, the Delta sigB mutants showed reduced pigmentation, accelerated sedimentation, and increased sensitivity to hydrogen peroxide during the stationary growth phase. A cytoplasmic protein missing in the Delta sigB mutants was identified as alkaline shock protein 23, and an extracellular protein excreted at higher levels in one of the Delta sigB mutants was identified as staphylococcal thermonuclease. Interestingly, most sigB deletion phenotypes were only seen in S, aureus COL and Newman and not in 8325, which was found to contain an 11-bp deletion in the regulator gene rsbU, Taken together, our results show that sigma(B) is a global regulator which modulates the expression of several virulence factors in S, aureus and that laboratory strain 8325 is a sigma(B)-defective mutant.