Natural human isolates of Staphylococcus aureus selected for high production of proteases and α-hemolysin are σB deficient

It has been reported that high production of proteases and α-hemolysin in the prototype Staphylococcus aureus strain 8325-4 was associated with its σB deficiency. Here we analyzed one fresh clinical isolate (KS26) and two ancient human isolates (Wood46 and V8) selected for high production of proteases and α-hemolysin. All three strains lacked yellow pigment and showed a low level of expression of sigB-dependent promoters, indicating σB deficiency. Nucleotide sequencing of the sigB operon revealed that KS26 and Wood46 had stop codons in rsbU and sigB, respectively, while V8 had an insertion of an IS element in rsbU. Complementation experiments with sigB on a plasmid reduced expression of proteases and α-hemolysin dramatically, indicating that the high production of these exoproteins was associated with σB deficiency. Although σB-deficient strains show attenuated virulence in some animal models, our results indicate that such strains can cause infection in humans.