Mechanistically, we determined that this group of isolates are deficient in the cysteine protease staphopain A. Supporting our data, previous reports showed that staphopain A induces host cell death in epithelial cells through a still unknown mechanism44, and that mutants in its transcriptional regulator Rsp present prolonged residence within host cells, reduced cytotoxicity and lethality in a pneumonia or sepsis murine model63. The gene discussed is CTSB; the disease is pneumonia.