| Staphylococcus aureus infects nearly every tissue and organ system in the human body, and this versatility has been attributed to an ability to rapidly alter virulence factor production such that a colonization phenotype can be rapidly exchanged for one favoring invasion. Cell surface FnBPs promote adhesion to traumatized host tissue, and are maximally expressed during exponential growth. Conversely, secreted V8 protease (SspA) expression is repressed during exponential growth, and upregulated as cells approach stationary phase. Herein, we hypothesized that the colonization and invasive phenotypes of S. aureus can be significantly affected by sued proteases, using FnBP and SspA as paradigms of these phases of infection.; Initially we addressed factors that contribute to the considerable variation in Fn-binding observed among S. aureus clinical isolates, as this may be important for developing adhesion-blocking antibodies. The fnb locus was polymorphic in a small number of clinical isolates in terms of the number of fnb genes and/or repeated Fn-binding D-motifs. Furthermore, functional antigenic variation was observed in the D3-like motif of the epidemic strain CMRSA-1. However, these observations were not related to variations in Fn-binding. In contrast, variation in Fn-binding among MRSA was inversely related to protease activity, and previous work in our lab demonstrated a role for SspA in degrading cell-surface FnBP. Therefore, we hypothesized that SspA modifies the colonization and invasive phenotypes of S. aureus. Non-polar inactivation of sspA in S. aureus SP6391 (sspA::ermAB) had a pleiotropic effect on the profile of secreted proteins, including accumulation of a 40-kDa precursor of the SspB cysteine protease that is normally processed by SspA to a 22-kDa form, increased alpha-toxin, and altered autolysin activity. SP6391 also displayed decreased aureolysin (aur) gene transcription, a metalloprotease that was previously shown to process SspA to an active form. SP6391 showed increased virulence in a mouse tissue abscess model, but did not differ in Fn-binding relative to the wild-type strain. Therefore, other proteases likely contribute to degradation of FnBP. These results define a cascade pathway of proteolytic activities, where metalloprotease is required for maturation of SspA, which then processes SspB and controls stability of secreted virulence factors. |
