CTSK and infection: MMP1 expression remained elevated at 15 DPI, whilst the MMP13 response decreased over the duration of the infection CTSK, which encodes a major collagenase associated most commonly with osteoclastic bone resorption but also with osteocytic osteolysis, was significantly upregulated by infection at both DPIs tested and whilst the effect was only present in the highest MOI tested at 7 DPI, the up-regulation became uniform across all MOI at 15 DPI (Figure 3I).