MMP9 and bacterial infectious disease: Our data support the following theories of MMP-9 secretion by M. pneumoniae: first, MMP-9 expression is associated with the MAPK/NF-κB/AP-1 cascade; second, histone acetylation appears to be critical in M. pneumoniae-induced inflammatory responses, indicating that epigenetic modifications induced by TLRs may be a notable inflammatory signal during bacterial infection.