Which microorganism is considered the keystone pathogen most strongly associated with chronic periodontitis?

Porphyromonas gingivalis is recognized as the keystone pathogen of chronic periodontitis. Despite being present in relatively low abundance, it dysregulates the local immune response by degrading complement proteins and hijacking TLR2 signaling, allowing the entire subgingival microbial community to become more virulent. It is a key member of the 'red complex' (along with Treponema denticola and Tannerella forsythia) — the group most strongly associated with advanced periodontitis. Aggregatibacter actinomycetemcomitans is primarily associated with aggressive/localized periodontitis.