Which organism is most strongly associated with chronic periodontitis?

Porphyromonas gingivalis is a gram-negative, anaerobic rod and the keystone pathogen most strongly associated with chronic periodontitis (now classified as Stage I-IV periodontitis). It is a member of the 'red complex' along with Tannerella forsythia and Treponema denticola. P. gingivalis produces virulence factors including gingipains (proteases), lipopolysaccharide, and fimbriae that promote inflammation and tissue destruction. S. mutans is associated with dental caries. S. aureus is not typically associated with periodontitis. P. intermedia is associated with acute necrotizing ulcerative gingivitis (ANUG) and pregnancy gingivitis.