All of the following are established periodontal pathogens associated with chronic periodontitis EXCEPT:

Correct answer: D. Streptococcus intermedius

• A. Incorrect (this IS a pathogen). T. forsythia is the second member of the red complex.
• B. Incorrect (this IS a pathogen). T. denticola, a spirochete, is the third member of the red complex.
• C. Incorrect (this IS a pathogen). P. gingivalis is a member of Socransky's RED COMPLEX and the archetypal keystone pathogen of chronic periodontitis.
• D. Correct — this is the exception. STREPTOCOCCUS INTERMEDIUS belongs to the Streptococcus anginosus (milleri) group. It is a facultative Gram-POSITIVE coccus associated with odontogenic abscesses, brain and liver abscesses, and infective endocarditis — not with the anaerobic Gram-negative consortium that drives periodontal destruction. Streptococci in general are associated with periodontal HEALTH and decrease proportionally as periodontitis progresses.

Note: Socransky's red complex = P. gingivalis + T. forsythia + T. denticola. Orange complex (Fusobacterium nucleatum, Prevotella intermedia, Campylobacter) bridges early colonisers to the red complex. Aggregatibacter actinomycetemcomitans is separately linked to aggressive/molar-incisor periodontitis.