Which organism is a Gram-negative facultative anaerobe in the oral cavity associated with localized aggressive periodontitis?

• A. Porphyromonas gingivalis
• B. Aggregatibacter actinomycetemcomitans
• C. Streptococcus salivarius
• D. Neisseria sicca

Answer: (B)

Understanding which organism in the oral cavity fits as a Gram-negative facultative anaerobe linked to localized aggressive periodontitis. Aggregatibacter actinomycetemcomitans matches this profile: it is a Gram-negative, facultatively anaerobic, non-spore-forming rod/coccobacillus found in dental plaque and subgingival sites of patients with localized aggressive periodontitis. Its pathogenicity is aided by leukotoxin production, which impairs immune cell function and promotes rapid tissue destruction, helping explain its strong association with this disease form, especially in younger individuals. The other organisms don’t fit this exact combination: Porphyromonas gingivalis is Gram-negative but an obligate anaerobe and is more classically linked to chronic periodontitis; Streptococcus salivarius is Gram-positive and a common mucosal inhabitant with no strong tie to aggressive periodontal disease; Neisseria sicca is a Gram-negative diplococcus typically a harmless commensal of the upper respiratory tract and not a primary periodontal pathogen.