Which organism is most commonly associated as the primary causative agent of aggressive periodontitis?

Aggregatibacter actinomycetemcomitans (formerly Actinobacillus actinomycetemcomitans, Aa) is the most strongly associated bacterial pathogen in aggressive periodontitis (now classified as 'Stage III/IV generalized periodontitis with a molar-incisor pattern' in the 2018 classification). Aa produces a potent leukotoxin that kills neutrophils, evades host defenses, and produces cytolethal distending toxin, causing direct tissue damage. It is found in particularly high concentrations in localized aggressive periodontitis affecting young patients (often pubescent), which characteristically involves the first molars and incisors. Porphyromonas gingivalis is the primary pathogen in chronic periodontitis.