Which microorganism is most commonly associated with aggressive periodontitis (Grade C, localized form)?

Aggregatibacter actinomycetemcomitans (Aa) is the primary pathogen associated with localized aggressive periodontitis (LAP), particularly in young patients. Aa produces leukotoxin that destroys neutrophils, a cytolethal distending toxin, and can invade periodontal tissues, making it difficult to eradicate with SRP alone. Porphyromonas gingivalis is more associated with generalized chronic and generalized aggressive periodontitis. Tannerella forsythia and Fusobacterium nucleatum are members of the red and orange complexes, respectively, associated with chronic periodontitis. Aa is the hallmark pathogen of the localized aggressive form.