In periodontal disease, the primary microbial component is usually:

The primary microbial component in periodontal disease is Porphyromonas gingivalis. This bacterium is considered a keystone pathogen in the development and progression of periodontitis. It has been extensively studied and is known for its ability to evade the host immune response, disrupt the balance of oral microbiota, and contribute to tissue destruction through the production of various virulence factors.

Porphyromonas gingivalis contributes to the inflammatory process in the periodontal tissues, leading to the breakdown of the supporting structures of the teeth. Its presence is often correlated with more severe forms of periodontal disease, making it a critical focus in both research and clinical practice concerning periodontal health.

The other options mentioned, while involved in oral health, are not primarily associated with periodontal disease in the same way. Streptococcus mutans, for example, is predominantly linked to dental caries rather than periodontal conditions. Actinomyces israelii and Fusobacterium nucleatum also play roles in periodontitis but are not as central as Porphyromonas gingivalis in the disease process itself. Given the significance of Porphyromonas gingivalis in the pathogenesis of periodontal disease, it is rightly identified as the primary microbial component.