Define the keystone pathogen concept in periodontitis and name the organism most associated with it.

Define the keystone pathogen concept in periodontitis and name the organism most associated with it.

• A. A keystone pathogen is a high-abundance microbe that drives dysbiosis and disease; Porphyromonas gingivalis is most associated.
• B. A keystone pathogen is a low-abundance microbe that prevents disease.
• C. A keystone pathogen is a virus that causes periodontal tissue damage.
• D. A keystone pathogen is a commensal that has no effect on disease.

Answer: (A)

Keystone pathogens in periodontitis are microbes that exert a large influence on the surrounding microbial community and the host immune response despite being present at low numbers. Their impact isn’t about being the most abundant organism; instead, they disrupt defenses and shift the ecosystem toward a dysbiotic, inflammation-driven state that favors tissue damage and disease progression.

Porphyromonas gingivalis is the classic example. It uses virulence strategies that manipulate the host immune system and remodeling of the microbial community, creating an environment where inflammatory and proteolytic bacteria can thrive. This disproportional effect—driving dysbiosis and disease from a minority presence—defines the concept.

So, the best-fitting description is a low-abundance microbe that drives dysbiosis and disease, with Porphyromonas gingivalis as the organism most associated with this role. The other ideas—being a high-abundance microbe, a virus, or a harmless commensal—do not capture how keystone pathogens function in periodontitis.