Fig. 1.

Dysbiosis of the subgingival microbiota.

Inflammatory mucosal diseases such as periodontal disease are induced under certain conditions by a polymicrobial community in which different members have distinct and synergistic roles that promote destructive inflammation. A keystone pathogen (e.g., P. gingivalis), with the help of accessory pathogens in terms of nutrition and colony formation, initially subverts host immunity leading to the emergence of dysbiotic microbiota, in which commensal bacteria–turned pathobionts overactivate the inflammatory response and cause tissue destruction. Pathobionts are organisms that are generally benign within an indigenous community but become pathogenic when host-microbe homeostasis breaks down under certain conditions, such as with antibiotic treatment and tissue injury, and especially in immunocompromised hosts. These conditions can potentially promote the outgrowth of pathobionts and disrupt the symbiotic microbiota, resulting in dysbiosis and inflammation. Dysbiosis can be promoted by these factors, either individually or in combination. A poorly controlled host immune response, in turn, can generate a self-perpetuating pathogenic cycle where dysbiosis and inflammation reinforce each other by forming a positive feedback loop. This figure is modified from Ref. [108].