High-throughput 16S rRNA gene sequencing has allowed the characterization of subgingival microbiome shifts from health to periodontitis identifying health-associated, periodontitis-associated and core species, which preserve their proportions from health to disease. The development of gingivitis is also characterized by distinct shifts. Microbiome shifts resemble microbial successions and result from interspecies interactions and community adaptation to the changing environment as inflammation ensues. Gingivitis-associated and core species are proposed as likely mediators of microbiome transitions.