The study of the microbiota associated with periodontal diseases has been impacted by changes in paradigms regarding the etiology and pathogenesis of periodontal diseases over the years. Contrary to the ‘dogma’ of gram-negative bacterial dominance in periodontitis, gram-positive anaerobic species exhibit a significant increase in deep diseased sites relative to healthy sites, and can be detected in greater abundance than gram-negative species in some studies. Many of these species are as yet to be cultivated. These organisms exhibit polymicrobial synergy. The Eubacterium recolonize in smokers than in non-smokers counterparts, with chronic periodontitis.