Bacteria involved in the initiation and progression of periodontal disease are classified into color-coded groups. The categories are based upon the pathogenicity of the bacteria and their role in the development of plaque . Species in the red complex (P. gingivalis, T. forsythia, and Treponema denticola) are strongly related with pocket depth and bleeding on probing. The red complex can be used as a clinical parameters that is considered the most meaningful in periodontal diagnosis. Species in the orange complex (Fusobacterium nucleatum, P. intermedia, Prevotella nigrescens, Peptostreptococcus micros, Streptococcus constellatus, Eubacterium nodatum, and Campylobacter gracilis) are closely associated with one another. The orange complex is closely related to the red complex. Similarly to the red complex, the orange complex is significantly associated with increasing pocket depth; a decreased level of these species lead to improvement in periodontal status. A. actinomycetemcomitans, which is classified in the green complex, is associated with localized aggressive periodontitis.