Overview: Bacteria of genus Aggregatibacter are moderately abundant in the mouth, making up 2% or less at most oral sites except for the tongue dorsum where they are in very low abundance. Members of the genera Haemophilus and Aggregatibacter are closely related and not always cleanly distinguished using short fragments of the 16S ribosomal RNA gene.

Ecological role/importance in health and disease: The Aggregatibacter species most abundant in the healthy mouth are A. aphrophilus, A. paraphrophilus, A. segnis, and the unnamed A. sp. HMT 458. The species A. actinomycetemcomitans is associated with periodontitis, including aggressive periodontitis, and was part of the "green complex" of Socransky et al. (1998).