<unk>  belongs to the phylum Bacteroidetes and is a nonmotile, Gram-negative, rod-shaped, anaerobic, pathogenic bacterium. It forms black colonies on blood agar.
It is found in the oral cavity, where it is implicated in certain forms of periodontal disease, as well as the upper gastrointestinal tract, respiratory tract, and in the colon. Collagen degradation observed in chronic periodontal disease results in part from the collagenase enzymes of this species. It has been shown in an in vitro study that P.<unk>  can invade human gingival fibroblasts and can survive in them in the presence of considerable concentrations of antibiotics. P.<unk>  also invades gingival epithelial cells in high numbers, in which cases both bacteria and epithelial cells survive for extended periods of time. 
High levels of specific antibodies can be detected in patients harboring P.<unk> .
In addition, P.<unk>  has been linked to rheumatoid arthritis. It contains the enzyme peptidyl-arginine deiminase, which is involved in citrullination. Patients with rheumatoid arthritis have an increased incidence of periodontal disease[citation needed] and antibodies against the bacterium are significantly more common in these patients.
P.<unk>  is divided into K-serotypes based upon capsular antigenicity of the various types.
Porphyromonas gingivalis
Aggregatibacter actinomycetemcomitans
Mansonella perstans
Cardiobacterium hominis
Clostridium botulinum
Pasteurella multocida
Streptococcus mutans
Mansonella ozzardi