Porphyromonas gingivalis belongs to the phylum Bacteroidetes

Bacteroidetes

The phylum Bacteroidetes is composed of three large classes of bacteria that are widely distributed in the environment, including in soil, in sediments, sea water and in the guts and on the skin of animals....

and is a non-motile, gram-negative

Gram-negative

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

, rod-shaped, anaerobic

Anaerobic organism

An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...

 pathogen

Pathogen

A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

ic 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

Periodontal disease

Periodontitis is a set of inflammatory diseases affecting the periodontium, i.e., the tissues that surround and support the teeth. Periodontitis involves progressive loss of the alveolar bone around the teeth, and if left untreated, can lead to the loosening and subsequent loss of teeth...

, as well as the upper gastrointestinal tract

Gastrointestinal tract

The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....

, respiratory tract

Respiratory tract

In humans the respiratory tract is the part of the anatomy involved with the process of respiration.The respiratory tract is divided into 3 segments:*Upper respiratory tract: nose and nasal passages, paranasal sinuses, and throat or pharynx...

, and in the colon

Colon (anatomy)

The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...

. Collagen degradation that is observed in chronic periodontal disease results in part from the collagenase enzymes of this species.

In patients harbouring Porphyromonas gingivalis one finds high levels of specific antibody in the serum.

Additionally Porphyromonas gingivalis has been linked to rheumatoid arthritis

Rheumatoid arthritis

Rheumatoid arthritis is a chronic, systemic inflammatory disorder that may affect many tissues and organs, but principally attacks synovial joints. The process produces an inflammatory response of the synovium secondary to hyperplasia of synovial cells, excess synovial fluid, and the development...

. Porphyromonas gingivalis contains the PAD (Peptidyl-Arginine Deaminase) which is involved in citrullination

Citrullination

Citrullination or deimination is the term used for the post-translational modification of the amino acid arginine in a protein into the amino acid citrulline. This reaction, shown below, is performed by enzymes called peptidylarginine deiminases...

. Patients with Rheumatoid Arthritis have an increased incidence of periodontal disease and antibodies to the bacterium are significantly more common in patients with Rheumatoid Arthritis.