Porphyromonas

From MicrobeWiki, the student-edited microbiology resource

A Microbial Biorealm page on the genus Porphyromonas

Porphyromonas gingivalis. From The Forsyth Institute and TIGR.

Classification

Higher order taxa:

Bacteria; Bacteroidetes/Chlorobi group; Bacteroidetes; Bacteroides (class); Bacteroidales; Porphyromonadaceae, Porphyromonas

Species:

Porphyromonas asaccharolytica, P. cangingivalis, P. canoris, P. cansulci, P. catoniae, P. circumdentaria, P. crevioricanis, P. endodontalis, P. gingivalis, P. gingivicanis, P. gulae, P. levii, P. macacae, P. salivosa

NCBI: Taxonomy Genome

Description and Significance

Porphyromonas, which are commonly found in the human body and especially in the oral cavity, were originally classified in the Bacteroides genus. Porphyromonas gingivalis are an oral anaerobe associated with periodontal lesions, infections, and adult periodontal disease. Approximately 70-90% of people pubescent and older have gingivitis, an oral inflammatory process and a possible precursor to adult periodontal disease, which is associated with Porphyromonas gingivalis. Gingivitis allows Porphyromonas gingivalis to further infect the areas near the root of the teeth causing tooth decay and infection.

Genome Structure

The genome of Porphyromonas gingivalis has been sequenced and studied by the The Forsyth Institute and TIGR. The genome size of this species of Porphyromonas was determined to be 2,176 kb according to Xba I restriction enzyme digests and 2,250 kb from Spe I digests. Information and a schematic representation of the Porphyromonas gingivalis W83 can be found at TIGR. A Porphyromonas genome is constituted approximately 46-57 mol % G-C. The P. gingivalis sequencing project was one of the first to look at the Gram-negative phylum containing P. gingivalis. Sequencing this bacteria was a major step to enhancing the knowledge of the metabolism of Gram-negative anaerobes and of bacterial diversity. The P. gingivalis genome project showed evidence of genetic exchange between these bacteria and mammalian host cells.

Cell Structure and Metabolism

Porphyromonas gingivalis. From The Forsyth Institute and TIGR.

Porphyromonas are Gram-negative, nonsporeforming, anaerobic, rod-shaped bacteria that produce porphyrin pigments (dark brown/black pigments). Like Bacteroides, Porphyromonas are more closely related to Gram-positive bacteria than other Gram-negative bacteria. Also like Bacteroides, Porphyromonas have an outer membrane, a peptidoglycan layer, and a cytoplasmic membrane.

The black pigmentation of P. gingivalis is from the accumulation of hemin used as an iron source for bacterial growth. This may be a reason that people with higher metal intakes, such as iron, have more of a risk for getting gingivitis and periodontitis. Also, cell surface adhesion molecules on the surface of Porphyromonas, which interact with other bacteria, epithelial cells, and extracellular matrix proteins, assist the bacteria in living in their human host. (See Pathology.) The mouth generally has a consistant flow sugars and other simple carbohydrates, so it is likely that P. gingivalis living in peridontal tissue receive their energy from these materials. However, another common idea is that the main source of P. gingivalis energy and cell materials come mainly from peptides instead of single amino acids. However, due to the complicated amino acid composition of peptides or proteins, the amino acid metabolic pathway of P. gingivalis has been difficult to determine. Also, some enzymes involved in amino acid metabolism in these bacteria are known to be oxygen labile (changes in the presence of oxygen), which further complicates the detection and analysis of P. gingivalis metabolic enzymes. To see a list of other metabolic pathways that occur within Porphyromonas gingivalis, see The Systems Biology Institute.

Ecology

P. gingivalis may be one of the natural bacterial flora in the oral cavity that is comprised of over 400 different species of microorganisms. However, isolating it in a health oral cavity has proven difficult. It constitutes approximately 5% of the bacterial flora in an oral cavity with gingivitis and more than 5% in a mouth with advanced periodontitis. P. endodontalis also does not appear in a healthy mouth but can be detected in a diseased mouth. Porphyromonas also favor a slightly alkaline environmental pH. Like other bacteria that live in the human mouth, Porphyromonas favor an average temperature of around 95 degrees and a 100% humidity. It has been reported that anywhere from 1,000 to 1 billion bacteria can live on each tooth surface. P. asaccharolytica has been isolated from many nonoral sites such as the cervix, ear, intestine, genitalia, and from many infections throughout the body (only limited reports of P. asaccharolytica in the mouth). Other strains have been found in samples of blood, amniotic fluid, umbilical cord, empyema, peritoneal and pelvic abscesses, endometritis, and infections.

Pathology

Cell surface adhesion molecules on the surface of Porphyromonas interact with other bacteria, epithelial cells, and extracellular matrix proteins; they are currently being studied for their pathogenic potential. P. gingivalis is thought to spread through tissue, destroy tissue, and evade host defenses by the use of secreted cell-bound proteases, immunoactive cellular compounds, and toxins. P. gingivalis cytotoxic metabolic end products, which include butyrate, propionate, have low molecular weights which allows them to easily penetrate periodontal tissue and disrupt the host cell activity.

In the past, more research papers and NIDR-funded grants have been devoted to P. gingivalis that to any other dental pathogens. This is due to the high frequency in which P. gingivalis is associated with peridontal lesions, infections, and periodontitis. Projects such as The Forsyth Institute and TIGR's Porphyromonas gingivalis genome project hope to switch the method of treating periodontal diseases by surgery and tooth scaling to antibiotic or vaccine therapies.

Diagram of gingivitis. From American Academy of Periodontology

Gingivitis, which is inflamation of the gums that causes bleeding and exposes the base of the teeth, allows Porphyromonas gingivalis to infect the areas near the root of the teeth causing tooth decay and infection. the most common type of gingivitis is brought on by the accumlation of microbial plaques in people who do not take proper care of their mouth. Pockets form around the teeth, lesions can form, bacterial infections occur, and, eventually, peridontal ligaments break down and destruction of the local aveolar bone occurs. The teeth then loosen and fall out or can be broken off. Once bacterial infections occur, the gingivitis takes on a new, infectous form called acute necrotizing ulcerative gingivitis (ANUG). ANUG can cause an accelerated destruction of affected tissues as well as local or systemic spread of infection. When ANUG spread beyond the gingiva (gums) and invades the local tissues of the mouth and face, the syndrome is called noma (cancrum oris).