Louise Parker Bench D 23/09/2016 
Higher order taxa
Bacteria – Bacteroidetes – Bacteroidia – Bacteroidales – Porphyromonadaceae - Porphyromonas - P. gingivalis
Species: Porphyromonas gingivalis
Strain: 2561 = ATCC 33277= CCUG 25893 = CCUG 25928 = CIP 103683 = DSM 20709 = JCM 12257 = NCTC 11834.
Description and significance
Porphyromonas gingivalis is a Gram-negative anaerobic bacteria. It is a major causative agent of the initiation and progression of periodontal disease, the main cause of tooth loss . P. gingivalis is an opportunistic pathogen found in the subgingival sulcus of the human oral cavity. Along with Treponema denticola and Tannerella forsythia, P. gingivalis forms the red bacterial complex which is often seen in advanced periodontal lesions. P. gingivalis is strongly associated with gingival recession, increased sulcular pocket depth and bleeding upon probing. The bacterium has also been found in healthy individuals, at a prevalence of 10-25% P. gingivalis is found in those with periodontitis at a prevalence of 79-90%. Epidemiological studies have shown that strains vary in association with human disease and virulence in animal models. Strains W83, W50, ATCC 49417, A7A1 have been classified as virulent, whereas avirulent strains include 381, 33277 and 23A4. P. gingivalis is obligately anaerobic and does not form spores. Cells are non-motile rods 0.5 by 1-2μm in broth. On blood agar, colonies are black-pigmented, smooth, shiny, convex and 1-2mm in diameter. Periodontal disease is the most common cause of infectious inflammatory disease. The majority of the population has been affected by mild periodontal disease and 5-8% suffer from more severe forms. Periodontal disease has also been linked with increased risk of cardiovascular disease.
The P. gingivalis strain W83 has a 2,343,479bp genome sequence. It has an average G+C content of 48.3%. 1,990 open reading frames were identified in the genome, making up 85% of the genome. 1,075 of these were assigned biological role categories. Repetitive elements make up approximately 6% of the genome and are mainly DNA repeats and transposable elements. Analysis of this genome has shown that P. gingivalis is able to metabolise many amino acids and generate products toxic to the human host which aids development of periodontitis.
Cell structure and metabolism
Cell wall: P. gingivalis has a gram-negative cell wall consisting of an inner membrane, thin peptidoglycan layer and outer membrane. The peptidoglycan layer contains lysine as the diamino acid.
Biofilm formation: In addition to infecting gingival cells, P. gingivalis is able to inhabit subgingival dental biofilms. The bacterium is a late coloniser, attracted to the biofilm by intermediate colonisers such as Fusobacterium nucleatum. P. gingivalis is able to increase the virulence of biofilms even in low numbers. It does this by subverting host responses, altering biofilm community structures and enabling an increase in bacterial load. For these reasons it is termed a ‘keystone’ pathogen. When studied in mice, P. gingivalis was not found to cause disease in isolation. Thus it is thought that relationships with other microbes are necessary for P. gingivalis to use its full potential for pathogenicity.
Fimbriae: Fimbriae are surface appendages that protrude from the outer membrane of most strains of P. gingivalis. The bacteria express fimbriae of two types, made up of FimA protein and Mfa protein. Both types are thought to contribute to periodontal disease and when fimbriae is not expressed, binding to and invasion of host cells is disrupted. Fimbriae also mediate adhesion to extracellular matrix proteins and commensal bacteria including Streptococci and Actinomyces viscosus.
Capsule: Capsule is also thought to contribute to virulence. While its chemical composition varies between strains, strains without capsule have been found to mainly cause non-invasive localised abscesses in animal models. Unencapsulated P. gingivalis are also associated with increased phagocytosis and killing by macrophages and dendritic cells. Virulent strains of P. gingivalis W83 and W50 have a thicker capsule than less virulent strains, which is linked to decreased production of leukocyctes. Capsule type also influences initial adhesion to periodontal pocket epithelial cells.
Metabolic function: P. gingivalis requires anaerobic conditions for growth, in addition to heme or hemin and vitamin K. The black colour seen when P. gingivalis is grown on blood media is associated with heme accumulating on the cell surface. When grown on heme-limited media, the bacteria become less virulent. P. gingivalis gains energy by fermentation of amino acids. This is crucial as sugars are not abundant in deep periodontal pockets where P. gingivalis is found.
P. gingivalis is an obligate anaerobe found mainly in the human oral cavity. It has been found among epithelial cells obtained from periodontal pockets, gingival crevices and buccal mucosa collected from individuals with periodontitis and those with healthy gingivae . P. gingivalis has also been found in the upper gastrointestinal system, respiratory tract, colon and vagina . P. gingivalis is found relatively uncommonly and in low numbers in healthy individuals compared to those with disease. As a keystone pathogen, P. gingivalis uses a number of virulence factors. Fimbriae, hemagglutinins, outer membrane proteins and gingipains are involved in colonisation and attachment. Gingipains are a group of cysteine proteases which have trypsin-like proteolytic activity when secreted from the cell. Gingipains cleave host proteins and result in tissue damage and disruption of the immune response. Capsule, lipopolysaccharide, complement proteases and fimbriae are used to evade the host response. To damage host tissues and spread through them, proteases, collagenase, fibrinolytic, keratinolytic and other hydrolytic enzymes are used .
P. gingivalis is one of the major causes of periodontal disease. Periodontal disease is the inflammation of the gingiva, alveolar bone, periodontal ligament and cementum. Periodontal diseases comprise gingival disease - the inflammation of gingival tissues - and the more severe periodontitis. Periodontitis is the irreversible inflammation of the periodontal tissues induced by plaque. It can destroy the periodontal ligament and alveolar bone and cause migration of the epithelial ligament. This in turn results in a periodontal pocket, upon which bacteria can colonise and subgingival plaque can form. In a study of patients with chronic periodontitis, 87.75% of subgingival plaque samples contained P. gingivalis.
Periodontal disease has been linked with rheumatoid arthritis. Rheumatoid arthritis is a systematic autoimmune disease associated with debilitating joint destruction and chronic inflammation. Periodontal destruction and joint destruction are known to have common pathogenic mechanisms and are thought to influence each other. Individuals with periodontal disease show increased chance of developing rheumatoid arthritis. Periodontal disease has also been linked with cardiovascular disease, and P. gingivalis has been found in atherosclerotic plaques.
Application to biotechnology
There is currently no vaccine in use for periodontitis prevention. However, a number of P. gingivalis antigens have been assessed for their potential as vaccine candidates. In a mouse model capsule was tested as an antigen in a poly-saccharide-fimbriae protein conjugate. This resulted in higher serum IgG and protection against P. gingivalis infection, and was more effective than the use of capsule or fimbriae alone. Two different outer membrane proteins were found to reduce lesion size in rats and mice and protect against P. gingivalis-induced alveolar bone loss. Gingipains, hemagglutinins among others have also been found to be protective.
Recently it was found that P. gingivalis being present at the time of HIV infection of macrophages significantly repressed replication of the virus. This effect is dependent on TLR4 signalling and is reversible. This finding is consistent with studies which established that Escherichia coli interferes with HIV replication at multiple stages of the cycle. For P. gingivalis, it appears that repression occurs during a step after pro-viral integration and transcription. The exact mechanism is still unknown, but a possibility is that interferons secreted in response to bacterial LPS have anti-viral activity.
Biofilm formation on dental implants can lead to disease and hinder the success of the implant. Titanium is a common material used for dental implants. A novel copper-bearing titanium alloy material was designed to kill bacteria and deter biofilm formation. Based on gene expression studies, biofilm growth observation and bacterial viability measurements, the alloy was found to have antimicrobial and antibiofilm activity against Streptococcus mutans and P. gingivalis.
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This page is written by Louise Parker for the MICR3004 course, Semester 2, 2016