User:S4323213

Richard Leung - 43232134 Bench C 23/9/2016 ^[1]

Classification

Higher order taxa

Bacteria – Bacteria – Bacteroidetes – Bacteroidetes – Bacteroidales – Porphyromonadaceae – Porphyromonas

Species

Porphyromonas gingivalis W83

Description and significance

Porphyromonas gingivalis is a gram-negative obligate anaerobe that is non-motile and pathogenic. The bacterium is rod-shaped, found in the oral cavity and has been strongly implicated as a pathogen in periodontitis ^[1], a damaging disease where the supporting structures of teeth and the gingiva are affected. It can also be found in the upper gastrointestinal tract, the respiratory tract, and the colon. P. gingivalis can be found under conditions of both health and disease, where the prevalences identified range from 10% to 25% in healthy individuals and 79% to 90% in those with periodontitis ^[2]. Previous studies in epidemiology have revealed that P. gingivalis strains differ depending on their human disease association ^[3]. Furthermore, studies using different animal models have also demonstrated that strains of P. gingivalis vary in their virulence, whether it be in soft tissue destruction or death, with some being classified as virulent (e.g. strain W83) and others being identified as being avirulent ^[4].

The virulent strain W83 has been cultured and can be obtained from the American Type Culture Collection (ATCC). This bacterium was originally isolated from an undocumented human oral infection in Bonn, Germany in the 1950s. P. gingivalis has been recognised as an opportunistic pathogen of the oral mucosa and can be found in oral biofilms. Onset and progression of chronic periodontitis has been associated with this bacterium. This bacterium produces an enzyme, collagenase, that degrades collagen observed in chronic periodontal disease. In addition, P. gingivalis can induce secretion of cytokines from immune cells when they invade into hosts. The cytokines that are released are present in inflamed gingiva and result in aggravation of the oral gingival tissues and alveolar bone leading to damage ^[5].

The structural components of P. gingivalis include the capsule, outer membrane containing various proteins, lipopolysaccharide (LPS), bacterial fimbriae and proteinases. These structures all have functions in the pathogenesis of periodontitis in individuals. The virulence factors of the pathogenic bacterium P. gingivalis (W83) should be further studied due to the microbes propensity to cause disease by invading gingival epithelial cells and evade host defences and immune responses ^[6]. In order to effectively control this bacterium in human pathology, more research needs to be conducted in order to find effective control measures against it.

Genome structure

The genome of P. gingivalis W83 is circular, 2,343,479bp long, and with an average G+C content of 48.3% ^[7]. Four ribosomal operons (5S-23S-tRNAAla-tRNAIle-16S), two structural RNA gases and fifty-three tRNA genes with specificity for all 20 amino acids ^[7]. Furthermore, 1,990 ORFs have been identified in the genome ^[8]. Of the 1,990 ORFs, 1,075 (54%) were identified to be involved in biological roles, 184 (9.2%) were conserved hypothetical proteins or conserved domain proteins, 208 (10.5%) were of unknown function, and 523 (26.3%) encoded hypothetical proteins ^[9].

It has also been identified that 6% of the genome are made up of repetitive elements and fall into two classes: DNA repeats and transposable elements ^[9]. The DNA repeats identified include uninterrupted direct repeats and clustered regularly interspaced short palindromic repeats (CRSPRs). Moreover, transposable elements detected include insertion sequence (IS) elements, miniature inverted-repeat transposable elements (MITEs) and large stretches of genes that are conjugal and mobilisable transposons ^[9].

Cell structure and metabolism

The cellular structure of the bacterium is made up of the capsule, fimbriae, LPS and outer membrane containing various proteins. The capsule of P. gingivalis has been identified to play an important role in aiding evasion of host immune system activation, promotes survival of the bacterium within the host cells, and increasing virulence ^[10]. Studies have determined that the capsule of W83 decreased the production of leukocytes indicating that the capsular structure and adhesion capacity increase virulence ^[11]

For the bacterium to get established in the oral cavity, first there must be adhesion to the teeth or mucosal surfaces. Adherence to host surfaces is facilitated by adhesins on the surface of the bacteria, as either cell wall components or associated with other surface structures that interact with receptors of cells in the mouth ^[12]. The adhesins found on the capsule functions as a method of providing resistance to the flow of saliva.

Studies have identified that P. gingivalis expresses two distinct fimbriae on its cell surface: FimA (long, major fimbriae, role in attachment and biofilms) and Mfa (short, minor or Mfa1 fimbriae, cell-cell adhesion and microcolony formation) ^[11]. Depending on the strain, the FimA protein varies and has been classified into six types: types I-V and Ib ^[11]. The W83 strain contains type IV FimA that are poorly fimbriated ^[11].

LPS is a key factor in the causation of periodontitis. In periodontitis lesions, gingival fibroblasts may directly interact with P. gingivalis and its bacterial products, including LPS ^[12]. This is due to LPSs ability to potently activate host inflammatory and innate immune responses by inducing pro inflammatory cytokines leading to periodontal tissue destruction.

The bacterial cell membrane acts as a selective barrier that offers protection and allows movements of substances through outer membrane porin proteins ^[13]. The few major proteins in the membrane serve as antigens that host immune cells recognise. Furthermore, the formation and maintenance of periodontal biofilms is a result of outer membrane proteins interacting with periodontal microflora ^[14].

The P. gingivalis strain W83 is capable of glycolysis and gluconeogenesis when in need of energy or glucose. In addition, the bacterium is also capable of performing the TCA cycle, oxidative phosphorylation. Fermentation of amino acids for energy is an important function required for survival in deep periodontal pocket, where sugar availability is low ^[15].

Ecology

The bacterium is an obligate anaerobe that can be found in the oral cavity. It can also be found in other parts of the body including the upper gastrointestinal system, respiratory tract, and the colon ^[15].

Plaque biofilm development relies on physical interaction of P. gingivalis with itself and other bacteria through congregation and coadhesion ^[16]. P. gingivalis colonises with other species to form plaque biofilms at and below the gingival margin and at deep crypts of the tongue ^[17]. Residence under the gumline places P. gingivalis and other dental plaque microorganisms in contact with the host epithelium and immune effectors that maintains healthy periodontal tissues in conjunction with proper oral hygiene. However, failure to control the plaque will result in a higher bacterial load and lead to the subsequent development of gingival inflammation and disease ^[17].

Pathology

Periodontal disease is characterised by inflammatory pathologic state of the gingiva and supporting structures of the periodontium including: gingival, alveolar bone, periodontal ligament and cementum in addition to others causes gingivitis as well as periodontitis. According to the periodontal disease classification system proposed by the American Academy of Periodontology (AAP), periodontal diseases are generally grouped into two major categories, gingival disease and periodontitis. The categories are determined by whether the destruction of periodontal attachment has occurred ^[18].

Gingival disease is defined by the inflammation of the gingival tissues that are caused by the accumulation of dental plaque. The disease is clinically characterised by symptoms that include redness, swelling and bleeding of the tissues. However, as the periodontal ligament and alveolar bone are not involved, the attachment of the teeth is not affected ^[19]. Individuals can suffer from gingivitis indefinitely and will not progress to periodontists. In order for progression to periodontitis, perturbations in local conditions or generalised host susceptibility must first occur ^[11].

Periodontitis on the other hand refers to an irreversible plaque-induced inflammation of the periodontal tissues leading to degradation of the periodontal ligament, alveolar bone, and migration of the epithelial ligament, where there is eventual destruction of aforementioned structures if not treated ^[11]. This subsequently causes formation of the main clinical feature of periodontitis, a periodontal pocket. The formed pocket is an ideal surface for bacterial colonisation and sub gingival plaque formation ^[11].

Furthermore, P.gingivalis has been linked to rheumatoid arthritis. The bacterium contains the enzyme peptide-arginine deaminase (PAD), which is involved in ctrullination whereby the amino acid arginine is converted to citruline ^[15]. Individuals with rheumatoid arthritis have an increased incidence of periodontal disease and antibodies against the bacterium can be found in higher levels ^[15].

Application to biotechnology

P. gingivalis is a major cause of periodontal disease where supporting structures of the teeth are damaged. The bacteria are involved in the initiation of the disease, however tissue destruction and clinical symptoms are a result of host immune response to the infecting agent ^[20]. It has been identified that the capsule is highly significant in virulence. Moreover, all K-antigen positive serotypes are known to be highly invasive where the mechanisms involve resistance to phagocytosis. There is evidence to suggest that the K-antigens can serve as potential target antigens for vaccine development due to antibodies being able to recognise all K-antigens ^[20].

Another potential target antigen is the fimbriae. The fimbriae promotes adherence to host surfaces. They are well characterised, highly immunogenic and important for the adhesion of the bacterium to tissues of the oral cavity ^[20]. As the structure is so exposed it has been considered as a promising candidate antigen for vaccine development ^[20]. Isolation of antibody secreting cells specific for P. gingivalis fimbriae cans be retrieved from inflamed gingival tissue ^[21].

Current research

Recent studies in clinical and epidemiology has revealed that there is strong evidence for association between rheumatoid arthritis and periodontitis. There has been significant evidence to suggest that P. gingivalis facilitates the development and progression of collagen induced arthritis ^[15]. Furthermore, P. gingivalis expresses the enzyme peptide-arginine deiminase that has the ability to convert arginine residues in proteins to citrulline. Citrullination of proteins results in an alteration of structure and function that may be involved in the deregulation of the host's signalling network and immune evasion ^[22]. There has also been accumulating evidence for the role of autoimmunity against citrullinated proteins in the development of rheumatoid arthritis ^[22].

In addition, it has been revealed that the effects of catechin epigallocatechin gallate found in tea has an effect on established biofilms and on biofilm formation by P. gingivalis. Research suggests that catechin epigallocatechin gallate destroys established biofilms and inhibits biofilm formation ^[23].