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Name: Jovin Choo Jia Ying
Bench: E
Date: 31 August 2016
Porphyromonas gingivalis

Classification

Higher order taxa

Bacteria – Bacteria – Bacteroidetes – Bacteroide – Bacteroidales – Porphyromonadaceae – Porphyromonas ^[1]

Species

Species name: Porphyromonas gingivalis

Type strains: 381, 2561, ATCC 33277, BCRC 14417, CCRC 14417, CCUG 25893, CCUG 25928, CIP 103683, Coykendall 2561, DSM 20709, JCM 12257, KCTC 5121, KDI, NCTC 11834, Slots 2561, SU63, W50, W83. ^[2]

Description and significance

Porphyromonas gingivalis is found in 86% of subgingival plaque samples from patients suffering from chronic periodontitis. P. gingivalis is a non-motile, asaccharolytic, obligate anaerobe, gram negative, rod shaped bacterium. It is known to form black pigmented colonies after being cultured for 6-10 days on blood agar due to accumulation of heme. It also requires iron for its growth. P. gingivalis is previously named Bacteroides gingivalis before reclassification into a new genus. It is a secondary colonizer of dental plaque adhering to primary colonizers such as Streptococcus gordonii and P. intermedia. ^[3]

As P. gingivalis is a prime etiological agent that causes severe forms of periodontitis, understanding the mechanism of the pathogenesis will allow us to develope treatment for periodontal disease and hopefully eradicate Porphyromonas gingivalis. ^[4]

Genome structure

W83 is a virulent strain of P. gingivalis with a genome size of 2,343,479 bp and an average GC content of 48%. It is found to have 4 ribosomal operons, 2 structural RNA gene and 53 tRNA genes. A total of 1990 ORFs were identified. Out of these, 54% were assigned to biological categories, 9.2% were conserved hypoethical or domain proteins, 26.3% were encoded hypoethical proteins while the remaining 10.5% were unknown. As iron is a major requirement for P. gingivalis, multiple systems relating to iron uptake such as iron chelate ABC uptake system, TonB-dependent iron receptors and FeoB ferrous iron uptake systems were discovered. Clusters of gene (PG1582-86) responsible for ensuring tolerance to oxygen in the oral cavity were also isolated. Genes encoding for adherence factors such as hemagglutinin (PG0411, 1326, 1674, 1427, 1548, 2198) and capsule were also identified in this strain (PG0106-0120, PG0435-0437, PG1140-49 and PG1560-1565). ^[1]

Cell structure and metabolism

Cell wall

Like any other Gram-negative bacteria, P. gingivalis has both outer (OM) and inner membrane (IM) separated by the periplasm, which contains the peptidoglycan layer. The IM is made up of a phospholipid bilayer with various integral IM proteins while OM is made up of an asymmetrical bilayer with phospholipids in the inner learflet and lipopolysaccharide on the outer. The OM is hypothesized to be highly associated with the formation and maintenance of biofilms within periodontal microflora, mediated by OM proteins. As the OM is the most exposed area of the cell, OM proteins such as LptO, RagA, RagB and OmpA-like proteins are also important providing adherence to host cells and secreting gingipains. ^[3]

Biofilm formation

Biofilm formation is employed by the bacteria for protection against environmental stresses. In P. gingivalis, it has been discovered that the polyphosphate kinase activity, encoded by the ppk gene is a requirement for biofilm formation. Stress-associated protein uspA is influenced by the production of intracellular polyphosphate and is postulated to play a role in the formation of biofilms. ^[5]

The formation of dental plaque is made up of biofilm formation of different species of microflora existing in commensal harmony with the host. P. gingivalis forms biofilms with Streptococcus gordonii through LuxS-dependent signalling, which is able to mediate interspecies communication in mixed species biofilms. Initial adherence between S. gordonii Ssp surface protein and P. gingivalis minor fimbriae is required prior to biofilm formation. ^[6]

Motility

P. gingivalis species are non-motile. ^[3]

Metabolic functions

P. gingivalis possesses a limited capacity to uptake and metabolize organic nutrients. Metabolic reconstruction analysis has shown that P. gingivalis poorly utilizes glucose and does not use carbohydrates to support its growth. ^[1] This could be due to the environment it localizes in – sugar availability is low in deep periodontal pockets. ^[3]

Instead, P. gingivalis prefers peptides as its carbon and nitrogen source. However, P. gingivalis was found to be able to metabolize other sugars like melibiose, galactose, starch and maltodextrin. The bacteria also possess hexose aminidases that degrades complex amino sugars from host cell, which results in a more susceptible host cell to degradation by bacterial proteases. ^[1]

11 amino acids were found to be substrates for energy production from degrading host tissues or breaking down other bacteria in the oral cavity. Major fermentation products of P. gingivalis were found toxic to host cells. P. gingivalis uses nucleosides and nucleobases for nucleic acid synthesis or carbon and energy sources. ^[1]

Iron acquisition is also important for survival and growth of P. gingivalis as it has several iron uptake systems. P. gingivalis uses sodium ions to drive its transport system, hence several metal ion transporters and ion/proton exchangers can be found encoded in the genome. ^[1]

Ecology

P. gingivalis is an obligate anaerobe. Its major habitat is found to be in the subgingival sulcus of the human oral cavity with preference to reside in deep periodontal pockets. Higher number of bacteria can be found in locations with periodontitis and in lower or non-detectable sites with subgingivalis health or plaque-associated gingivitis. ^[3] P. gingivalis have been isolated and cultured from supragingival plaque, oral mucosal surfaces, dorsum of the tongue, saliva as well as the pharynx. P. gingivalis is not found in any other microbiota of any body site. ^[7]

Microbe/Host interactions

Bacterial adherence begins in the oral cavity before prolierating in the dental plaque. Fimbriae, proteases, hemagglutinins and LPS partitipate in the adherence of P. gingivalis to host cells. Fimbriae are able to bind to multiple sites such as epithelial cells, fibroblasts, salivary components, hemoglobin and several extracellular matrix (ECM) proteins of the human host. Coaggregation with other plaque-forming bacteria also assists in colonization of P. gingivalis. P. gingivalis invades by the membrane ruffling mechanism of host cells and is internalized through vacuoles. Activation of human epithelial cells, spleen cells and peripheral blood monocytes by P. gingivalis fimbriae results in the release of IL1, IL6, IL8 and TNF A. ^[8]

Pathology

P. gingivalis is a major cause of infections that affects the structures around the teeth which is also known as periodontal disease. Prolong progression will result in the loss of supportive connective tissue and bone, leading to tooth loss. ^[1]

P. gingivalis is categorized based on their ability to form abscesses. The invasive strain of P. gingivalis is more pathogenic as compared to the non-invasive strain. The cause of advanced periodontal disease is dependent on the immunocompetency of the host, and if P. gingivalis is working alone or together with other pathogens. P. gingivalis found in high frequency in adult periodontitis lesions suggests that it works with other pathogenic microbes to cause further progression of the disease. ^[3]

P. gingivalis is also able to cause non-oral infections and diseases such as endocarditis as well as abscesses in lungs, head, neck and abdominal areas. ^[7]

Virulence Factors

To survive and replicate in the host, P. gingivalis must first pass through the oxygen-rich oral cavity before settling in an anaerobic environment. Colonization of P. gingivalis occurs in the presence of virulence factors such as the fimbriae, capsule, lipopolysaccharides, proteases and outer membrane proteins (OMP). Virulence factors are expressed accordingly to the environment the bacteria are in.^[3]

Capsule: Adhesins on surface of bacteria and receptors on oral surface mediates adherence of P. gingivalis and host tissues. Most adhesions are associated with capsule or fimbriae found on bacterial cell walls. Capsules facilitate attachment of bacteria to host cells. It is observed that strains that are encapsulated are more virulent as compared to those that are not, due to resistance to phagocytosis. The type of capsule structure and adhesion capacity can also characterize the difference in virulent strains.^[3]

Fimbriae: P. gingivalis expresses FimA and Mfa protein on its cell surface and are postulated to contribute to progression of periodontal inflammatory responses. It has been reported that strains with type IV fimbriae are poorly fimbriated as oppose to those with type I FimA. These fimbriae adhere to a variety of oral substrates and molecules.^[3]

Lipopolysaccharides (LPS): The bacterial LPS consist of O antigen and lipid A, which are responsible for maintaining the cellular and structural integrity of the cell and controlling the entry of hydrophobic molecules and toxic chemicals. Lipid A interacts with toll-like receptor 2 and 4, which causes deregulation of host innate immune system.^[3]

Proteases: P. gingivalis’s proteases is known to be strongly involved in the progression of periodontal disease. Proteases from P. gingivalis are most commonly known as gingipains, which has high resistance to the host defence system. Gingipains are also able to degrade ECM proteins and deregulate host inflammatory response.^[3]

Outer Membrane Protein (OMP): The OMP is believed to mediate interactions between the microflora of the oral cavity to form biofilms. The 40-kDa OM protein on the cell surface is an important aggregation factor and is found in several strains of P. gingivalis at both the cell surface and extracellular vesicle.^[3]

Application to biotechnology

Drug targets

Antimicrobial peptides was discovered to possess inhibition activities against the growth of P. gingivalis. A biosynthetic peptide, Pep-7 forms pores at cytoplasmic membranes poles of P. gingivalis. Its selective activity against the Porphyromonas sp., through alteration of the permeability barriers of, proves it to be a good candidate for periodontal treatment. ^[9]

Vaccine target antigen

Virulence factors of P. gingivalis has been studied extensively for the possibility of using them to create vaccines.

Capsule - The capsule of P. gingivalis is recognized by antibodies and elicits IgG response. It can also be utilized as glycoconjugates in the form of a carrier in vaccines to overcome its insufficient immunogenecity as a single antigenic determinant. ^[10]

Cysteine Proteases (Gingipains) - Combination of antigenic domains of the enzymes can be exploited to induce sufficient immune response when used in a vaccine. ^[10]

Fimbriae - P. gingivalis’s fimbriae has been well characterized and is known to be highly immunogenic which is highly promising as a vaccine candidate. Studies have shown that it is more advantageous and effective when it is used as an adjuvant instead of a single antigenic determinant in vaccine production. An increase in FimA specific IgG antibodies and fimbriae specific IgA and IgG antibodies was seen when it was used as an adjuvant in vaccines. ^[10]

Outer Membrane Protein (OMP) - Major factor in colonization and aggregation of P. gingivalis. OmpA-like protein and 40-kDa OMP have been extensively studied for their immunization possibilities. 40-kDa OMP stimulates serum IgG, IgA, and salivary IgG response when delivered orally, nasally and transcutaneously. ^[10]

Current research

Vaccine and Treatment

Recent studies have investigated outer membrane vesicles (OMVs) as a potential mucosal immunogen due to its wide range of surface antigens. Small doses of OMVs as vaccine antigen elicits protective immunoglobulin on mucosal surface of the oral cavity. Coupled with a mucosal adjuvant, results show feasibility in developing a safe and effective intranasal vaccine. ^[11]

Currently undergoing phase 1 clinical trials, DNA vaccines are also potential targets for periodontal disease. Researchers are also cloning potential genes for vaccine development from periodontopathic bacteria and candidate vaccine antigens are currently being trialled in mouse models. ^[10]

In another recent study, inhibition of quorum sensing was discovered to weaken the formation of biofilm, leading to a decrease in colonization of pathogenic and commensal microbes. Through the utilizations of AI2 inhibitors, biofilm formation and virulence expression can be controlled and perhaps prevent P. gingivalis growth, stopping the occurrence of periodontitis. ^[12]

References

1. Nelson, K.E., Fleischmann, R.D., Deboy, R.T., Paulsen, I.T., Fouts, D.E., Eisen, J.A., et al. (2003) Complete Genome Sequence of the Oral Pathogenic Bacterium Porphyromonas gingivalis Strain W83. J Bacteriol 185: 5591–5601.

2. LPSN

3. How, K.Y., Song, K.P., and Chan, K.G. (2016) Porphyromonas gingivalis: An overview of periodontopathic pathogen below the gum line. Front Microbiol 7: 1–14.

4. Mysak, J., Podzimek, S., Sommerova, P., Lyuya-Mi, Y., Bartova, J., Janatova, T., et al. (2014) Porphyromonas gingivalis: Major periodontopathic pathogen overview. J Immunol Res 2014: 1–8.

5. Kuramitsu, H., Chen, W., and Ikegami, A. (2005) Biofilm Formation by the Periodontopathic. J Periodontol 2005: 2047–2051.

6. McNab, R., Ford, S.K., El-sabaeny, A., Barbieri, B., Cook, G.S., and Lamont, R.J. (2003) LuxS-Based Signaling in Streptococcus gordonii: Autoinducer 2 Controls Carbohydrate Metabolism and Biofilm Formation with Porphyromonas gingivalis. J Bacteriol 185: 274–284.

7. Asikainen, S., and Chen, C. (1999) Oral ecology and person-to-person transmission of Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis. Periodontol 2000 20: 65–81.

8. Amano, A. (2003) Molecular Interaction of Porphyromonas gingivalis with Host Cells: Implication for the Microbial Pathogenesis of Periodontal Disease. J Periodontol 74: 90–96.

9. Suwandecha, T., Srichana, T., Balekar, N., Nakpheng, T., and Pangsomboon, K. (2015) Novel antimicrobial peptide specifically active against Porphyromonas gingivalis. Arch Microbiol 197: 899–909.

10. Grover, V., Kapoor, A., Malhotra, R., and Kaur, G. (2014) Porphyromonas gingivalis Antigenic Determinants - Potential Targets for the Vaccine Development against Periodontitis. Infect Disord - Drug Targets 14: 1–13.

11. Nakao, R., Hasegawa, H., Dongying, B., Ohnishi, M., and Senpuku, H. (2016) Assessment of outer membrane vesicles of periodontopathic bacterium Porphyromonas gingivalis as possible mucosal immunogen. Vaccine 34: 4626–4634.

12. Cho, Y.-J., Song, H.Y., Amara, H. Ben, Choi, B.-K., Eunju, R., Cho, Y.-A., et al. (2016) In Vivo Inhibition of Porphyromonas gingivalis Growth and Prevention of Periodontitis With Quorum-Sensing Inhibitors. J Periodontol 87: 1075–1082.

This page is written by Jovin Choo Jia Ying for the MICR3004 course, Semester 2, 2016