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Broadening our understanding of periodontal pathogens may open the door to better treatment strategies.

By increasing the knowledge base of how the most virulent periodontal pathogens work, hopefully new strategies can be implemented to reduce their damage on oral health.

The secretion system of a cell plays a significant role in its ability to proliferate within a particular ecological niche by enabling nutrition storage, communicating between cells, defending against antagonists, and inducing virulence factors.25 The first Gram-negative bacterium’s secretion system was discovered in the 1970s, revealing Escherichia coli’s ability to produce hemolysin-A and secrete it into the environment. Since then, eight more secretion pathways have been discovered and are referred to as type x secretion systems (T1SS  to T9SS).

Porphyromonas gingivalis’ secretion system (T9SS) — most recently discovered — uses a two-step process, during which the substrates first pass the inner membrane into the periplasm and then are guided to the outer membrane translocation channel.1,2 The T9SS has two different roles: one that is related to bacterial gliding motility, but also a sinister one used by the pathogens fostering dysbiosis. In the latter case, in a pathogenic bacterium such as P. gingivalis, the protein secretions can remain attached to the surface of the outer membrane, released into the extracellular environment, or penetrate the cytoplasm of a target cell.2

Additional Mechanisms

P. gingivalis’ outer membrane is composed of lipopolysaccharide (LPS) — a vital structure necessary to maintain its cell shape and ability to carry out cell functions. LPS functions as a barrier, blocking and preventing assaults or elimination by the host complement.

The outer membrane LPS houses enclosed, endocytic-derived outer membrane vesicles that contribute to toxin transport and pathogenicity.2,3 Recent findings demonstrate that bacterial pathogens like P. gingivalis activate outer membrane vesicles as a means of communication, manipulation, and disruption of the normal immune response of the host, revealing once again the profound ingenuity and capabilities of this microorganism.4

Peptidylarginine deiminase (PPAD) is a protein-modifying enzyme converting arginine residues to neutral citrulline residues, altering protein structure and function. This process, known as citrullination, is a major factor influencing colonization and biofilm growth. Thus far, P. gingivalis is the only bacterium in the human body known to generate and secrete PPAD,1,5 and its precise role in periodontal diseases and bacterial biochemistry is yet unknown, studies suggest that ammonia—a byproduct of PPAD activity—aids P. gingivalis’ resistance to elimination through acidic cleansing, a process initiated by the host immune response.6

This view is supported by the restriction of P. gingivalis’ growth on protein substrates at low pH, and observation of citrullination and amino acid fermentation of lysine and arginine providing a favorable environment for its survival. PPAD can be secreted with other virulence factors (gingipains) directly through the T9SS or indirectly through outer membrane vesicles.1,5,7

P. gingivalis is an especially dangerous and capable periodontal pathogen, affecting not only oral but also systemic health. Ongoing research will reveal more insights into how the bacterium lives and succeeds as one of the most notable periodontal pathogens.


  1. Lunar Silva I, Cascales E. Molecular strategies underlying Porphyromonas gingivalis virulence. J Mol Biol. 2021;433:166836.
  2. Lasica AM, Ksiazek M, MadeJ M, Potempa J. The type ix secretion system (t9ss): highlights and recent insights into its structure and function. Front Cell Infect Microbiol. 2017;7:215.
  3. Zhang Z, Liu D, Liu S, Zhang S, Pan Y. The role of Porphyromonas gingivalis outer membrane vesicles in periodontal disease and related systemic diseases. Front Cell Infect Microbiol. 2021;10.
  4. Nara PL, Sindelar D, Penn MS, Potempa J, Griffin WST. Porphyromonas gingivalis outer membrane vesicles as the major driver of and explanation for neuropathogenesis, the cholinergic hypothesis, iron dyshomeostasis, and salivary lactoferrin in Alzheimer’s disease. J Alzheimers Dis JAD. 2021;82:1417–1450.
  5. Vermilyea DM, Ottenberg GK, Davey ME. Citrullination mediated by PPAD constrains biofilm formation in P. gingivalis strain 381. NpJ Biofilms Microbiomes. 2019;5:1–11.
  6. Moradali MF, Ghods S, Angelini TE, Davey ME. Amino acids as wetting agents: surface translocation by Porphyromonas gingivalisISME J. 2019;13:1560–1574.
  7. Bereta G, Goulas T, MadeJ M, et al. Structure, function, and inhibition of a genomic/​clinical variant of Porphyromonas gingivalis peptidylarginine deiminase. Protein Sci. 2019;28:478-486.

This information originally appeared in Marsh I, Matthews A. The pathogenicity of Porphyromonas gingivalis. Dimensions of Dental Hygiene. 2022;20(10)34-37.

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