(Coykendall et al. 1980) Shah and Collins 1988
It is found in the oral cavity, where it is implicated in certain forms of periodontal disease, as well as the upper gastrointestinal tract, respiratory tract, and in the colon. Collagen degradation that is observed in chronic periodontal disease results in part from the collagenase enzymes of this species. It has been shown in an in vitro study that P. gingivalis can invade human gingival fibroblasts and can survive in them in the presence of considerable concentrations of antibiotics. P. gingivalis also invades gingival epithelial cells in high numbers, in which cases both bacteria and epithelial cells survive for extended periods of time. High levels of specific antibodies can be detected in patients harboring P. gingivalis.
In addition, P. gingivalis has been linked to rheumatoid arthritis. P. gingivalis contains the enzyme peptidyl-arginine deiminase (PAD), which is involved in citrullination. Patients with rheumatoid arthritis have an increased incidence of periodontal disease and antibodies against the bacterium are significantly more common in these patients.
Arg- gingipains have been found to play a key role in the collection of nutrients for P. gingivalis survival. Grenier et al. in 2001 found that Rgp degrades large peptides of the host organism to provide P. gingivalis with an abundant Nitrogen and Carbon source from human albumin serum P. gingivalis can also degrade transferrin (TfR) within host cells which provides the organism with an abundant iron source needed to perform multiple cellular functions.
The gingipains are also responsible for a number of necessary functions related to host invasion and colonization. Furuta et al. characterized Rgp gingipains to be necessary for adhesion and invasion as they processed precursor proteins of long fimbriae. The P. gingivalis genes encoding RgpA, Kgp and hemagglutinin A (HagA) were strongly expressed after incubation with T. denticola. The hemagglutinin adhesion domain-containing proteins act to increase adhesive capacities of P.gingivalis with other bacterial species (Meuric V, Martin B, Guyodo H, Rouillon A, Tamanai-Shacoori Z, Barloy-Hubler F, Bonnaure-Mallet M. Treponema denticola improves adhesive capacities of Porphyromonas gingivalis. Mol Oral Microbiol 2013;28(1):40-53). They are also associated with coordinating the integrity of the biofilm in the developing and maturation phase. Lys- gingipains (Kgp) can bind to immobilized matrix proteins fibrinogen and fibronectin and may have a role in host colonization.
Gingipains also have the ability to degrade multiple signals of the host immune response. They have the ability to cleave subclass 1 and 3 IgG antibodies  as well as pro inflammatory cytokines such as IL-1β, IL-2, IL-6, TNF-α and IL-8 in regions of high P. gingivalis concentration., which impairs host immune response function. Rgp can inhibit IL-2 accumulation in T-cells, which enables it to evade the host adaptive immune response, by modulating T-cell communication and proliferation.
Gingipains are key factors in tissue damage symptoms of Periodontitis, which results from the degradation of matrix metalloproteins, collagen, and fibronectin. Degradation of these substrates interferes with interactions between host cells and the extracellular matrix, therefore impeding wound healing and causing destruction of periodontal tissues. Rgp is responsible for eliciting the host inflammatory response via the p38α MAPK transduction pathway. This response likely contributes to the inflammatory nature of periodontitis and is involved in tissue and bone destruction.
The encapsulated strain of P. gingivalis is much more virulent than the non-encapsulated strain in a mouse abscess model. The capsule is a capsular polysaccharide (CPS) and when present down regulates cytokine production especially pro inflammatory cytokines IL-1β, IL-6, IL-8, and TNF-α indicating host evasion responses. However, other studies have found the CPS to elicit host immune responses like PMN migration and dose and time dependent expression of cell migration chemokines like MCP-1, KC, MIP-2 and RANTES in CPS challenged murine peritoneal macrophages. These conditions are likely to contribute to the inflammatory lesions observed in periodontitis.
Ongoing studies have provided evidence that Vaccine’s made from P. gingivalis CPS impairs oral bone loss in murine models. These vaccines have been able to elicit potent immune responses such as increased IgM and IgG responses that recognizes P. gingivalis whole organisms.
Fimbriae is an appendage that is involved in cellular attachment and greatly contributes to virulence. It is found on many gram negative and some gram positive bacterium.
P. gingivalis virulence is heavily associated with fimbriae as fimbriae have been characterized to be key factors in adhesion, invasion, and colonization. Fimbriae’s are also responsible for invasion of MV’s (Membrane Vesicle) into host cells. They were found to bind to cellular α5β1 integrins, which mediated adherence and impaired the homeostatic controls of host cells. Fimbriae were also found to be associated with modulating β2 integrin adhesive activity for uptake by monocytes using the CD14/TLR2/PI3K signaling complex, which may contribute to intracellular evasion tactics, by P. gingivalis. P. gingivalis have long fimbriae, short fimbriae, and accessory components each of which have distinct functions.
Long fimbriae (FimA) also known as major fimbriae are long, peritrichous, filamentous components. They have a role in initial attachment and organization of biofilms as they act as adhesins that mediate invasion and colonization of host cells contributing to P. gingivalis virulence.
Short fimbriae (Mfa1) also known as minor fimbriae have distinct roles from long fimbriae and are characterized to be essential for cell-cell auto aggregation and recruitment for micro colony formation. Short fimbriae are involved in cell-cell adhesion with other dental commensals. It was found to coadhere and develop biofilm in conjunction with Streptococcus gordonii by interaction with SspB streptococcal surface polypeptide. This interaction may be essential in the invasion of dentinal tubules by P. gingivalis.
Fim C,D, and E accessory components associate with the main FimA protein and have a role in binding with matrix proteins and interaction with CXC- chemokine receptor 4. Loss of function experiments by Pierce et al. has confirmed that P. gingivalis mutants deficient for Fim C, D, or E have drastically attenuated virulence.
Evasion of host defenses and immune responses
P. gingivalis has many ways of evading host immune responses which affects its virulence. P. gingivalis does this by using a combination of gingipain proteases, a capsular polysaccharide, induction of host cell proliferation and the cleavage of chemokines responsible for neutrophil recruitment.
Virulent P. gingivalis further modulates leukocyte recruitment by proteolysis of cytokines and chemokines that are secreted by the host cells. The Arg-gingipain and Lys-gingipains are responsible for this proteolysis. In a study using a mouse model P. gingivalis was specifically found to down regulate IL-8 induction causing delayed neutrophil recruitment. Prevention of neutrophil recruitment may inhibit the clearance of the bacterium from the site of infection allowing for colonization.P. gingivalis is able to evade opsonophagocytosis from PMN’s by using Gingipain K (Kgp) to cleave IgG 1 and 3. This further modulates immune response by impairing signaling. Other studies have found that P. gingivalis can subvert the complement pathway through C5αR and C3αR, which modulates the killing capacity of leukocytes allowing for uncontrolled bacterial growth. P. gingivalis was also found to inhibit pro inflammatory and antimicrobial responses in human monocytes and mouse macrophages by fimbrial binding to CXCR4, inducing PKA signaling and inhibiting TLR-2 mediated immune response.
Once in the host cells, P. gingivalis is capable of inhibiting apoptosis by modulating the JAK/Stat pathway that controls mitochondrial apoptotic pathways. A proliferative phenotype maybe beneficial to the bacterium as it provides nutrients, impairs host cell signaling, and compromises the integrity of the epithelial cell layer allowing for invasion and colonization
P. gingivalis is a “red” bacteria and is a “keystone” bacterium in the onset of chronic adult periodontitis. Though it is found in low abundance in the oral cavity it causes a microbial shift of the oral cavity allowing for uncontrolled growth of the commensal microbial community. This leads to periodontitis through the disruption of the host tissue homeostasis and adaptive immune response. After using using laser capture microdissection (LCM) plus qRT-PCR to detect P. gingivalis in human biopsies, colocalization of P. gingivalis with CD4+ T cells was observed ** .However , the infection mechanism of T cells by P. gingivalis remains unknown.
P. gingivalis has been associated with increasing the virulence of other commensal bacterium in both in vivo and in vitro experiments. P. gingivalis outer membrane vesicles (OMV’s) were found to be necessary for the invasion of epithelial cells of Tannerella forsythia. P. gingivalis short fimbriae was found to be necessary for coculture biofilm formation with Streptococcus gordonii  Interproximal and horizontal alveolar bone loss in mouse models are seen in co infections involving P. gingivalis and Treponema denticola. The role of P. gingivalis as a community activist in periodontitis is seen in specific pathogen free (SPF) mouse models of periodontal infections. In these models P. gingivalis inoculation causes significant bone loss which is a significant characteristic of the disease. In contrast, germ free (GF) mice inoculated with a P. gingivalis mono infection causes no bone loss indicating that P. gingivalis alone cannot induce periodontitis 
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