**2. Pathophysiological links between periodontitis and CVD**

### **2.1 Subgingival microbiota and periodontitis development**

A highly organized subgingival microbial community is involved in the transition from a periodontal health condition to a dysbiotic pathogenic status namely periodontitis characterized by a complex shift of the microbiota composition, abundance, and arrangement [10, 11]. Three members of the subgingival consortium included in the "Red complex" have been constantly identified in the subgingival microbiota of periodontitis patients are: *Porphyromonas gingivalis* (*P. gingivalis*), *Treponema denticola* (*T. denticola*), and *Tannerella forsythia* (*T. forsythia*) [12]. Previously viewed as true periodontopathogens, they are now rather considered as pathobiont organisms because these bacteria are normally present in low numbers in the subgingival microbiota of periodontal healthy individuals [13]. *Filifacter alocis* and bacteria of genera *Parvimonas, Fusobacterium*, and *Prevotella* are nowadays considered as pathobionts [14]. Pathobionts expand within the microbial community once ecological changes take place, which initiates and favor periodontitis development [11]. Dynamic and synergistic interactions between subgingival microbiota and the host shape and stabilize dysbiotic communities within their subgingival habitat [15]. Gingival inflammation triggers local tissue destruction that releases subgingivally specific nutriments corresponding to the growth of highly demanding Gram-negative bacteria, which amplify local dysbiosis [16].

A paradox remarqued by the specialists is that dysbiotic microbiota requires inflammation to sustain their nutrition in the context in which localized inflammatory reaction developed by the host is a normal function addressed to reestablish local homeostasis through inhibiting bacterial development [11]. The behavior of *P. gingivalis* could explain this paradox through the manipulation of the periodontal immune reactions [17].

A low proportion of *P. gingivalis* in the subgingival microbiota in both periodontal health and periodontitis has been identified. The bacterium is considered a keystone pathogen in the dysbiotic microbiota creating a pro-inflammatory, antiphagocytic environment favorable to the growth and development of pathobionts [18].

Upregulation of many virulence genes of *P. gingivalis* in healthy sites before progression of the attachment loss has been described. In contrast, *T. denticola* and *T. forsythia* upregulate only very few virulence genes and only in the later stages of periodontitis evolution. It seems that *P. gingivalis* serves as a microbial driver in the transition from periodontal healthy status to periodontitis [16].

Microbiota within the subgingival biofilm is a complex highly dynamic architectural arrangement of bacteria rather than a random static positioning of the microbial cells next to each other. Within the biofilm, biochemical interaction, signaling, and genetic exchanges between bacteria take place. Facultative aerobes within subgingival biofilms can sequester oxygen and create anaerobic niches that favor the expansion of anaerobic protobionts and thus the transition toward dysbiosis [11]. The bacterial load in periodontal pockets is exponentially greater compared to the flora of the sulcus at the expense of the dominant species rather than due to the replacement of early colonizers [16]. Four distinct layers have been described in the subgingival biofilm from periodontitis patients. The basal layer of the subgingival biofilm, located in the proximity of the tooth surface, is formed by *Actinomyces* spp. *Fusobacterium* and *Tannerella* reside in the intermediate layers of the biofilms while *Prevotella* and

*Porphyromonas* are localized in both the apical and intermediate layers [19]. Bacterial cells of the *Cytophaga*-*Flavobacterium*-*Bacteroides* cluster were located in the apical layers and *Treponema* was located above the densely packed biofilm. Microbial cells of the genus *Synergistes* have been described closely arranged to polymorphonuclear leukocytes indicating direct physical interactions between biofilm microbes and host immune cells [19]. However, close relationships between polymorphonuclear leucocytes and subgingival microbes have been previously highlighted through transmission electron micrographs [20].

#### **2.2 Pathogenetic mechanisms linking periodontitis and CVD**

Atherosclerosis is a chronic, vascular inflammatory condition being a major cause of CVD. In atherosclerosis, the deposition of lipids in the artery walls results in plaque build-up. Its progression results in the reduction of blood flow and consecutive ischemia of organs and tissues and promotes clot formation [21]. The American Heart Association pointed out the relationships between periodontitis and CVD although causality remains unproven [22, 23].

A wide range of studies has investigated the causal relationships between periodontitis and CVD which have resulted in two essential hypotheses explaining this link. One hypothesis sustains the role of systemically disseminated periodontopathogens and of their by-products to induce atheroma formation through the infection of blood vessels [24]. Disseminated bacteria and their products including lipopolysaccharide (LPS) challenge the immune system inducing systemic inflammatory reactions [3].

Several data communicated over time have reported the ability of bacteria from subgingival plaque to migrate and localize to vascular walls and atheromatous plaques. Recent information provided by a systematic review and a meta-analysis showed that the DNA of periodontopathogens was present in atheromatous plaque samples from patients with myocardial infarction [25]. However, less than 5% of patients receiving surgery for atherosclerotic vascular disease presented bacterial DNA in isolated samples which were not statistically significantly different from patients receiving surgeries for rheumatic heart disease [26].

The second hypothesis considers that longstanding periodontal inflammation overlaps with existing chronic systemic inflammation through the dissemination of locally abundantly produced molecules (pro-inflammatory cytokines, chemokines, and gingiva-derived C-reactive protein CRP) promoting atherosclerosis development and CVD [24, 27].

Moreover, a recent emerging hypothesis considers that periodontopathogens and periodontal inflammation promote modifications in oral and gut microbiome which in turn may influence the evolution of both periodontitis and atherosclerosis [24].

In periodontitis patients, the processing of antigenic structures by the liver induces a systemic acute phase response associated with an increased plasma CRP [3]. Periodontitis patients have statistically significantly higher high sensitivity CRP plasma values compared to periodontally healthy subjects amounting to 1.56 mg/L. This difference is biologically relevant since it could drive patients into the high CRP risk category (>3 mg/L) [28]. IgG antibodies against specific periodontal bacteria were associated with all-cause and CVD mortality [29].

A vascular endothelial activation is a central event for atherosclerosis development and a connecting link to periodontitis [30]. Circulating bacterial LPS, outer membrane vesicles and fimbriae, as well as inflammatory cytokines induce the

*Periodontitis and Heart Disease: Current Perspectives on the Associative Relationships… DOI: http://dx.doi.org/10.5772/intechopen.102669*

up-regulation of cell-surface receptors and the expression of adhesion molecules on the vascular endothelium, which recruit peripheral blood monocytes at the surface of the activated endothelium. On the other hand, antibodies targeting specific bacterial proteins behave as auto-antibodies through "molecular mimicry" and induce damage of the vascular endothelium [31]. Monocytes follow a chemoattractant gradient and migrate into the sub-endothelial space, become tissular macrophages, capture oxidized low-density lipoprotein cholesterol (LDL) and develop into foam cells. Apoptosis of the latter favor the accumulation of lipids in the sub-endothelial space forming the atheromatous plaques, which become coated by a fibrous shelter and promote platelet adhesion. Apoptosis of endothelial cell exposes the fibrous cap which in association with the enzymatic destruction of the extracellular matrix induce plaque rupture, exposure of pro-thrombotic plaque components, and subsequent thrombus formation that leads to vascular occlusion and CVD related events (myocardial infarction or stroke) [3].

Periodontitis patients have increased platelet recruitment and platelet hyperactivation as sustained by the augmented plasma concentration of platelet factor 4 (PF4) [32, 33]. Moreover, periodontitis has been frequently accompanied by a prothrombotic state. In patients with mild, moderate, or severe periodontitis D-dimer values were found to be increased by 1.62-fold, 2.06-fold and 2.54-fold, respectively than in healthy patients [34].

### **3. Epidemiologic relationships between periodontitis and CVD**

Although, the relationship between periodontitis and CVD has long been the subject of debate in the scientific literature, the existence of a moderate association between periodontitis and CVD was firstly reported by a systematic review in 2003 [35]. Since then, many other systematic reviews and meta-analyses highlighted this idea. Periodontitis patients had 19%, 15% and 14% increased risk of developing CVD, respectively [36–38].

Individuals with active periodontitis had a nearly 2–2.5-fold higher risk of developing acute myocardial infarction than those without the disease [39, 40].

A positive association between both severe and moderate periodontitis and acute myocardial infarction exists, which suggests a relationship between periodontitis severity and CVD [41].

A significant association was found between calcified carotid artery atheroma and the presence of periodontitis both in acute myocardial infarction (OR, 1.51; 95% CI, 1.09–2.10) and controls (OR, 1.70; 95% CI, 1.22–2.38) [42]. Periodontitis was associated with a 45% higher risk of acute myocardial infarction (OR, 1.45; 95% CI, 1.10–1.91), whilst patients with both periodontitis and calcified carotid artery atheromas had a 75% higher risk for acute myocardial infarction then those without these conditions [42].

An increased CVD burden in patients with significant periodontal destruction had been suggested in different studies. Data from the Oral Infections and Vascular Disease Epidemiology (INVEST) study showed that the number of missing teeth was significantly associated with carotid artery plaque after adjustment for CVD risk factors [43]. Moreover, the Periodontitis and Its Relation to Coronary Artery Disease (PAROKRANK) study reported an increased risk of myocardial infarction in patients with periodontitis (odds ratio of 1.28, 95% confidence interval (CI) 1.03–1.6) after adjustment for some behavioral, social and medical risk factors. The radiographic

bone loss was used to quantify periodontal destruction [44]. However, concerns have been raised in relation to the difficulty to control for all possible confounders and the possibility that periodontitis could still be a surrogate for other risk factors not highlighted by these studies [45].

Periodontitis may also represent a risk factor for stroke, especially in ischemic events. However, new studies with a robust design are necessary for a reliable conclusion [46].
