**6. The evolution from a healthy periodontium to periodontitis**

In the oral cavity, the tooth surface offers a niche for bacteria colonization and biofilm forma‐ tion resulting in a varied polymicrobial community. A healthy environment is maintained if the multiplication of symbiotic microbiota is regulated. Periodontal diseases are related to a shift from symbiotic microbiota to dysbiotic microbiota, and this shift is related with the accumula‐ tion of dental plaque or biofilm. Biofilm elaboration consists of four sequential phases. Phase 1 consists of the adsorption of different molecules to a surface to condition the biofilm formation. Phase 2 consists of single organism adhesion. Phase 3 consists of growth of extracellular matrix production and multiplication of adhering bacteria and phase 4 consists of sequential adsorption of further bacteria to form a more complex and mature biofilm (**Figure 4**) [32]. The microbial eti‐ ology of gingivitis and periodontitis has been established for several decades. In 1994, Haffajee and Socransky adapted Koch's postulates to be used in the identification of periodontal patho‐ gens. In 1996, at the World Workshop in Periodontics three species of pathogens were identified as causative factors of periodontitis *Aggregatibacter actinomycetemcomitans*, *Porphyromonas gingivalis*, and *Tannerella forsythia*; however, these three species cannot be considered to be the only causative pathogens of periodontitis, but we are certain that they participate in the disease [32].

significant release of granules. The hierarchical mobilization of neutrophil granules and secre‐ tory vesicles depends on intracellular Ca2+ level. Gradual elevations in intracellular Ca2+ are

Neutrophil stimulation can also undergo a mechanism called NETosis. Although NETosis has previously been described as a special form of programmed cell death, there are forms of NET production that do not end with the demise of neutrophils. NETosis leads to the release of decondensed chromatin into the extracellular space. The chromatin forms a trap for patho‐ gens that looks like a net, which is why they are called neutrophil extracellular traps (NETs). NETs also contain histones, cytoplasmic proteins, and antimicrobial granular molecules. NETs formation mechanisms are still unknown, nevertheless, NADPH oxidase activation, reactive oxygen species (ROS) production, myeloperoxidase (MPO), and neutrophil elastase

In periodontal health, the interaction between symbiotic microbial community and neutrophils is strongly controlled to prevent tissue damage. This interaction has been evaluated in studies with germ‐free mice and specific pathogen‐free mice. Results of these studies showed that oral sym‐ biotic commensal microbiota has no impact on the structure of gingival tissue of germ‐free mice, while gut commensal microbiota is fundamental on the structural formation of the intestinal tissue [29]. Periodontal tissue recruits neutrophils by means of the chemotactic receptor CXCR2. This receptor has two ligands CXCL1 and CXCL2. Both ligands are expressed in the junctional epithelium of germ‐free and specific pathogen‐free mice, but there is a significant increase on CXCL2 in the epithelium of specific pathogen‐free mice. Therefore, oral bacterial community induces an increase in neutrophil recruitment via CXCL2 [29]. Neutrophils play a key role in preserving oral health, since low neutrophil counts as well as deficiency in neutrophil functional responses have been associated with periodontal disease. As mentioned before, neutrophils kill pathogens by phagocytosis, degranulation, or NETs formation (**Figure 1C**). Neutrophils are very efficient phagocytic cells and have a very efficient antimicrobial mechanism to do so, the respira‐ tory burst response. In this response, high consumption of oxygen results in the production of reactive oxygen species (ROS), through the activation of the NADPH oxidase complex (**Figure 3**) [5]. Patients with chronic granulomatous disease, a rare genetic disorder that consist on muta‐ tions in the NADPH oxidase, are inefficient in mounting a respiratory burst response. As a con‐ sequence, these patients present early in life recurrent infections [30]. These patients present higher bacteria colonization and gingivitis; however, they do not present periodontitis [31].

induced by ligation of L‐selectin, CD11b/CD18, and the fMLP receptors [26].

**5. Neutrophil interactions with symbiotic oral bacteria**

**6. The evolution from a healthy periodontium to periodontitis**

In the oral cavity, the tooth surface offers a niche for bacteria colonization and biofilm forma‐ tion resulting in a varied polymicrobial community. A healthy environment is maintained if the

**4.3. Neutrophil extracellular traps (NETs)**

74 Role of Neutrophils in Disease Pathogenesis

(NE) release (**Figure 3**) are required [25].

**Figure 4.** Biofilm elaboration consists of four sequential phases. Phase 1 consists of the adsorption of different molecules to a surface to condition the biofilm formation. Phase 2 consists of single organism adhesion. Phase 3 consists of growth of extracellular matrix production and multiplication of adhering bacteria and phase 4 consists of sequential adsorption of further bacteria to form a more complex and mature biofilm. The first two phases are representative of health, phase 3 of gingivitis, and phase 4 of periodontitis.

#### **6.1. Balanced inflammation**

Neutrophils are the main leukocytes recruited to the gingival sulcus. Neutrophils exit the gingival blood vessels and travel through the gingival junctional epithelium until they reach the sulcus [8]. In the sulcus, neutrophils create a barrier against the growing bacteria biofilm to prevent bacteria from invading the underlying tissues. Neutrophil migration from vessels toward the gingival sulcus requires CXCR2 binding to CXCL2. Migration is controlled by gradients of chemokines and adhesion molecules such as IL‐8, ICAM‐1, and E‐selectin [29]. Neutrophil presence in the sulcus is necessary to preserve oral health since patients with altered neutrophil production and distribution develop severe periodontitis at early ages [33]. Chédiak‐Higashi syndrome, Papillon‐Lefèvre syndrome, neutropenias, and leukocyte adhesion deficiency (LAD) are some examples of neutrophil diseases. In Papillon‐Lefèvre syndrome neutrophils have defective chemotaxis, as a consequence, they are not efficiently recruited to the sites of inflammation and infection [34]. Neutropenia, a persistent reduction of neutrophil numbers in circulation, is frequently associated with susceptibility to infections. In many neutropenic conditions, severe periodontal disease is recurrently seen since primary dentition eruption [35]. CXCR2‐deficient mice cannot recruit neutrophils to oral tissues. These mice also experience periodontitis and periodontal bone loss early in life [36]. Leukocyte adhesion deficiency is a group of inherited disorders, in which neutrophils fail to transmi‐ grate from the circulation to the site of inflammation or infection. Neutrophils of patients with leukocyte adhesion deficiency have defective expression and function of adhesion mol‐ ecules like integrins. Therefore, neutrophils cannot adhere firmly to the vascular endothelium to transmigrate. Even though the presence of neutrophils is necessary to control infections, plenty of neutrophils on a site of infection is not always protective. In fact, neutrophil num‐ bers in inflamed periodontal tissues correlate with the severity of the lesions [37], and tissue destruction seems to be a collateral damage of hyperactive neutrophils [38].

#### **6.2. Periodontitis**

Periodontal diseases cause the destruction of the tooth supporting tissues, gingiva, periodontal ligament, cement, and alveolar bone and may eventually lead to tooth loss. Severe periodontitis affects approximately 10% of the global population [39]. Periodontal disease is the consequence of a shift in oral microbiota population from a symbiotic to a dysbiotic microbial community in the mouth. Periodontal disease begins when some factors that promote the growth of selected symbiotic bacteria, induce host inflammatory pathways [40, 41]. Periodontitis not only severely deteriorates people's quality of life by impairing the dentition but also adversely affect systemic health. A clear correlation between periodontal disease and atherosclerosis has been estab‐ lished in clinical observations and in animal models. In particular, polymicrobial infection with *Treponema denticola*, *Porphyromonas gingivalis*, *Tannerella forsythia*, and *Fusobacterium nucleatum* has been shown to promote progression of atherosclerosis [42]. Another correlation between peri‐ odontitis and diabetes also has been well documented. Higher plaque levels and higher incidence of chronic gingivitis are both found in adults and in children with diabetes [43, 44]. Periodontal treatment showed a beneficial effect on metabolic control of type 2 diabetic patients.Other various systemic diseases such as diabetes, cardiac disease, low birth weight, renal diseases, metabolic syndrome, obesity, Parkinson's disease, and Alzheimer's disease have been also proposed to be linked with periodontal disease on the basis of systemic inflammation [40, 41, 45].

#### **6.3. Inflammation in periodontitis**

**6.1. Balanced inflammation**

76 Role of Neutrophils in Disease Pathogenesis

**6.2. Periodontitis**

Neutrophils are the main leukocytes recruited to the gingival sulcus. Neutrophils exit the gingival blood vessels and travel through the gingival junctional epithelium until they reach the sulcus [8]. In the sulcus, neutrophils create a barrier against the growing bacteria biofilm to prevent bacteria from invading the underlying tissues. Neutrophil migration from vessels toward the gingival sulcus requires CXCR2 binding to CXCL2. Migration is controlled by gradients of chemokines and adhesion molecules such as IL‐8, ICAM‐1, and E‐selectin [29]. Neutrophil presence in the sulcus is necessary to preserve oral health since patients with altered neutrophil production and distribution develop severe periodontitis at early ages [33]. Chédiak‐Higashi syndrome, Papillon‐Lefèvre syndrome, neutropenias, and leukocyte adhesion deficiency (LAD) are some examples of neutrophil diseases. In Papillon‐Lefèvre syndrome neutrophils have defective chemotaxis, as a consequence, they are not efficiently recruited to the sites of inflammation and infection [34]. Neutropenia, a persistent reduction of neutrophil numbers in circulation, is frequently associated with susceptibility to infections. In many neutropenic conditions, severe periodontal disease is recurrently seen since primary dentition eruption [35]. CXCR2‐deficient mice cannot recruit neutrophils to oral tissues. These mice also experience periodontitis and periodontal bone loss early in life [36]. Leukocyte adhesion deficiency is a group of inherited disorders, in which neutrophils fail to transmi‐ grate from the circulation to the site of inflammation or infection. Neutrophils of patients with leukocyte adhesion deficiency have defective expression and function of adhesion mol‐ ecules like integrins. Therefore, neutrophils cannot adhere firmly to the vascular endothelium to transmigrate. Even though the presence of neutrophils is necessary to control infections, plenty of neutrophils on a site of infection is not always protective. In fact, neutrophil num‐ bers in inflamed periodontal tissues correlate with the severity of the lesions [37], and tissue

destruction seems to be a collateral damage of hyperactive neutrophils [38].

Periodontal diseases cause the destruction of the tooth supporting tissues, gingiva, periodontal ligament, cement, and alveolar bone and may eventually lead to tooth loss. Severe periodontitis affects approximately 10% of the global population [39]. Periodontal disease is the consequence of a shift in oral microbiota population from a symbiotic to a dysbiotic microbial community in the mouth. Periodontal disease begins when some factors that promote the growth of selected symbiotic bacteria, induce host inflammatory pathways [40, 41]. Periodontitis not only severely deteriorates people's quality of life by impairing the dentition but also adversely affect systemic health. A clear correlation between periodontal disease and atherosclerosis has been estab‐ lished in clinical observations and in animal models. In particular, polymicrobial infection with *Treponema denticola*, *Porphyromonas gingivalis*, *Tannerella forsythia*, and *Fusobacterium nucleatum* has been shown to promote progression of atherosclerosis [42]. Another correlation between peri‐ odontitis and diabetes also has been well documented. Higher plaque levels and higher incidence of chronic gingivitis are both found in adults and in children with diabetes [43, 44]. Periodontal treatment showed a beneficial effect on metabolic control of type 2 diabetic patients.Other various systemic diseases such as diabetes, cardiac disease, low birth weight, renal diseases, metabolic Periodontitis is associated with a change in oral microbiota from symbiotic bacteria to dysbiotic anaerobic microorganisms, which have adapted to succeed in an inflammatory environment (**Figure 4**). Pathogenic bacteria, such as *Porphyromonas gingivalis*, induce changes in the normal microbiota of the gingival crevicular fluid, leading to increased biofilm deposition in the gin‐ gival sulcus. The gingival sulcus is the space between the tooth surface and the free gingiva. Pathogenic bacteria also induce moderate inflammation known as gingivitis (**Figure 4**). When this moderate inflammation is not well resolved, a chronic inflammatory state is established, which results in the formation of pathologically deepened gingival sulcus also called periodon‐ tal pockets, followed by extensive tissue destruction, including bone loss (**Figures 2** and **4**). These last events are induced by the accumulation of dysbiotic bacteria in the periodontal pock‐ ets and are thought to be the initial trigger for periodontitis [46]. Accumulation of dysbiotic bacteria biofilm leads to an increase in the inflammatory infiltrate, composed mainly by neu‐ trophils into oral tissues. There, neutrophils form a barrier that prevents bacteria from invading deeper tissues and are essential for maintaining healthy oral tissues. In the case of neutrophils deficiencies, severe periodontitis appears with a concomitant inflammation state. On the con‐ trary, excess numbers of neutrophils induces a chronic inflammatory state. Thus, inflammation is an important element in periodontitis that is deregulated when neutrophil homeostasis is altered. Periodontitis in the absence of neutrophils has traditionally been explained by the lack of neutrophil control on bacterial infections. Patients with leukocyte adhesion deficiency pres‐ ent frequent infections and develop early severe periodontitis. However, this type of periodon‐ titis does not usually respond to treatment with antibiotics or mechanical removal of bacteria biofilm, suggesting that other mechanisms are at work. Recently it was shown that the driving force for this type of periodontitis involves the production of IL‐23 and IL‐17. In leukocyte adhesion deficiency type 1 patients, T cells were identified as the main producers of IL‐17 [47]. IL‐17 not only stimulates fibroblasts to produce G‐CSF but also promotes inflammation and stimulates osteoclasts, leading to bone loss. These findings are in agreement with the neutrostat mechanism discussed above. When apoptotic neutrophils are phagocytosed by macrophages, anti‐inflammatory signals are produced that lead to less IL‐23 production, which is a strong inducer for IL‐17 production. IL‐17 in turn induces G‐CSF production (**Figure 1**).

Neutrophils can be found in large numbers in inflamed periodontal tissues, and their pres‐ ence correlates with the severity of the periodontal destruction. Therefore, this destruction seems to be collateral damage of hyperactive neutrophils [48, 49]. Neutrophil recruitment is at least in part regulated by Del‐1 and LFA‐1 interactions. Del‐1 blocks LFA‐1 binding to its ligand ICAM‐1 and prevents neutrophil transmigration [50]. Neutrophil recruitment is also triggered with elevated IL‐17 levels, which resulted to be responsible for the tissue dam‐ age, because antibodies against IL‐17 prevented inflammation and bone loss. High levels of IL‐17 could be responsible for the bone loss in chronic periodontitis, by stimulating osteoblast expression of RANKL, an important osteoclastogenesis factor.

#### **6.4. Dysbiotic bacteria**

Diverse environments present in the oral cavity allow symbiotic and dysbiotic microbiota to find the best niche that fits their growth requirements, resulting in the formation of unique microbial biofilm communities. Periodontal disease microbiota includes a large number of microorgan‐ isms including *P. gingivalis*, *Tannerella forsythia*, and *Treponema denticola* [51]. Fortunately, nucleic acid screening and 16S pyrosequencing techniques have made more efficient finding changes in microbiota of healthy and of periodontal disease patients [52]. Screenings have been made at nine different oral sites including the oral epithelium, the maxillary anterior vestibule, the dorsum and lateral tongue surface, the hard and soft palate, the tonsils, the tooth surfaces, and the subgingival plaque [53, 54]. There are between 100 and 300 bacterial species in a single indi‐ vidual. Our general idea is that infectious diseases are caused by the action of a single foreign pathogen. However, periodontitis is originated by the complex association and interaction of a diverse polymicrobial community [37, 51, 55]. Data obtained from oral biofilm studies using checkerboard DNA‐DNA techniques link the different stages of the disease to a specific bacterial group or complex with the presence of the triad of bacteria composed by *P. gingivalis*, *T. forsythia*, and *T. denticola*, which are strongly associated with increased severity of periodontitis [56]. Other microorganisms have also been identified such as *F. alocis*, a Gram‐positive bacterium, which is present in periodontal disease sites, while *Veillonella sp*, a Gram‐negative uncultivated bac‐ terium, is associated with healthy periodontal sites. This data indicates that the general idea of Gram‐negative anaerobic bacteria being the pathogen population is not completely correct.

In a healthy gingival tissue, the local symbiotic microbiota is less diverse and rich, with neutro‐ phil recruitment to clear the infection and resolving the inflammation with no collateral dam‐ age to the host (**Figures 4** and **5**). The progression from health to periodontitis is now explained as the transition from a symbiotic microbiota to a polymicrobial dysbiotic microbiota. Several risks factors, such as smoking, tissue injury, diet changes, an immunocompromised host, or the colonization of the oral cavity by pathogenic bacteria can modify the oral ecosystem resulting in a dysbiotic polymicrobial community. In consequence, the host's response toward the poly‐ microbial dysbiotic microbiota challenge is more robust and not regulated, transitioning from a controlled/stable immune response into a nonresolving chronic inflammatory response [57].

Polymicrobial dysbiotic microbiota has an arsenal of self‐defense mechanisms, which can be directed to attack against neutrophils or camouflage the biofilm (**Figure 5**) [58]. Microbiota has an intermicrobial communication called quorum sensing, that enables the dysbiotic microbiota to optimize the biofilm conditions and ensure nutrient supply. Among the defense mechanisms, the production of bacterial surfactants by *P. aeruginosa* biofilms causes rapid cell death in neutrophils [59]. Additionally, quorum sensing molecules control neutrophil ROS response and penetration into *P. aeruginosa* biofilms [60]. Similarly, *Aggregatibacter actinomycetemcomitans* and *S. aureus* produce bacterial toxins that induce neutrophils lysis and degran‐ ulation [61–63]. In addition to directly attacking neutrophils, dysbiotic microbiota in biofilms can render themselves resistant to neutrophil‐mediated killing by disguising their immuno‐ genicity. NET formation within *Haemophilus influenzae* biofilms does not harm the biofilm. This is presumably due to their expression of certain lipooligosaccharide glycoforms, which shield pathogen‐associated molecular patterns (PAMPs) and thus inhibit recognition and

**6.4. Dysbiotic bacteria**

78 Role of Neutrophils in Disease Pathogenesis

Diverse environments present in the oral cavity allow symbiotic and dysbiotic microbiota to find the best niche that fits their growth requirements, resulting in the formation of unique microbial biofilm communities. Periodontal disease microbiota includes a large number of microorgan‐ isms including *P. gingivalis*, *Tannerella forsythia*, and *Treponema denticola* [51]. Fortunately, nucleic acid screening and 16S pyrosequencing techniques have made more efficient finding changes in microbiota of healthy and of periodontal disease patients [52]. Screenings have been made at nine different oral sites including the oral epithelium, the maxillary anterior vestibule, the dorsum and lateral tongue surface, the hard and soft palate, the tonsils, the tooth surfaces, and the subgingival plaque [53, 54]. There are between 100 and 300 bacterial species in a single indi‐ vidual. Our general idea is that infectious diseases are caused by the action of a single foreign pathogen. However, periodontitis is originated by the complex association and interaction of a diverse polymicrobial community [37, 51, 55]. Data obtained from oral biofilm studies using checkerboard DNA‐DNA techniques link the different stages of the disease to a specific bacterial group or complex with the presence of the triad of bacteria composed by *P. gingivalis*, *T. forsythia*, and *T. denticola*, which are strongly associated with increased severity of periodontitis [56]. Other microorganisms have also been identified such as *F. alocis*, a Gram‐positive bacterium, which is present in periodontal disease sites, while *Veillonella sp*, a Gram‐negative uncultivated bac‐ terium, is associated with healthy periodontal sites. This data indicates that the general idea of Gram‐negative anaerobic bacteria being the pathogen population is not completely correct.

In a healthy gingival tissue, the local symbiotic microbiota is less diverse and rich, with neutro‐ phil recruitment to clear the infection and resolving the inflammation with no collateral dam‐ age to the host (**Figures 4** and **5**). The progression from health to periodontitis is now explained as the transition from a symbiotic microbiota to a polymicrobial dysbiotic microbiota. Several risks factors, such as smoking, tissue injury, diet changes, an immunocompromised host, or the colonization of the oral cavity by pathogenic bacteria can modify the oral ecosystem resulting in a dysbiotic polymicrobial community. In consequence, the host's response toward the poly‐ microbial dysbiotic microbiota challenge is more robust and not regulated, transitioning from a controlled/stable immune response into a nonresolving chronic inflammatory response [57].

Polymicrobial dysbiotic microbiota has an arsenal of self‐defense mechanisms, which can be directed to attack against neutrophils or camouflage the biofilm (**Figure 5**) [58]. Microbiota has an intermicrobial communication called quorum sensing, that enables the dysbiotic microbiota to optimize the biofilm conditions and ensure nutrient supply. Among the defense mechanisms, the production of bacterial surfactants by *P. aeruginosa* biofilms causes rapid cell death in neutrophils [59]. Additionally, quorum sensing molecules control neutrophil ROS response and penetration into *P. aeruginosa* biofilms [60]. Similarly, *Aggregatibacter actinomycetemcomitans* and *S. aureus* produce bacterial toxins that induce neutrophils lysis and degran‐ ulation [61–63]. In addition to directly attacking neutrophils, dysbiotic microbiota in biofilms can render themselves resistant to neutrophil‐mediated killing by disguising their immuno‐ genicity. NET formation within *Haemophilus influenzae* biofilms does not harm the biofilm. This is presumably due to their expression of certain lipooligosaccharide glycoforms, which shield pathogen‐associated molecular patterns (PAMPs) and thus inhibit recognition and

**Figure 5.** Neutrophil response in periodontal health and disease. Health: Symbiotic bacteria community adheres to molecules of the salivary pellicle that are bound to the tooth surface. Few neutrophils patrol the gingival sulcus, and as the bacterial burden increases, neutrophils regularly exit the blood stream entering the connective tissue layer beneath the junctional epithelium and the tooth and kill some of the associated microbes (thin arrows). Neutrophils maintain bacterial concentration so there is no inflammation or tissue damage (arrow heads). Disease: Following the presence of a risk factor (smoking, poor diet, injury, etc.; thick arrow) dysbiotic bacteria (big oval) can colonize the symbiotic microbial community. Following colonization, the sulcus is invaded by dysbiotic bacteria which shut down the IL‐8 production. Neutrophils enter the connective tissue, but do not get to the sulcus. This causes many neutrophils to accumulate in the connective tissue. As some neutrophils transmigrate to the dysbiotic biofilm increase inflammation is conducted by neutrophil degranulation, reactive oxygen species (ROS) production, and NETosis that damages the host tissue.

opsonization. This molecule can provide protection against antimicrobial peptides [64]. One important microbial defense mechanism is the evasion of phagocytosis. Prevotella strains were recognized by neutrophils but not phagocytosed, depending on whether they produced mannose‐rich exopolysaccharides as part of their extracellular matrix [65]. *S. aureus* is able to survive after being phagocytosed by neutrophils [66]. *S. aureus* is known to be potent triggers for NETosis and degranulation. Therefore, it can be assumed that implementation of such survival strategies coexists with the elimination of bacteria by neutrophils. Finally, inflamma‐ tion and tissue destruction mediated by neutrophils evoke frequent gingival bleeding, which these bacteria may use as an additional source of nutrients, such as iron and vitamin K.

### **7.** *Porphyromonas gingivalis*

*Porphyromonas gingivalis* are anaerobic, Gram‐negative, nonmotile, asaccharolytic rods that usually exhibit coccal or short rod morphologies. It is part of the black‐pigmented Bacteriodes group [32]. *P. gingivalis*, even in low colonization levels, can induce the shift from symbiotic microbiota to dysbiotic microbiota followed by inflammatory bone loss. This bacteria uses different mechanisms to destabilize neutrophil homeostasis, inhibition of phagocytic kill‐ ing, resistance to granule‐derived antimicrobial agents and to the oxidative burst, impaired recruitment and chemotaxis, promote inflammatory response, and delay of neutrophil apop‐ tosis. *P. gingivalis* has a number of virulence factors related to the subversion of the innate immune system. This ability is what often characterizes a successful pathogen, as it tends to disable the overall host response while simultaneously enhancing the pathogenicity of a poly‐ microbial community. *P. gingivalis* are resistant to oxidative killing [67] and recruit hyperac‐ tive neutrophils with an enhanced response, which is characterized by the release of reactive oxygen intermediates, several cationic peptides, and enzymes such as matrix metalloprotein‐ ases (MMPs). All this responses increased tissue damage [48]. *P. gingivalis* also can manipulate both complement and TLR signaling to induce bacterial persistence.

*Porphyromonas gingivalis* gingipains are able to trigger the expression of proinflammatory sur‐ face receptor TREM‐1 on neutrophils, and several periodontopathogenic species can induce IL‐8 gene expression in gingival epithelial cells and fibroblasts [68, 69].

## **8.** *Treponema denticola*

*Treponema denticola* is an anaerobic, Gram‐negative, motile, spirochete that can be poorly detected in the gingival plaque of healthy individuals. However, it is present in very high numbers in the subgingival periodontal pocket and is associated with the dysbiotic microbiota biofilm for‐ mation in periodontal lesions. *T. denticola* limits neutrophil chemotaxis, and inhibits junctional epithelial cells to secrete IL‐8. Additionally, this pathogen is able to degrade IL‐8 that is already present at the infection site, which disables the neutrophil chemotactic gradient. *T. denticola* major outer sheath protein (Msp) is one of its most important virulence factors in contribut‐ ing to the disease progression. This membrane protein modulates neutrophil signaling path‐ ways involved in cytoskeletal dynamics that are relevant in chemotaxis and phagocytosis [70]. Msp controls neutrophil cytoskeletal functions like migration, adhesion, and cell shape. It also causes extracellular matrix degradation by stimulating the release of activated MMPs from neutrophils.
