Studies have identified several species of oral bacteria, mainly gram-negative anaerobes, which are responsible for periodontitis (
Fig. 1). These oral bacteria can be classified into five major complexes, namely the green, purple, yellow, orange, and red complexes[
64–
66]. The “red complex” and “orange complex” are commonly associated with periodontitis. The “red complex” species, consisting of
P. gingivalis,
Treponema denticola, and
Tannerella forsythia, exhibit very strong associations with periodontitis, indicating that these pathogenic bacteria might be the primary pathogens in periodontitis. Many studies have been conducted to investigate the pathogenic mechanisms and characterize the virulence factors of the three pathogens. A “keystone-pathogen hypothesis” has been proposed in which
P. gingivalis in the periodontium drives the progression of periodontitis[
67].
P. gingivalis-derived gingipains, phosphoserine phosphatase SerB and lipopolysaccharide (LPS) are potent virulence factors, which can impair the innate host defense systems and induce a destructive response[
68]. Gingipains are arginine-specific cysteine proteinases that can cleave complement component C5 into active fragments C5a locally and activate C5a receptors (C5aR) on leukocytes; C5aR signaling then triggers inflammation and subverts leukocyte killing capacity[
69]. Phosphoserine phosphatase SerB secreted by
P. gingivalis contributes to the inhibition of IL-8, thus delaying the recruitment of neutrophils and leading to overgrowth of oral bacterial species[
70]. LPS of
P. gingivalis exhibits specific lipid A structures, which acts as a Toll-like receptor 4 (TLR4) antagonist to inhibit the activation of TLR4-dependent antimicrobial pathways in leukocytes[
71]. The subversion of host defense caused by these virulence factors collectively lead to dysbiotic microbiota, inflammation and bone resorption, resulting in a self-feeding ‘vicious cycle’[
67]. Although the “red complex” species exert keystone effects in disruption of periodontal tissue homeostasis, the putative oral bacteria in the “orange complex” that consists of
F. nucleatum,
Prevotella intermedia,
Prevotella nigrescens, and
Peptostreptococcus micros, have moderately pathogenic effects on the development of periodontitis. For example,
F. nucleatum can influence the secretion of inflammatory factors, including TNF-α, IL-6 and IL-8, from immune cells or the oral epithelium[
72,
73], and therefore modulates the host inflammatory response.
F. nucleatumcan also interacts directly with
P. gingivalis and increases the invasion ability of
P. gingivalis[
74]. Furthermore, the virulence properties of
P. gingivalis can be enhanced by another “orange complex” organism:
Peptostreptococcus micros through promoting the activity of
P. gingivalis gingpains[
75]. In addition to the above bacteria in the “red complex” and “orange complex”,
Aggregatibacter actinomycetemcomitans (
A. actinomycetemcomitans) has been shown to cause periodontitis as well[
76]. The virulence factors of
A. actinomycetemcomitans such as leukotoxin, cytolethal distending toxin, and LPS, can cause tissue destruction and bone resorption by promoting inflammation and interacting with the immune system (
Fig. 1).