Colonizing in the intestinal tract of humans, the gut microbiota is a vast and complex microbial community. Gut microbiota makes a crucial contribution to metabolism in the human body by producing enzymes that the human genome cannot encode, such as the synthesis of dietary carbohydrates, bile acids (BAs), and vitamins. Besides, in healthy individuals, gut microbiota may protect the permeability of the gastrointestinal mucosa, thereby supporting the function of the intestinal barrier to prevent LPS from entering the bloodstream from gut microbes (10). The gut microbiota present in the human body is mainly comprised of six phyla including Firmicutes, Bacteroidetes, Proteobacteria, Actinobacteria, Verrucomicrobia, and Fusobacteria, and the predominant types are Bacteroidetes and Firmicutes (18). A recent metagenome study divided gut microbiome into three clusters: Bacteroides >30% to enterotype I, Prevotella >15% to enterotype II, and the rest to enterotype “others” (III). This research discovered that the order Lactobacillales is remarkably enhanced while the phylum Bacteroidetes (Bacteroides + Prevotella) is declined among patients with coronary heart disease (17), suggesting a connection between gut microbiota and AS.

The microflora in the oral cavity mainly includes Firmicutes, Bacteroidetes, Proteobacteria, Fusobacteria, and Actinobacteria (10, 18). Oral microbiota has an impact on activating immune responses. For example, periodontitis-associated microbiota such as Porphyromonas gingivalis (P. gingivalis), a main pathogen causing periodontosis by initiating the immune response of the host (18). Furthermore, oral microbiota may participate in the progress of AS (24). A study examined the diversity of microbiota present in atherosclerotic plaque by utilizing 16S rRNA gene pyrosequencing. The results indicated that the combined abundances of Streptococcus and Veillonella in atherosclerotic plaque were closely linked to those in the oral cavity. Besides, the study found that some specific bacterial phylotypes were observed in both the atherosclerotic plaque and gut or oral samples obtained from the same individual (1). Numerous epidemiological researches have demonstrated that periodontal disease may elevate the risk of AS (25–27). Additionally, several studies have established a relationship between oral microbiota and the onset of AS (1, 10, 28).

The impairment of endothelial function is an early sign of AS, and it can be initiated by various factors (45, 46), including the invasion of periodontal pathogens (24). At the DNA, RNA, or antigen level, various types of oral bacteria have been found in atheroma tissues (24). Live P. gingivalis and A. actinomycetemcomitans were detected in atheromatous tissue by Kozarov et al. (47). The genomic DNA of A. actinomycetemcomitans, P. gingivalis, and other microorganisms were first detected by using bacterial 16S rRNA-specific PCR analysis (48). Oral microbiota affects the occurrence and progression of AS by directly invading vascular endothelial cells and inducing endothelial dysfunction (ED) (24). Extensive research has been conducted on the mutual effect between P. gingivalis and cardiovascular cells (24). Adherence to and invasion of host cells by P. gingivalis is mediated by adhesins, including the major fimbriae (FimA) gingipains, and hemagglutinins (49, 50). In addition to adhesins, the adhesive and invasive capacity of P. gingivalis into endothelium is also facilitated by other microorganisms in the oral cavity, a case in point is Fusobacterium nucleatum (51). After P. gingivalis invades human aortic endothelial cells, it affects the occurrence and development of AS through several mechanisms. For example, inducing the pro-coagulant effects in endotheliocytes through enhancing AS-related plasminogen-activator inhibitor-1(PAI-1) levels and biological activity (29), eliciting a platelet aggregation response (52). Besides, altering the genes responsible for mitochondrial function in endothelial cells may lead to mitochondrial dysfunction (30, 31). Inhibiting the down-regulation of genes in the PPAR-signaling pathway may promote the development of AS by preventing plaque regression (32). Additionally, interference with reverse cholesterol transport by P. gingivalis may also impede the regression of atherosclerotic plaque (9).

As a crucial virulence factor of gram-negative microbiota, LPS, commonly known as endotoxin, are responsible for systemic endotoxemia when it translocating to the bloodstream (53). The bioactive lipid A of LPS, either as individual molecules or in aggregates, is identified by the LPS receptor complex, which is composed of toll-like receptor 4 (TLR4), cluster of differentiation 14 (CD14), and myeloid differentiation 2 (MD-2). This complex is expressed in a variety of cells, such as monocytes, macrophages, neutrophils, epithelial cells, and fibroblasts (10). Lipid A of P. gingivalis mainly binds to TLR4 and is either inert or antagonistic in response to TLR4 activation (33). This activation mediates systemic inflammatory response, affects the vessel walls and atheromatous lesions (54). Endotheliocytes in the vasculature are prone to AS and can be activated by LPS originating from periodontal pathogens, resulting in the adhesion of monocytes. What's more, LPS is one of the main types of pathogen-associated molecular patterns (PAMPs) that are known to elicit severe immune reactions in the event of a pathogen trespassing the epithelial barrier and reaching the bloodstream (34). The relevant immune response is related to the development of AS. Additionally, LPS can stimulate host cells to form cytokines such as TNF-α, IL-1, IL-6, and IL-12, which in turn increase leukocyte adhesion to the endothelium and vascular permeability (35, 36). As a result of vascular inflammation, coagulation may be activated, which can potentially lead to thrombosis (55). What's more, LPS may promote foam cell formation and LDL accumulation (56). Besides, LPS also stimulates the MMP's production in endothelial cells, macrophages, and mast cells within plaques, giving rise to the rupture of fibrous caps in plaques (57–59).

Owing to the abundance of gram-negative bacteria in the subgingival microbiota, individuals suffering from periodontitis experience endotoxemia (60). Endotoxemia can contribute to the formation of AS by elevating LDL concentrations and reducing high-density lipoprotein (HDL) concentrations. In a meta-analysis, individuals with periodontitis have been implicated in having significantly elevated LDL and reduced HDL in comparison with the normal group (7). Endotoxemia correlates with a predominance of the LDL phenotype, which is highly proatherogenic as it facilitates their accumulation within the macrophages and promotes the form of subendothelial foam cells—a signal of early AS (54, 61, 62). Small, dense LDL fragments decrease their affinity to the LDL receptor, allowing them to easily penetrate the arterial wall, and undergo oxidative modifications (ox-LDL), issuing in rapid absorption and cholesterol accumulation in the macrophagocytes (37). Furthermore, oligopeptide fragments of the LDL protein, apoB100, can function as autoantigens, triggering T-cell activation and adaptive immune responses that promote inflammation and macrophage activation (54). HDL is regarded as anti-atherosclerotic owing to characteristics beyond its role in reverse cholesterol transport, other protective effects include stimulating endothelial nitric oxide synthase (eNOS) generation and exhibiting anti-inflammatory, antithrombotic, and anti-apoptotic properties, which may directly safeguard the endothelial cells (38).

Endothelial cell dysfunction may be indirectly induced in patients with periodontal infection through a systemic inflammatory state. Levels of inflammatory mediators and acute phase proteins including TNF-α, IL-1, IL-6, IL-8, IFN, and C-reactive protein (CRP) are increased following blood exposure to bacterial antigens (28). CRP can directly impact vascular vulnerability via multiple mechanisms, such as modulating the localized expression of adhesion molecules, reducing endothelial nitric oxide (NO) bioactivity, and modifying LDL uptake by macrophages (18). Ishikawa et al. proposed that CRP within atherosclerotic plaques directly contributes to the inflammatory process of AS (63). Once in circulation, these mediators can participate in the immune alteration of endotheliocytes from an antithrombotic to a prothrombotic state, which contributes to formulating atherosclerotic lesions (64, 65). What's more, researchers have revealed that respondents with periodontitis have raised adhesion molecules including macrophage chemoattractant peptide-1(MCP-1), macrophage colony stimulating factor (MCSF), E-selectin, ICAM-1, and VCAM-1 in comparison with the normal group (39, 40). These inflammatory cytokines have been linked to ED and the progression of atheromatous plaques in several observational studies (44). Among them, MCP-1 is responsible for recruiting monocytes, MCSF converts monocytes into macrophages, and macrophages eventually transform into foam cells, thereby inducing ED and triggering AS (66). E-selectin acts as a receptor for carbohydrate ligands on leukocyte surfaces. Its role is to attract circulating leukocytes and bind them to locate at the surface of endothelial cells. Following this, the transmembrane glycoprotein ICAM-1 engages with integrins on leukocyte surfaces, facilitating robust adhesion to the endothelium. Consequently, this process contributes to AS via the inflammatory effects on the vascular endothelium (44).

P. gingivalis can exist in immune cells, including dendritic cells and macrophages, escape the immune reaction, and enable the bacteria to spread, thus bacteremia and endotoxemia may occur (67). The detection of bacteremia and endotoxemia is primarily carried out by host cells through the signaling of microorganism-related molecular patterns-pattern recognition receptors (MAMP-PRR). These include the peptidoglycan receptor nucleotide-binding oligomerization domain-containing protein 1 (NOD1), diacylated and triacylated lipopeptides receptors TLR2 and TLR6, and CpG oligodeoxynucleotides receptor TLR9. Direct engagement of PRR by MAMP contributes to the development of AS (41).

A report has revealed that patients with infectious agents possess advanced plasma fibrinogen, which can stimulate the increase of protein that is inflamed and elicit blood clotting (68). Furthermore, elevated PAI-1 was found in samples with severe periodontitis (69). PAI-1, a protease inhibitor, reduces fibrinolysis by inhibiting tPA and uPA, indirectly promoting blood clotting (70). Other platelet activation markers, such as CD18, sP-selectin, P-selectin, and activated glycoprotein IIb/IIIa, were found to have a connection with periodontitis and AS. Research has demonstrated that platelets contribute to atheroma formation and thrombosis due to their aggregation and pro inflammatory mediator release after activation (42, 43).

Oral bacteria induce the production of matrix metalloproteinases (MMP) and myeloperoxidase (MPO) (28, 44). MMP can trigger atherosclerotic plaque rupture by degrading collagen in fibrous caps (71). The enzyme MPO, whose substrate can be inflamed and oxidized, has been linked to atheroma plaque development and suppresses the expression of endothelial effect in numerous observational studies (44).

Gut microbiota is capable of adjusting the BAs synthesis, the final metabolite of cholesterol, which may induce AS (1). Cholesterol is synthesized into cholic acid and chenodeoxycholic acid in the liver. Subsequently, in the distal ileum, the process of converting cholic acid into deoxycholic acid and chenodeoxycholic acid into lithocholic acid is facilitated by the gut microbiota. BAs are generated by the liver and reserved in the gallbladder. During eating, they are delivered to facilitate the intestinal absorption of various compounds, including triglycerides, cholesterol, and fat-soluble vitamins. About 95% of BAs are reabsorbed by the intestinal wall and transported back to the liver, forming a process known as hepato-enteric circulation. BAs can play a role in lipid homeostasis, immunity, and heart function. During this process, gut microbiota can activate signaling pathways that regulate the metabolic levels of total BA, deoxycholic acid, and lithocholic acid, which may influence the development of AS (76).

Studies have shown that LPS can trigger inflammatory responses by activating the TLRs pathway, inducing vascular oxidative stress, ED, and vascular inflammation (77, 82). These effects of LPS can accelerate the formation of plaques and thrombosis. Besides, the lipid A structure of LPS vary from diverse types of microbiomes, resulting in different biological activities (18, 78).

Gut microbiota can engage with the immune system by inducing immune tolerance and reducing inflammation (77, 79). The innate immune system is responsible for detecting and eliminating pathogenic microbes to reduce inflammation, which is accomplished through the secretion of various factors such as secretory immunoglobulin A (SIgA), TLR5, and inflammasome (18, 83). In adaptive immunity, gut microbiota can participate in mediating neutrophil migration and affecting the T cells' maturation cycle. However, the activation of the immune system induced by pathogenic microorganisms developed in a diet high in salt or fat with inadequate fiber may increase the risk of AS. This effect occurs by stimulating the generation of Th17 cells, which then migrate to damage vascular function and water-salt balance (84).

Oral microbiota may cause variations to the gut microbiota, contributing to a concurrent increase in proinflammatory cytokines and gut permeability that are followed by systemic disease progression (10, 85, 86). The component of the gut microbiota was changed after oral gavage of P. gingivalis in vivo. Specifically, in the ileal contents, there was an elevation in the percentage of Bacteroides and a decrease in Firmicutes (87). This observation suggests that the invasion of oral microbiota can potentially impact gut microbiota by promoting the proliferation of oral pathobionts including Klebsiella/Enterobacter species (88). Amassed oral pathobionts migrate to the lower gastrointestinal tract and are ectopically located in the intestine through the enteral route or hematogenous route (88, 89), where they activate the inflammasome-mediated IL-1 signaling in colonic mononuclear phagocytes, causing inflammation (88). Oral pathobiont K. aerogenes likely induces IL-1b production via caspase-11-mediated, non-canonical inflammasome activation, therefore exacerbate colitis (88, 90).

The presence of periodontitis can increase the production of Th17 cells in the mouth. These cells have an affinity for the gut and migrate there in the presence of inflammation, which establishes a connection between oral and gut health (88). Once in the intestine, these Th17 cells are stimulated by periodontal pathogens that have translocated, leading to the occurrence of colitis. What is noteworthy is that these cells are not activated by commensal gut microbes (88, 91). Periodontitis links oral and intestinal health through the introduction of pathogenic T cells and colitogenic pathobionts that aggravate intestinal inflammation, creating a dual mechanism involving both microbial and immune systems (88).

However, oral pathobionts do not colonize the gastrointestinal tract of healthy individuals. In order for oral pathobionts to successfully colonize the gut, two conditions need to be met: disruption of colonization resistance of gut-resident microbiota and oral inflammation. The first condition involves the disruption of colonization resistance. This resistance is normally provided by the resident microbiota in the gut, which prevents the invasion of foreign microbes. In the presence of gut inflammation, this resistance is perturbed, allowing ingested oral pathobionts to outcompete and displace the resident bacteria. The inflammatory milieu in the gut also provides a favorable environment for the growth of Enterobacteriaceae, including bacteria translocated from the oral mucosa. However, it is important to note that the loss of colonization resistance or gut inflammation alone is not sufficient to facilitate ectopic gut colonization by oral pathobionts (90, 92, 93). The second condition that must be met is oral inflammation. There appears to be a threshold for the number of ingested oral pathobionts that must be reached for them to successfully transition from the oral cavity to the gut. Periodontal inflammation plays a crucial role in raising the abundance of oral pathobionts and increasing the likelihood of their successful passage through the acidic environment encountered in the stomach. In summary, both gut inflammation and oral inflammation are necessary for oral pathobionts to colonize the gut. Gut inflammation disrupts colonization resistance, allowing the oral microbes to invade the gut, while oral inflammation increases the abundance of oral pathobionts and facilitates their transition from the oral cavity to the gut (94, 95).

Dietary interventions are an effective way to control the microbiota in our bodies. The food we eat is also the food of bacteria, so we can choose the types of bacteria by changing our diet (105). An experimental study demonstrated that dietary fats and oral infection may significantly contribute to the dysbiosis of gut microbiota related to AS by reduced community complexity and excessive growth of specific symbiotic microbiota. And after oral infection with P. gingivalis, the diet-induced lipid-lowering effect to control plaque accumulation was attenuated (112). Furthermore, high-salt diets could enhance local and systemic inflammation by elevating pro-inflammatory cytokines and altering intestinal permeability, some of which may come from specific microbiota (75). Some studies have shown that Mediterranean diet affects the microbiota and the relevant metabolism (106). This dietary pattern was related to a lower proportion of Bacteroidetes, Firmicutes, Streptococcus and higher Bifidobacterium, Catenibacterium and fecal SCFAs (75). The high-fiber diet is beneficial for cardiovascular health (107).

Probiotics have great potential for treating various diseases by promoting nutrient absorption and regulating the balance of gut microbiota. Studies have found that the Bifidobacterium can inhibit the development of AS to a certain extent, so it is possible to treat AS by using probiotics to maintain the balance of gut microbiota (108). Prebiotics are a kind of organic matter that can provide nutrients to probiotics, promote their reproduction, and regulate the composition and function of microbiota. At the same time, prebiotics improves glucose tolerance and blood lipid (109).

Many experiments have demonstrated that the TMAO pathway is associated with AS, so microbial enzymes that produce TMA during metabolism may be a potential target (73). By developing enzyme inhibitors that restrain the activity of related enzymes, the levels of TMA and TMAO in the body can be reduced, thus reducing the risk of AS (110).

Studies have indicated that oral administration of anti-CD3 antibody therapy can induce Th3 and Foxp3 + Treg, inhibiting the formation of AS through a TGF-β-dependent mechanism (111). Using the gut microbiota as a therapeutic target to prevent AS, regulating the Treg and DCs in intestinal immunity may serve as a new strategy to prevent AS (8, 72). Another study reported a significant negative correlation between the abundance of the Bacteroides genus and fecal LPS levels. And there might be a novel therapeutic strategy for preventing AS through increasing Bacteroides abundance to reduce fecal LPS levels and suppress immune responses (78).