It has previously been demonstrated that individuals with CVD have a differential microbiome composition compared to healthy individuals (22–24). Specifically for atherosclerosis, it was shown that microbial diversity is different between patients with stable and unstable plaques, while dysbiosis also affects lipid metabolism and systemic lipid levels (25, 26). As such, it was found that lower amounts of beneficial bacteria including Bifidobacteria and less short-chain fatty acids (SCFA)-producing bacteria are linked to a higher risk of CVD (24, 27, 28). Further, studies have observed that atherosclerosis associated with less Bacteroides, such as Roseburia intestinalis and Prevotella copri, and an increase in Firmicutes, including Ruminococcus gnavus (29, 30). Additionally, the microbial species Streptococcus and Veillonella have been shown to be present in atherosclerotic plaques, indicating systemic bacterial translocation (31).

One important factor that might contribute to intestinal dysbiosis in CVD is caloric intake and diet composition (32, 33). For example, it was described that diets containing low-fibre are considered to worsen the risks for development of CVD, including through remodelling of the microbiome (34, 35). In contrast, high-fibre diets lowered blood pressure and CVD severity in humans (36). Additionally, heart failure patients showed altered microbial richness, with increased pathogenic bacterial growth - such as Salmonella, Campylobacter and Candida species –, lower SCFA-producing bacteria (e.g., Blautia and Ruminococcus) and reduced alpha- and beta-diversity (37, 38). These changes are all linked to the heightened release of pro-inflammatory signals which can be modulated by the presence of certain pathogenic microflora, or their metabolites. Indeed, several microbiome-dependent metabolites are directly linked to the development of CVD. The most important ones will be discussed in the next section (see Figure 1).

SCFA such as acetate, propionate, and butyrate are primary microbial metabolites produced from the fermentation of dietary fibres in the gut (24). Previous research described that SCFA supplements can reduce cardiovascular disease risk factors, including hypertension and atherosclerosis, by fostering beneficial gut bacteria and improving gut health (24, 39). Moreover, studies recognised that hypertensive individuals have aberrant acetyl-CoA production, which is important for synthesising SCFA (40). Specific gut bacteria such as Bacteroides acidifaciens have been linked to reduced blood pressure and improved heart function in animal models, where acetate and propionate supplements alleviate cardiac hypertrophy (41). Butyrate-producing bacteria like Roseburia intestinalis have also been shown to reduce atherosclerosis in mice by strengthening gut barrier function, which decreases translocation of inflammatory molecules into circulation (42, 43).

SCFA primarily act through receptors such as OLFR78, GPR41, and GPR43, which are involved in blood pressure regulation and vascular health (44–46). For example, propionate triggers a hypotensive effect by modulating Gpr41 and Olfr78 expression (47). Butyrate and acetate also enhance endothelial function by increasing nitric oxide bioavailability, contributing to improved vascular health (48). In human studies, high dietary fibre intake has been associated with lower blood pressure, and soluble fibres specifically have shown similar protective effects (49, 50). Mechanistically, SCFAs are also implicated in anti-inflammatory processes. Butyrate is thought to reduce inflammation by modulating gut barrier integrity and inhibiting histone deacetylases, leading to beneficial epigenetic changes in gene expression related to inflammation (51, 52). SCFA have been shown to reduce the production of pro-inflammatory cytokines such as IL-1β and TNF-α in animal models (53). Yet, the benefits of SCFA are complex and can vary. For instance, elevated circulating SCFA levels as a result of a diet high in both fibre and protein correlated with higher LDL cholesterol and blood pressure, and lower HDL cholesterol, potentially raising CVD risk (54). Therefore, while SCFA generally exhibit protective cardiovascular effects, some variations in their impact are noted depending on the fibre source, SCFA type, and the diet's overall composition. Hence, further research, especially in human clinical settings, is needed to better define how specific SCFA and fibre types influence cardiovascular risk, as well as to understand any potential pitfalls associated with high SCFA levels in certain diets.

Trimethylamine N-oxide (TMAO) is one of the most studied microbiota–host co-metabolite in relation to CVD (23). It is derived from phosphatidylcholine, L-carnitine and choline, and produced by microbial enzymes containing high amounts of trimethylamine (TMA) (24, 55). TMAO and its precursors, particularly L-carnitine, choline, and betaine, have been linked with an increased atherosclerotic burden, showing that higher TMAO levels correlate with increased risk for adverse cardiovascular outcomes (56, 57). Studies have consistently shown a dose-dependent increase in CVD risk with elevated levels of TMAO precursors across diverse populations and meta-analyses involving over 26,000 individuals (23, 58). Also, preclinical studies employing animal models like ApoE^−/− mice receiving microbiota transplants from TMAO-producing mice exhibit heightened atherosclerosis, an effect that decreases when plasma TMAO is reduced (59, 60). TMAO production also varies by diet; omnivores generally show greater TMAO synthesis from L-carnitine than vegetarians or vegans, emphasising the influence of dietary habits on gut microbial metabolism (56). Despite the robust evidence connecting TMAO with CVD risk, some inconsistencies remain. Notably, studies investigating the effect of TMAO-rich diets have reported neutral or even beneficial cardiovascular effects of TMAO and its precursors. Observational studies in asymptomatic individuals have found no association between TMAO levels and atherosclerosis progression, suggesting that TMAO may not be an early CVD predictor (24, 61).

Mechanistically, TMAO appears to promote atherosclerosis through several pathways, including effects on cholesterol transport, LDL modification, foam cell formation, and increased platelet reactivity, which enhance clot formation (23, 24, 62, 63). Additionally, TMAO has been linked to increased inflammation markers such as C-reactive protein, IL-1β, and activated monocytes, with higher TMAO levels corresponding to greater inflammatory activity (23, 64, 65). In human studies, TMAO has been associated with atherosclerotic plaque instability and rupture, indicating a role in local vascular remodelling (66). Furthermore, in Western diet-induced obesity models, TMAO has been tied to cardiac remodelling, including hypertrophy and fibrosis, which can impair heart function, and these effects have been diminished by antibiotic use to reduce TMAO levels (23, 63, 67). In Ldlr^−/− mice on a high-choline diet, Icam-1, Il-6 and Cox-2 expression was elevated in the aorta, all of which are promoting atherosclerosis through stimulation along the MAPK and NFκB pathways (68). Furthermore, specific inhibitors targeting TMA lyase, the enzyme responsible for TMAO production, have shown promise in reducing TMAO-related atherosclerosis risk in preclinical studies in mice (69, 70). This research exemplifies how microbial metabolism influences CVD risk and suggests that targeting TMAO pathways could be a potential strategy to mitigate cardiovascular risk in susceptible individuals.

The microbiome is also involved in the production and composition of bile acids (BAs), which are saturated or hydroxylated steroids that aid absorption of dietary fats, lipophilic vitamin uptake, and metabolic regulation of lipids, glucose, and energy (24, 57, 71). Initially, primary BAs are derived from cholesterol that is metabolized in the liver. Then, BAs are transported to the gallbladder, from where they enter the duodenum via the bile (57, 72). The gut microbiota deconjugates these primary BAs to produce secondary BAs. Secondary BAs are produced by several bacteria, such as Lactobacillus, Bacteroides (gram-negative), Enterococcus and Clostridium (24, 73). When BAs enter the bloodstream, their associated receptors can influence signalling pathways for metabolism, which has been shown to be involved in CVD risk (74). A higher ratio of secondary to primary BAs has been linked to hypercholesterolemia and cardiovascular diseases, where it may correlate with worse survival in heart failure patients (75). In human studies, circulating levels of BAs, especially lower primary and secondary BA concentrations, are associated with higher severity of coronary artery disease (CAD) (76).

The main pathway by which BAs can influence CVD is through their interactions with nuclear and membrane receptors such as the Farnesoid X-activated receptor (FXR) (24, 57). FXR activation has shown both protective and detrimental effects on atherosclerosis, depending on the experimental model, which suggests that FXR's effects may depend on complex factors such as receptor location and specific BA interactions. For instance, it was shown that FXR activation in certain atherosclerosis-prone mice reduces plaque formation, while in other murine models worsened disease was reported (77–79). Furthermore, FXR also plays a role in the TMAO pathway by regulating FMO3, an enzyme involved in lipid metabolism and inflammation (80). Another membrane receptor, TGR5, has been found to exert anti-inflammatory effects relevant to CVD (74). Activated by secondary BAs, TGR5 can inhibit NFκB signalling, which in turn reduces foam cell formation and inflammation in atherosclerotic lesions (81). In contrast, BAs-mediated signalling via the PXR appears to aggravate atherosclerosis by increasing levels of lipoproteins and upregulating CD36 expression in macrophages (82). Inhibition of PXR has been shown to reduce lipid uptake and alleviate plaque formation, underscoring its complex role in lipid metabolism and plaque development (83).

Taken together, these findings suggest that BAs and their receptors are integral to cardiometabolic regulation, connecting gut, liver, and cardiovascular health. Given these complexities, further investigation is needed to confirm the therapeutic viability of targeting BA and their receptors in human CVD, particularly considering the variable impacts of receptor activation across different tissues and metabolic contexts.

Besides BA formation from cholesterol in the liver, the gut microbiota directly uses cholesterol to form coprostanol, a non-absorbable sterol eliminated in faeces (84, 85). Currently, the main bacterial genera identified with this cholesterol-reducing ability are Eubacterium (e.g., E. coprostanoligenes) and Bacteroides (e.g., Bacteroides strain D8), though there are likely more undiscovered strains (84). Importantly, animal models support the cholesterol-lowering potential of coprostanol-producing bacteria (86, 87). For instance, hypercholesterolemic rabbits administered these bacteria showed a significant drop in plasma cholesterol that continued beyond 34 days after the last treatment (88). However, human studies on cholesterol conversion to coprostanol have encountered limitations due to small sample sizes, narrow demographic diversity, and unsuccessful strain isolation, which hinder broader conclusions and mechanistic studies (89). Additionally, the specific genes or enzymes involved in intestinal cholesterol conversion remain ill-identified, underscoring the need for studies to better understand microbial contributions to cholesterol metabolism and CVD prevention (89–91).

Succinate is a C4-dicarboxylic acid that is produced by human and gut microbial cells as an intermediate metabolite during the Krebs cycle and glutamine metabolism (92). Additionally, succinate can be produced via fermentation of oligosaccharide and polysaccharides, where it acts an intermediate product of propionate synthesis. The main bacterial strain producing succinate are Bacteroidetes (93).

Succinate has been linked to atherosclerosis, as it can function as an inflammatory signal ligand via its receptor SUCNR1, that becomes activated under certain cellular circumstances, such as tissue damage or hypoxia (94). The binding of succinate to its receptor leads to the expression of HIF-1α, IL-1β and other pro-inflammatory cytokines (95, 96). Moreover, succinate induces high levels of ROS in the mitochondria, which supports the conversion of pro-inflammatory macrophages (92, 95, 96). One study showed that the increase in serum IL-1β correlated with succinate in coronary heart disease (97). Additionally, extracellular succinate can interact with GPR91, which is expressed on naïve DCs and macrophages, where it could activate HIF-1α and in turn IL-1β (93, 98). Succinate is also implicated in the NFκB pathway, as it was shown that HUVECS increase NLRP3 and Caspase-1 after stimulation with succinate combined with LPS compared to LPS alone (97). In summary, elevated succinate might amplify the inflammatory response, thereby worsen atherosclerosis.

Imidazole propionate (ImP) is a microbial metabolite derived from the amino acid histidine, undergoing bacterial transformation from urocanate, a compound human cells produce from histidine. While humans do not convert urocanate further, gut bacteria in some individuals metabolize it into ImP (99). This ability to produce ImP has been linked to insulin resistance and T2DM, as studies show that individuals with elevated ImP also often have markers of poor glycaemic control and inflammation, independent of body mass index, chronic kidney disease, or insulin resistance (23, 100). Longitudinal research suggests that individuals who later develop T2DM frequently have elevated plasma urocanate levels prior to diagnosis, indicating ImP production as an early metabolic disruption that may be targetable for intervention (101). ImP production is also associated with a gut bacterial profile enriched in species linked to coronary artery disease, including Clostridium bolteae, Clostridium symbiosum, and Ruminococcus gnavus (23, 29). Dissimilar, the presence of Bacteroides and butyrate-producing bacteria is negatively associated with ImP levels, suggesting a protective effect of these bacteria against ImP production (23, 102).

Mechanistically, ImP has been shown to disrupt insulin signalling by activating the p62-mTORC1-S6K1 and AKT-AMPK pathways, processes involved in insulin resistance (23, 100, 103). Markedly, these mechanisms may extend to CVD risk, since p38γ/δ and mTORC1 signalling are also involved in CVD development (104). Experimental studies found that ImP may even interfere with the effects of metformin, a commonly prescribed diabetes medication, thus compounding metabolic dysregulation (103). In a recent study, ImP levels were notably elevated in individuals with CVD, independent of other traditional risk factors, suggesting a possible link between microbial histidine metabolism and cardiovascular health (105).

Tryptophan (Trp) is a crucial amino acid that is processed into several metabolites by microbial species in the gastrointestinal tract (106). Mostly, this is done by Clostridium sporogenes, Ruminococcus gnavus, Lactobacillus reuteri, and members of the Bacteroides, Bifidobacteria and Escherichia coli families (107). Many Trp metabolites can interact with aryl hydrocarbon receptor (AHR) and thus regulate epithelial cell function in the intestine (106, 108). This receptor is expressed on the cell surface of DCs, innate lymphoid cells, macrophages, neutrophils and Th17 cells, which also links Trp to several inflammation-driven diseases (107, 109). For example, in HIV patients, Trp has been implicated in promoting atherosclerotic lesions progression, where it was correlated with immune and T cell activation (110, 111). Additionally, Trp has been implicated in macrophage polarisation and effectiveness through AHR (106). In relation to this, a previous study examined that indole metabolites, and related Trp reactions, cause accumulation of IL-10 and downregulation of IFN-α, in macrophages stimulated by LPS (112).

In the context of atherosclerosis, a study demonstrated the athero-protective potential of Trp catabolism. The authors found that the absence of indoleamine 2,3-dioxygenase 1 (IDO) – the rate-limiting enzyme in the kynurenine pathway of tryptophan catabolism – was marked by increased intestinal expression of IFN-y and TNF-α and pro-inflammatory atherosclerotic plaques, characterised by large necrotic cores and high amounts of CD3⁺ T cells (113, 114). Additionally, the study described that Parabacteroides distasonis, an indole-producing bacterium, was less abundant in IDO deficiency and inversely correlated with atherosclerotic plaque size (114). Overall, this study presents a vital connection between gut Trp, Trp-dependent inflammation and atherosclerosis.

The Trp metabolite Indole-3-propionic acid (IPA) was also investigated in human atherosclerosis. It was found that Peptostreptococcus and Clostridium species, responsible for converting Trp to IPA, were diminished in atherosclerosis patients and associated with lower IPA serum levels (115). Additionally, they demonstrated in mice that IPA was crucial for reducing atherosclerosis burden and linked to higher ABCA1 and reduced miR-142-5p levels – important players for reverse cholesterol transport (RCT) in macrophages (115). Lastly, the lowered serum IPA in CAD patients was found to correlate with the results shown in mice, where the ABCA-1/miR-142-5p signalling pathway was impaired. To summarise, these data suggest that lowered IPA in serum takes part in RCT in macrophages and increased foam cell formation in atherosclerosis patients.

Probiotics are defined as ingestible living microorganisms that mediate the intestinal microbiota balance to result in health benefits for the host (116). As a viable substance, probiotics need to be able to withstand gastric juice and BAs to keep their viability intact and exert the desired effect when attached to the intestinal lumen (117). The most commonly used strains for probiotics comprise Lactobacillus, Saccharomyces, Enterococcus, Bifidobacterium, Bacillus, Streptococcus and Escherichia (23, 39, 117). The mode of action for probiotics is towards metabolic pathways, mainly the replenishment of beneficial bacteria and thus changing the microbial composition towards a healthier state (21). Furthermore, probiotics are capable of aiding digestion and lactose hydrolysis, facilitating mineral absorption of calcium, iron, manganese and zinc, and upregulating various vitamin biosynthesis, including Vitamin K and riboflavin (118, 119). Probiotics also exert pro-apoptotic, anti-oxidative and anti-proliferative influences on the gut microbiome. Many Lactobacillus- and Bifidobacterium-containing probiotics have also been observed to produce SCFA, thereby potentially improve gut health and its metabolic function (120, 121). Recent studies demonstrated several probiotics to have antihypertensive effects, prominently Lactobacillus plantarum, and that Lactobacillus rhamnosus was capable of lowering cholesterol to improve MASLD (122–124).

Most studies using probiotics in the context of atherosclerosis focus on targeting the traditional risk factors, such as reducing dyslipidaemia, restoring endothelial function, regulating secretion of inflammatory markers and macrophage polarisation (125). A few studies found that atherosclerosis was reduced in ApoE^−/− mice when treated with L. acidophilus (ATCC 4356 and 4962), L. rhamnosus GR-1 and A. muciniphila (126–128). Additionally, the total cholesterol and non-HDL cholesterol levels were lowered. In line, A. muciniphila was shown to diminish intestinal permeability and systemic inflammation in ApoE^−/− mice (128). These studies demonstrate that probiotics can be considered as a treatment option for CVD.

Prebiotics are fermented agents that are taken up and processed by the resident microorganisms within the consumer, resulting in distinct microbial profile changes and health benefits (116). Most prebiotics are dietary fibres, which are mainly undigested nor absorbed in the small intestine, but rather are fermented in the distal and large intestines (117). The fermentation releases the prebiotic substances that are then taken up as nutrients for the beneficial Lactobacilli and Bifidobacteria located there (117). Some of the commonly employed carbohydrates include oligofructose, galacto-oligosaccharides and inulin (129). Also, polyphenols and polyunsaturated fatty acids are considered prebiotics (117, 130). These chemicals are converted to conjugated fatty acids to enhance proliferation of beneficial bacteria within the gut. Because prebiotics are differentially degraded by all bacteria, they can be used to selectively change the microbial composition (117).

Importantly, in relation to CVD, treatment with inulin-type fructans in ApoE^−/− mice was able to improve the arterial endothelial function (131). Moreover, beta-glucans were capable of enhancing endothelial vascular reactivity and lower total and LDL cholesterol (132). Another study demonstrated that atherosclerosis development could be reduced by alteration of caecal bacteria using a cyclic polymer of glucose, while mannose oligosaccharides were able to reduce serum cholesterol levels and thus halt lesion progression (133, 134). Taken together, these studies demonstrate the potential for arresting atherosclerosis development using prebiotics.

Antibiotics treatment is one of the most wide-spread techniques to control the gut microbial flora, which has been found to exert various effects on cardiometabolic diseases. An antibiotics mixture consisting of ampicillin plus sulbactam (1 g/L), vancomycin (500 mg/L), ciproflaxin (200 mg/L), imipenem (250 mg/L) and metronidazole (1 g/L) was shown to mediate cholesterol metabolism in ApoE^−/− mice and humans by modulation of propionate levels and cholesterol transporter Niemann-Pick C1-like 1 (135). Additionally, the reduction of phytosterol levels by a similar antibiotic treatment (ampicillin 1 g/L, metronidazole 1 g/L, neomycin 1 g/L and vancomycin 0.5 g/L) has also been linked to altered cholesterol metabolism in mice (136). Another study demonstrated that orally administered vancomycin correlated with a reduction in infarct volume and aided in post-infarct cardiac function in rats (137). Other antibiotic applications resulted in lessened inflammation, bacterial translocation, vascular dysfunction and myocardial injury in mice (138, 139).

While these data indicated beneficial outcomes, antibiotics treatment did not affect disease progression in a number of clinical trials (140). In contrast, a longitudinal study in ∼36,000 adult women described that long-time use of various antibiotics associated with heightened risk for CVD via chronic alterations of the microbiome, including depletion of probiotic bacteria (141). The same risk indication of prolonged antibiotics use was also demonstrated in a study in MASLD patients (142). Moreover, in a recent cross-omics analysis employing a human cohort of atherosclerosis patients and ApoE^−/− mouse models, it was found that broad-spectrum antibiotics (ampicillin 1 g/L, metronizadole 1 g/L, neomycin 1 g/L and vancomycin 0.5 g/L) worsen atherosclerosis, independent of the type of diet given to the mice (143). Despite antibiotics-mediated loss of bacterial diversity, enhanced atherosclerosis associated with the presence of Lachnospiraceae, Ruminococcaceae, Porphyromonadaceae and Prevotellaceae, which could be better targets for more specific treatments (143). Thus, the impact of antibiotics on CVD may be type- and combination-dependent. One study in metabolic syndrome patients showed that vancomycin has severe effects on microbiota composition, bile acid metabolism and insulin sensitivity compared to amoxicillin which had no effect (144). Also, macrolides such as azithromycin have been correlated with increased risk of cardiovascular death and myocardial infarction, while others like erythromycin or roxithromycin were not (145). Taken together, this indicates some antibiotics are more efficient for intervention than others and that the type, dosage, and combination of antibiotics used appears to be a crucial factor for potential therapeutic applications in CVD.

While faecal microbiota transplantation (FMT) is widely used to investigate the role of the gut microbiome in health and disease, it also has been tested as an intervention approach in several studies (13, 21). Due to the positive results in regards of Clostridium difficile infections, FMT is currently being studied for other conditions, among them atherosclerosis. One study created an CTRP9-knockout atherosclerotic-prone mouse model, where they performed intragastrical FMT with faecal matter from WT donor mice (146). This study showed that atherosclerosis severity was reduced in mice after FMT and that the transfer of harmful microbiota provoked atherosclerosis development (146). Another study investigating the gut-immune axis in CVD found that germfree mice with FMT from hypertension patients led to increments in blood pressure and inflammation, when compared to germfree mice that received FMT from “healthy” individuals (147). The germfree mice which received the hypertensive FMT displayed an increase in markers for LPS production, which is often triggered by gram-negative bacteria including Klebsiella and Prevotella (10, 147). Both species were previously shown to be enriched in the microbial flora of hypertensive patients, linking dysbiosis and the microbiome to inflammation in CVDs (10, 147). Taken together, these studies demonstrate that FMT could be a potential strategy for atherosclerosis management, via altering or restoring the microbiome.

Certain small molecules have also been developed to specifically target the bacterial growth to limit disease. One study identified two such molecules, specifically cyclic D- and L-α-peptides, which can transfer the bacterial membrane and inhibit bacterial growth (148). These cyclic molecules demonstrated a reduction in atherosclerotic lesion size by 37% and 48% and also lessened cholesterol levels by 37% and 36% in Ldlr^−/− mice (148). Furthermore, the peptides led to downregulated expression of pro-inflammatory chemo- and cytokines. These data indicate that cyclic peptides targeting microbial strains could be a promising therapeutic option for atherosclerosis.

Further, the influence of inhibition of TMAO production by commensal bacteria using 3,3-dimethyl-1-butanol (DMB) on atherosclerosis was studied (69). The administration of DMB via the drinking water resulted in lower foam cell formation from macrophages and reduced atherosclerotic plaque sizes in ApoE^−/−mice, suggesting that targeting of microbial enzymes could reduce atherosclerosis burden.