Over the past decade, there has been a growing realization of the crucial role played by the gut microbiota, the microbial community residing in our gastrointestinal tract, in human metabolism, immunity, and disease processes, including coronary artery disease (CAD). The potential role of alterations in the gut microbiota and their impact on the progression of CVD and cardiometabolic diseases has been widely acknowledged. While the effects of the gut microbiota's metabolism of a protein-rich diet have been extensively studied, the association between these microorganisms and cardiovascular risk remains a topic of debate. The complex ecology of the gut microbiota and its biochemical influences and metabolic functionality has attracted the attention of clinicians and researchers. This review aims to explore the link between gut microorganisms and CVD.

Significant microorganisms associated with cardiovascular diseases

CVD can arise from various infectious and non-infectious agents, each with its implications [2]. In cases of endocarditis, where the inner lining of the heart becomes inflamed, a significant proportion (around 70%) are identified as blood culture-negative endocarditis (BCNE) [3].

BCNE is often observed in patients who have previously received antibiotic treatment [4]. Furthermore, slow-growing or intracellular microorganisms like Staphylococcus aureus, Streptococcus equi, Streptococcus oralis and Bartonella quintana, can also contribute to BCNE [4, 5].

Numerous infectious agents have been identified as potential contributors to atherosclerosis, including Helicobacter pylori, Cytomegalovirus, Hepatitis C virus, Chlamydia pneumoniae, and Porphyromonas gingivalis [6]. Intriguingly, a study conducted by Mitra et al. revealed differences in the microbiota composition between symptomatic and asymptomatic atherosclerotic plaques. Asymptomatic plaques exhibited an increased abundance of microbial families associated with the host microbiome, such as Porphyromonadaceae, Bacteroidaceae, Micrococcaceae, and Streptococcaceae [7]. In contrast, symptomatic atherosclerotic plaques displayed a higher prevalence of pathogenic microbial families, including Helicobacteraceae, Neisseriaceae, and Thiotrichaceae [7].

Disruption of the overall state of gut microbiota, known as dysbiosis, has been linked to increased inflammation, which is closely associated with the development of atherosclerosis [8]. Recent evidence has also connected alterations in gut microbiota and its metabolites to hypertension and vascular dysfunction [9, 10]. Specific microbial species, such as Escherichia coli, Klebsiella pneumoniae, and Streptococcus viridans, have been implicated in heart failure [11]. Another study demonstrated that patients with symptomatic stroke and transient ischemic attack exhibited an altered gut microbiota with an increased presence of opportunistic pathogens like Enterobacter, Megasphaera, Oscillibacter, and Desulfovibrio [12].

Furthermore, the gut microbiota can significantly influence blood lipid composition, which in turn can impact the development of coronary artery disease [13,14,15].

Staphylococcus aureus-induced myocarditis can lead to sepsis (bacterial infection in the bloodstream) and the formation of abscesses in the heart [6].

Blood-borne parasites, such as Borrelia burgdorferi, Ehrlichia species, and Babesia species, have also been linked to myocarditis [10].

Additionally, certain fungal species, notably Aspergillus fumigatus, A. flavus, and A. nidulans, have been frequently implicated in heart diseases [11].

Mechanisms of gut microbiome induction in cardiovascular disease

The gut microbiome serves as an endocrine organ, producing bioactive metabolites that can impact host physiology. Dysbiosis, which refers to changes in the composition of the gut microbiome associated with disease, has been linked to conditions such as atherosclerosis, hypertension, heart failure, chronic kidney disease, obesity, and type 2 diabetes [16]. Alterations in the gut microbiota can have significant effects on the regulation of host biochemistry and metabolism. Understanding the interplay between gut microflora, the inflammasome, the innate immune system, bile acids, and gut permeability can potentially inform preventive strategies against cardiovascular disease and shed light on the role of microorganisms in the pathogenesis of autoimmune diseases [17, 18, 19, 20, 21, 22].

Indeed, gut microorganisms play a crucial role in modulating systemic immune responses and metabolic dysfunctions, particularly in individuals with obesity, and contribute to the development of atherosclerosis [23,24]. By converting common nutrients into metabolites, the resident microflora in the intestinal tract can act as filters for dietary components. Specific microbial-associated metabolites such as trimethylamine-N-oxide (TMAO), short-chain fatty acids (SCFAs), and secondary bile acids have been implicated in influencing the development of cardiovascular disease [23]. Lower gut microbial diversity has been associated with elevated levels of white blood cells and C-reactive protein (hsCRP), whereas higher loads of gut microorganisms are inversely correlated with various markers of low-grade inflammation, including hsCRP and interleukin-6 (IL-6) [24,25,26,27].

Extensive research utilizing advanced technologies has provided substantial insights into the impact of gut bacteria on the

development of cardiovascular disease (CVD) [28,29,30]. One study demonstrated that gut bacteria, specifically Escherichia coli, can convert dietary products like L-carnitine and phosphatidylcholine into trimethylamine (TMA), which is further metabolized in the liver to form trimethylamine-N-oxide (TMAO). This conversion promotes atherosclerosis and cardiovascular diseases [30,31,32,33,34]. Additionally, an elevated level of the gut bacterium Collinsella has been observed in the carotid artery of patients with symptomatic atherosclerosis. Conversely, certain probiotics such as Lactobacillus rhamnosus GR-1, Bifidobacterium infantis, and Methanomassiliicoccus luminyensis B10 have shown effectiveness in treating heart failure in mouse models, further emphasizing the central role of gut microflora in CVD development [35].

Periodontal pathogens, including Porphyromonas gingivalis, Prevotella intermedia, and Fusobacterium nucleatum, have been found to exacerbate high-fat diet-induced systolic and diastolic arterial pressure in diabetic mice, contributing to the development of cardiovascular complications. Lipopolysaccharide (LPS) derived from P. gingivalis has been implicated in inflammation-induced CVD through the promotion of oxidative stress (increased reactive oxygen species) and mitochondrial dysfunction [36,37]. Furthermore, an intriguing study utilizing a mouse model has shed light on the mechanisms by which microorganisms can modulate pathogenic inflammation in the heart and influence novel innate immune responses [38,39].

Gut bacteria express various receptors such as Lipopolysaccharide (LPS) and pattern recognition receptors (PRRs), which play a role in stimulating and modulating the host immune response [44]. Several pathways, including the trimethylamine (TMA)/trimethylamine N-oxide (TMAO) pathway, short-chain fatty acids (SCFAs) pathway, and primary and secondary bile acid (BAs) pathways, are known to interact with host endocrine hormones such as ghrelin, leptin, and glucagon-like peptide 1 (GLP-1) [40,41,42].

In terms of linking adaptive immunity to cardiovascular disease (CVD), the glucocorticoid-induced leucine zipper (GILZ) protein has been reported to suppress immune and inflammatory responses, which can contribute to the development of myocardial infarction (MI). This suppression is associated with reduced levels of Th-17 cells and elevated levels of anti-inflammatory cytokine IL-10 positive cells [43]. Cytokines, including interferon-γ, transforming growth factor-β1, and interleukin-1β, also play a role in this process [44].

Figure 1: Shows Association of Dysbiosis. Conditions associated with dysbiosis include Atherosclerosis, Obesity, Hypertension (HTN), Chronic Kidney Disease (CKD), Type 2 Diabetes (T2D), and Heart Failure.

Figure credit: segments of the figure were generated by making use of pictures available from Servier Medical Art and Pixabay, accessed from Servier and Pixabay, and licensed under a Creative Commons Attribution 3.0 unported

Diagnostic modalities

In the field of microbiology, several advanced techniques and molecular biology tools have proven to be highly valuable in the detection of microorganisms implicated in heart diseases. Molecular methods such as polymerase chain reaction (PCR), 16S rRNA sequencing, and next-generation sequencing (NGS) have demonstrated their effectiveness in directly detecting microbial pathogens associated with cardiovascular diseases, including infective endocarditis [45, 46,47]. These molecular tools offer significant advantages over traditional culture-based methods, particularly in identifying viable but non-culturable pathogenic microorganisms [48,49,50].

The rise in research on the gut microbiome can be credited to the advancement of affordable and efficient next-generation sequencing (NGS) technology, along with the availability of various "omics" data, including human genomic, metabolomics, and proteomic data. The combination of NGS technology and breakthroughs in bioinformatics has transformed the microbiome field, replacing traditional culture-based methods and enabling the analysis of progressively intricate microbiome characteristics. Nonetheless, certain constraints persist. To illustrate, the utilization of 16S rRNA sequencing may result in a limited perspective on bacteria alone, overlooking other essential life forms such as fungi, protozoa, and viruses. Metagenomic studies offer a broader, multi-kingdom view, but they also have their limitations. A significant portion of the data, particularly viral data, cannot be assigned a specific function due to a lack of close matches in reference databases [48].

To advance our understanding and progress toward establishing causality in the field of microbiome research, it is crucial to build a comprehensive knowledge base that consolidates fragmented information. Additionally, incorporating innovations such as natural language processing, text mining, taxonomic representations, and standardizing the vocabulary used across the microbiome research field can expedite our comprehension [49].

Therefore, continued research efforts aimed at improving quality control, methodologies, and pipelines are imperative for the development of comprehensive models that elucidate the dynamics of the gut ecosystem of cardiovascular disease on a global scale. These advancements will contribute to understanding the complexity between the gut microbiome and cardiovascular health, laying the groundwork for potential therapeutic interventions and preventive strategies

Prospects for the future

Emerging evidence suggests a strong association between the gut microbiome and the incidence of cardiovascular disease (CVD). Research indicates that the microbiota interacts with the host through various pathways, and disruptions in the composition of the gut microbiota or its metabolites may contribute to increased risks of CVD and related pathological changes. As a result, innovative therapeutic targets and strategies have been developed to leverage the potential of gut microbiota in preventing and treating CVD.

Significant efforts are currently underway to explore the potential applications of microbiota in CVD and other human diseases more broadly. Firstly, there is a shift towards identifying specific strains of microbes, rather than focusing solely on the overall bacterial community, to better understand the contributions of individual microorganisms to disease progression. Secondly, future research may place greater emphasis on investigating microbiome-mediated metabolites and their downstream functional consequences, as current studies primarily focus on microbial composition. Thirdly, personalized approaches for modifying the microbiota are of utmost importance and are actively being pursued. This endeavour can be aided by conducting microbiome profiling of individual patients to identify metabolomic biomarkers.

These ongoing advancements in understanding the role of the gut microbiome in CVD, along with the exploration of specific microorganisms, metabolites, and personalized interventions, hold great promise for the development of effective therapeutic strategies.

Acknowledgments
None

Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Author contribution statement

Aroop Mohanty: Conceptualization (lead); writing – original draft (lead); formal analysis (lead); writing – review and editing (equal). Parul Singh: conceptualization, Software (lead); writing – review and editing (equal). Ankita Kabi: Methodology (lead); writing – review and editing (equal). Amogh Verma: Methodology (lead); writing – review and editing (equal). Ranjit Sah: Conceptualization (lead); writing – original draft (lead); formal analysis (lead); writing – review and editing (equal).

Data availability statement
Data included in article/supp. material/referenced in article.

Additional information
No additional information is available for this paper.

Declaration of competing interest
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Arooo Mohanty and Ranjit Sah are authors of this paper. Therefore, the peer review process was managed by alternative members of the Editorial Board and the authors had no involvement in the decision-making process.

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