**3.4 Cardiovascular disease**

The abnormal interactions between the microbiota and the host compromise homeostatic mechanisms. Most cardiovascular risk factors, such as age, obesity, diet, and lifestyle, can generate gut dysbiosis, which is associated with intestinal inflammation and poor integrity of the intestinal barrier [7, 23].

Diets rich in fat lead to the stimulation of mast cells from the intestinal mucosa, generating inflammatory mediators, such as histamine, which can amplify intestinal permeability [140]. However, high carbohydrate diets can also raise intestinal permeability and endotoxins [141].

Cardiovascular diseases (CVD), the number one cause of death worldwide, are influenced by smoking, dyslipidemia, diabetes mellitus, and arterial hypertension [23].

Dysbiosis is involved in numerous pathophysiological chains of events, leading to different conditions, and cardiovascular afflictions making no exception. The perturbation of the gut microbiota can favor a pro-inflammatory state in the human body, therefore promoting the atherosclerotic process [7, 23, 142].

Atherosclerosis is, unfortunately, a frequent chronic inflammatory process, which comprises endothelial dysfunction, dysfunction of vascular smooth muscle cells differentiation, infiltration with inflammatory cells, and subendothelial lipid accumulation [143].

Microorganisms, such as *Chlamydophila* pneumoniae, *P. gingivalis, Helicobacter pylori*, Influenza A virus, Hepatitis C virus, cytomegalovirus, and human immunodeficiency virus, were associated with a high risk for developing CVD [23, 144]. Infections can influence atherosclerosis through arterial wall inflammation, favoring plaque formation, or through the production of pro-inflammatory mediators, which are the result of infections of various sites in the body [23, 145].

High blood levels of lipopolysaccharides (LPS) have been linked to adverse cardiac events in patients with CVD such as atrial fibrillation [146]. LPS are endotoxins, byproducts of gut microbiota that can reach systemic circulation through the intestinal mucosa [147]. A decrease in gut bacteria, such as *Bacteroides spp*, has been negatively correlated with atherosclerotic plaque progression and endothelial dysfunction, thus promoting inflammation [148].

Atherosclerosis is associated with trimethylamine-N-oxide (TMAO), a vasculotoxic metabolite resulting from L-carnitine, choline, and phosphatidylcholine. TMAO was indicated to promote the development of aortic lesions in apolipoprotein E (apoE) in mice by modifying bile acid profiles. TMAO inhibits the production of bile acids through the farnesoid X nuclear receptor (FXR) and small heterodimer partner (SHP) [149].

Elevated serum levels of TMAO have been shown to predict CVD outcomes in heart failure. Individual TMAO formation is dependent on microbial gut composition. A red meat diet consumption rich in choline and an omnivorous diet with high carnitine may account for TMAO levels elevation [150]. In an observational study of 155 patients with heart failure, elevated plasma levels of TMAO were found in chronic HF patients with higher levels in NYHA class III and IV and were associated with worse prognoses [151].

Microbiota in the colon metabolizes secondary bile acids (BA) from un-recycled bile acids through bile-salt hydrolase (BSH). BA synthesis is an important pathway for cholesterol elimination, thus having an athero-protective function. Composition of bile acids is altered in heart failure patients with a decrease in the primary to secondary bile acids ratio. A decrease in BSH levels subsequently causes cholesterol buildup and progression of CVD. Microbial BSH modulates stimulation of hepatic FXR, which acts as a bile acid signaling receptor and a potential target for bile acid therapy in reducing cardiovascular complications [152, 153].

Moreover, probiotic supplements may improve intestinal balance and select probiotics could have a cardioprotective role. Altered bacterial diversity was observed in two heart failure with reduced ejection fraction (HFrEF) cohorts with an increase in *Prevotella* genus and a decrease in genera belonging to *Lachnospiraceae* family and *Rumminococcaceae Faecalibacterium* and *Bifidobactericeae Bifidobacterium* [154]. Similar cohorts had increases in pathogenic bacteria, such as *Campylobacter, Shigella, Yersinia enterolytica*, and *Candida* species, associated with an increase in gut permeability [155]. The *Firmicutes/Bacteroidetes* ratio (F/B) in hypertensive patients is higher than in the normotensive individuals, by lower levels of *Bacteroidetes* [156]. *Roseburia*, one of the main producers of butyrate, is diminished in hypertensive patients. However, *Roseburia* can also produce linoleic acid, which has anti-inflammatory properties and a possible role in lowering blood pressure values, together with linolenic acid [156–159]. According to CARDIA study, *Robinsoniella* and *Catabacter* were positively associated with hypertension [160].

Animal studies suggest that gut dysbiosis is associated with arterial hypertension both directly and indirectly. Change in microbial diversity such as the ratio of *Firmicutes* to *Bacteroidetes* in the intestine yields a potential mechanism in hypertension formation and a pathway for future treatment. By fermentation of fibers, these bacteria produce short-chain fatty acids (SCFAs) such as propionate and butyrate [161].

SCFAs play an important role in homeostasis, including blood pressure variations, through their interaction with certain receptors: G-protein-coupled receptors (GPCRs), such as Gpr41 or Olfr78. Studies on mice null for Olfr78 led to the conclusion that those animals were hypotensive, while mice null for Gpr41 were hypertensive [162].

In a metabolomic analysis of prehypertensive and hypertensive patients, it was shown that overgrowth of opportunistic bacteria, such as *Klebsiella* and *Prevotella copri,* was present in prehypertensive (pHTN) patients compared to healthy individuals, where higher levels of *Faecalibacterium, Bifidobacterium, Roseburia* and *Butyrivibrio* were found. This suggests alteration of the microbial profile occurs

*Intestinal Microbiomics in Physiological and Pathological Conditions DOI: http://dx.doi.org/10.5772/intechopen.110642*

well before clinical findings. Probiotics and antibiotics could be proven as potential therapies for BP. Furthermore, small-scale fecal transplant from hypertensive patients to germ-free mice has led to higher blood pressure levels compared to controls [163].

Atrial fibrillation (AF) is another important CVD that has been linked in recent studies with dysbiosis. Patients with persistent AF manifest an increase in *Ruminococcus, Streptococcus,* and *Enterococcus,* and bacteria, such as *Faecali bacterium, Oscillobacter,* and *Biliophilus,* were decreased [164]. An imbalance of microbiota leads to damage in the intestinal barrier function that in turn can promote atrial electrical remodeling by increasing the activity of NLRP3 inflammasome [165, 166].

A metagenomic analysis by Zhang et al. 2021 in a cohort of patients with AF showed that species with SCFA-synthesis enzymes such as *Coprococcus catus* and *Firmicutes bacterium* were decreased in the gut of AF patients compared to controls. Furthermore, homeostasis of gut microbiota metabolites such as bile acids can modulate the risk of AF [167].

### **3.5 Obesity and diabetes mellitus**

The microbiota of obese individuals significantly differs in composition and function from that of healthy individuals [168]. Thus, the microbiota of obese people is characterized by an increased ratio of *Firmicutes* vs. *Bacteroidetes*, mainly *Ruminiococcus, Candida*, and *Lactobacillus* [169, 170], increased amount of *Actinobacteria*, which produce SCFA and *Proteobacteria* [171]. Human studies have shown that obese people had more *Firmicutes* and approximately 90% fewer *Bacteroidetes* and a low-fat or low-carbohydrate diet can restore the *Firmicutes* to *Bacteroidetes* ratio but never be the same as the people that were lean from the beginning [169]. Some other studies demonstrated that a higher caloric intake increased *Firmicutes* by 20% and reduced *Bacteroidetes* by 20%, leading to a gain in body weight [172]. Studies on infants observed that obese children have a lower level of *Bifidobacterial* and a higher level of *Staphylococcus aureus* [173].

As it is already known, the diet has an important role in modulating microbiota composition, in both healthy and obese people. Some types of diets, like the Western diet, can modify microbiota, especially by increasing *Firmicutes* levels, leading to dysbiosis, metabolic stress, and obesity [174, 175]. Compared to the Western diet, a diet based on dietary fiber, plant polysaccharides, and lower fat and animal protein is characterized by a lower level of *Firmicutes* and a higher level of *Bacteroidetes* [28, 176]. Importantly, some mice and human studies underlined that a high-fat/highsugar Western diet can modify the microbiota in just 1 day [177, 178]. Chen J et al. 2019 have shown that dietary intake has more impact on microbiota changes in mice than genetic etiology [179]. Moreover, Pols et al. 2011 have demonstrated that an improper diet has significantly negative consequences leading to the disappearance of species and strains of microbiota [180].

The obesity-microbiota relationship and its mechanisms have been studied for a long time [168] Many studies have shown that alterations in the microbiota community modify the process of energy extraction from food and consequently the adiposity of the body [176]. The gut microbiota of obese people has a larger capacity for absorbing energy from meals, thus their gut bacteria lead to weight growth [170]. Some studies have shown that gut microbiota can influence adiposity by modulating host gene expression, metabolic and inflammatory pathways, and gut-brain axis [181]. Inflammation mediated by gut microbiota can increase circulating lipopolysaccharide (LPS) levels and gut permeability and thus adipose tissue inflammation,

commonly seen in obesity [182]. Microbiota metabolites like SCFA are increased in obese people, being involved in glucose homeostasis (improving glucose sensitivity) and lipid metabolism through free-fatty acid receptors, leading to activation of hepatic gluconeogenesis and lipogenesis [183] and inhibition of fatty acid oxidation in muscles [184]. Nondigestible carbohydrates can increase SCFA levels, which can modify the level of enteric hormones [185]. Alterations of the microbiota can reduce organisms that temper CD36 expression, such as products produced by *Clostridia*, which can increase lipid absorption, leading to obesity and metabolic syndrome [186]. Microbiota dysbiosis can reduce fasting-induced adipose factor expression, being involved in lipoprotein lipase (LPL) activation with lipid accumulation in adipose tissue [187]. Gut bacteria influence two key signaling pathways, glycemic reaction component binding domain, and cholesterol control component related proteins causing fat accumulation in the liver, where lipids can be then absorbed via visceral fat, thanks to LPL [170]. A lack of dietary fiber and poorly digestible carbohydrates reduce the diversity of bacterial flora [188]. Some studies have shown that lower microbiota diversity is associated with increased abdominal adiposity [189], but can be reversible in humans with cardiorespiratory fitness [190]. Human studies underlined that obese humans have a low fecal bacterial diversity, promoting adiposity, dyslipidemia, impaired glucose homeostasis, and higher low-grade inflammation [191]. Hormonal, neurological, and immunological pathways connect the brain with the microbiota [170]. Microbiota can modulate the synthesis of neuropeptides like dopamine, which regulate gastrointestinal function and thus can influence cognitive activity and increase hunger [192]. Among the metabolites secreted by the microbiota, serotonin, and γ-aminobutyric acid (GABA) control appetite and body weight regulation [193]. Alterations of the intestinal microbiota can modify the secretion of gastrointestinal hormones, such as glucagon-like-peptide-1 (GLP-1), which is involved in food intake control [194]. The dysbiosis of the microbiota in obese people can increase the level of acetate, enhancing the secretion of glucose-stimulated insulin and ghrelin, consequently increasing obesity [195]. Some studies underlined that the risk of obesity is associated with prenatal and perinatal antibiotic use by influencing microbial colonization and maturation [196].

Obesity-microbiota relationship and especially dysbiosis is associated with the risk of developing some other health problems, like diabetes mellitus (DM) [168, 197].

Schwartz et al. 2016 included for the first time gut microbiota modification as a mechanism implicated in DM [198]. The gut microbiota has an important role in influencing the immunologic system and developing type 1 DM (T1DM), as also as in developing metabolic disorders such as type 2 DM (T2DM) [197]. DM is considered an inflammatory clinical entity, characterized by inflammatory mechanisms that involve lipid accumulation, cytokines synthesized by a dysfunctional adipose tissue, a dysregulated immune system, as also as increased levels of inflammatory markers, such as C-reactive protein, Tumor Necrosis Factor-α, interleukins 6, 17 and 23, and Transforming Growth Factor β [199–201].

Studies have underlined that SCFAs, bile acid, branched-chain amino acids, imidazole propionate, and LPS have an important role in DM, among these the release of LPS with pro-inflammatory effects and decrease in SCFA production is the phenomena discussed in DM patients [197, 202].

In the case of dysbiosis, the LPS secreted by gram-negative bacteria from the gut generates a low-grade inflammatory state by interacting with type 4 toll-like receptors, increasing the risk of insulin resistance [203]. Physiological, the intestinal wall prevents the passage of LPS into the systemic circulation. High-fat diets increase the

### *Intestinal Microbiomics in Physiological and Pathological Conditions DOI: http://dx.doi.org/10.5772/intechopen.110642*

permeability of the intestinal wall and LPS circulation, by influencing the distribution of binding protein complexes and excessive and chronic production of biliary acids [197]. LPS binds then with the lipopolysaccharide-binding proteins and interacts with a membrane protein of differentiation 4, allowing the activation of TLR. A signaling cascade is then stimulated and focal adhesion kinase is phosphorylated and activated. In systemic circulations, LPS binds the TLR-4 in the membranes of immune and adipose cells, including pancreatic betta-cells, releasing TNF-α, IL-1, and IL-6, which can induce insulin resistance [204, 205].

Increased levels of *Firmicutes* in obese individuals, as was already mentioned, generate energy harvest, positive energy balance, and higher caloric bioavailability, leading to weight gain [197]. Modifications of *Firmicutes* to *Bacteroidetes* ratio have also been present in DM patients, being characterized by increased levels of *Bacteroidetes* [206], which are associated with decreased levels of *Akkermansia municiphila* [207]. Studies have observed an increased level of *Clostridium* and *Veillonella* genre in kids with T1DM, which ferment glucose and form propionate, succinate, and acetate from lactate and increase gut permeability [208]. Patients with DM and chronic pancreatitis have a low level of *Fecalibacterium prausnitzii*, which has anti-inflammatory properties and stimulate the synthesis of binding proteins [209]. Low levels of *R. bromii* have been observed in patients with DM, leading to the production of butyrate and energy [210]. T2DM is characterized especially by increased levels of *Bifidobacterium* and *Bacteroides* and to a lesser extent by *Faecalibacterium, Akkermansia, Roseburia, Ruminococcus, Fusobacterium,* and *Blautia* [211]. In patients with gestational diabetes mellitus, it was observed an increase in *Firmicutes* levels and a decrease in *Bacteroidetes* and *Actinobacteria* levels [212].

SCFAs are involved in T2DM by their immunomodulatory functions, but also stimulate the secretion of peptides that regulate the appetite and satiety, like GLP-1, the YY peptide, and ghrelin [213, 214]. In dysbiosis induced by a high-fat diet, it has been observed a decreased level of *Lactobacillus* and an increased level of *Bacteroides, Bukholderia*, and *Clostridium*, leading to an increased level of GLP-1 [215] and SCFA acetate, which affects insulin secretion, leading to obesity, hyperlipidemia, and insulin resistance [197, 216]. Studies have shown that increased levels of *Eubacterium* and *Roseburia* intestinalis in association with abnormal production and absorption of propionate, as also as postprandial insulin secretion and propionate generation in feces stimulated by butyrate, can increase the risk of T2DM [202].

Gut microbiota plays an important role in obesity and DM, especially in the case of dysbiosis, which influences the inflammatory and immune response, but also their pathophysiology. Throughout life gut microbiota is influenced by a lot of factors and has an important role in energy balance, being connected to obesity. Greater levels of LPS and lower levels of SCFA are the main characteristics of DM patients. Many mechanisms implicated in an obesity-microbiota-DM relationship were discussed in studies, a lot of them being still unwell known, so future research needs to investigate the function of the intestinal flora and its link to obesity and DM [170, 217].

### **3.6 Dermatological conditions**

The skin, together with the intestinal epithelium, represent the largest interfaces between the body and the external environment, being the place where the most important processes of immune tolerance take place, allowing their colonization with essential commensal microorganisms that form the skin and gut microbiota [218, 219]. Thus, their alterations are associated with the appearance or progression of

numerous inflammatory dermatological diseases, such as psoriasis, atopic dermatitis (AD), hidradenitis suppurativa (HS), acne, rosacea, alopecia areata, skin cancers, and seborrheic dermatitis [218]. Although most research groups have focused on the changes in the skin microbiota associated with dermatological diseases, recent studies have also observed alterations also in intestinal microbiota, probably through the systemic modulations determined by secreted molecules with the hormonal role and through the cells of the immune system [219, 220].

One of the most studied dermatological conditions associated with changes in the intestinal microbiota is psoriasis, a chronic inflammatory dermatosis, characterized by numerous pruritic, erythematous-scaly patches and plaques, distributed especially on the extension areas, associated or not with articular involvement [221]. Thus, a study conducted on a group of 30 patients with psoriasis and 30 healthy volunteers that evaluated the composition of the intestinal microbiota, observed that, although there is no difference statistically significant in terms of the type of bacteria in the analyzed samples (alpha diversity), their proportion is statistically significantly different between the two groups. Thus, the group with psoriasis showed an increase in the proportion of the families *Veillonellaceae* and *Ruminococcaceae* (p < 0.05) and of the genera *Faecalibacterium* and *Megamonas* (p < 0.05) compared to the healthy group [222]. The number of some of the microorganisms (*Bacteroides, Escherichia*, respectively *Dialister*) also seems to correlate negatively with different paraclinical markers like complement 3 (C3) (p < 0.01) respectively Interleukin 2 Receptor (IL2R) (p < 0.001). Moreover, *Prevotella*, respectively *Phascolarctobacterium* positively correlates positively with the level of C3 (p < 0.01), respectively IL2R (p < 0.001) [222]. Tan et al. 2015, observed a decrease in the classes of microorganisms *Mollicutes* and *Verrucomicrobiae* and the genus *Akkermansia* (species *Akkermansia muciniphila*), as well as an increase in the genera *Enterococcus* and *Bacteroides* in a study conducted on a group of 14 patients with psoriasis and 14 healthy volunteers [223].

Another study conducted by Hidalgo-Cantabrana et al. 2019 on a group of 19 patients with psoriasis and 20 healthy patients also highlighted the presence of the same phyla as in a healthy population, similar to the studies above. However, unlike Tan et al. [76], the populations of *Bacteroidetes* and *Proteobacteria* were lower than in the control group (p < 0.001), and *Actinobacteria* and *Firmicutes* were in a larger number (p < 0.001). This study also highlighted a decrease in *Verrucomicrobacteria* [224]. Scher et al. 2015 evaluated the variability of the microbiota in patients with early psoriatic arthritis, compared to patients with psoriasis and healthy patients, and found a decrease in *Akkermansia* and *Ruminoccocus* in those with psoriatic arthritis compared to patients with psoriasis. In the latter, a decrease in *Bacteroidetes* and *Coprobacillus* was observed. Also, lower levels of medium-chain fatty acids (involved in cell signaling) were found in patients with psoriatic arthritis (p < 0.05) and in those with psoriasis (p < 0.01) compared to the control group [225].

Regarding atopic dermatitis (AD), numerous studies evaluate both the changes in the microbiota, as well as the impact of the administration of probiotics on the evolution and severity of the disease. Thus, it was found that 1-week-old newborns who were later diagnosed with IgE-mediated eczema showed a decrease in *Enterobacteriaceae, Escherichia-Shigella* (statistically insignificant), and *Ruminococcaceae* (p = 0.0047). It was also found that the mothers of these children had an increased level of microorganisms from the *Bacilli* class and the *Streptoccocus* genus [226, 227]. AD was also associated in patients under 20 years, with a decrease in *Clostridium, Streptococcus, Enterobacteriaceae,* and *Bifidobacterium* (p = 0.006). Moreover, more severe forms of the disease were associated with a lower number of

### *Intestinal Microbiomics in Physiological and Pathological Conditions DOI: http://dx.doi.org/10.5772/intechopen.110642*

*Bifidobacterium* (p = 0.046) and a higher number of *Bacteroides* (p = 0.0443) compared to children with average manifestations of AD [228]. Another study carried out on a pediatric population (28 children aged 6 months old with AD) demonstrates the existence of a statistically significant correlation between the severity of the disease and the decrease in the number of bacterial species in the microbiota (r = −0.54, p = 0.002). Moreover, the administration of hydrolyzed casein in these patients led to an improvement in the clinical score and the composition of the microbiota [229].

Another dermatological condition with a significant impact on the quality of life, in which the microbiota seems to play an important role is hidradenitis suppurativa (HS). Thus, in those patients, a decrease in the diversity of the intestinal bacterial flora was also found, but with an increase in *Ruminoccocus gnavus*, which also appears to increase in other inflammatory digestive or articular diseases [230]. Kam et al. also observed a decrease in the phylum *Firmicutes* compared to the healthy population (p = 0.03), with changes in the genera *Lachobacterium* and *Veillonella* in the same direction (p = 0.019, respectively p = 0.005). The genera *Biophila* and *Holdemania* were found in a higher proportion of these patients, although the small number of patients on which the study was conducted (3) makes it difficult to interpret the data [231]. Another difference between the microbiota of HS patients compared to healthy ones was highlighted by Lam et al. 2021 in a study carried out on 17 patients with HS. He observed colonization with *Robinsoniella* only in patients with HS, not in the healthy group, but also a greater number of microorganisms from the *Sellimonas* genus in these patients. The latter was also associated with the presence of several inflammatory joint diseases [232].

The immunological, neurological, and biochemical interrelations between skin and gut, explained by the existence of the skin-gut axis are also reflected in the way in which microbiota alterations are present in various dermatological inflammatory pathologies. Although the current studies show changes in the proportions of bacteria from the intestinal microbiota, the small groups of patients, as well as the contradictory data from some studies prevent us from drawing clear conclusions and associating changes in specific genera or species with certain diseases.
