**3.3 Gut microbiota composition during type 2 diabetes**

*Parasitology and Microbiology Research*

LPS entry into circulatory system resulting in endotoxemia. Rise in LPS, modulates the integrity of the tight junctions of the intestinal membrane increasing LPS in circulation. Therefore, GM is a complex system having both advantageous and dangerous microbes, and understanding the GM and host integration system provides a generalized idea about the function of each unit of the GM-host system [83, 84].

**3.1 Gut microbiota and carbohydrate metabolism during type 2 diabetes**

**3.2 Gut microbiota and lipid metabolism during type 2 diabetes**

Our body metabolism, inflammatory processes and innate immune system are regulated by dietary lipids [100]. The dietary lipids can also act as (proinflammatory) ligands which can bind to nuclear receptors [101]. The nuclear receptors are peroxisome-proliferator-activated receptors (PPAR) and liver X receptors (LXR) which regulate metabolic and inflammatory pathways. Hence the dietary lipids can improve insulin action and down-regulate secretion of pro-inflammatory cytokines [102, 103]. Lipids can also activate G-protein coupled receptors (Gpcr) such as Gpr43 when activated by dietary-metabolite acetate lipolysis in adipocytes is decreased leading to reduced plasma-free fatty acids. Gpr43 can be considered as a potential target for regulation of lipid metabolism [104]. Inflammation and lipid accumulation are characteristic features of atherosclerosis [105]. Recent evidences provide sufficient link between atherosclerosis and GM variety [106]. Short-term antibiotic administration can alter the composition of GM which can convert dietary choline and l-carnitine to trimethylamine (TMA). TMA is later oxidized into TMAO by the action of hepatic Flavin monooxygenases [107]. Dietary choline is highly available in foods rich in lipid phosphatidylcholine, lecithin, such as in eggs, red meat, milk, poultry, liver, and fish [108]. Bile acids are key modulators of lipid and cholesterol metabolism, and they facilitate intestinal absorption and transport of nutrients, vitamins, and lipids. Production of bile occurs in the liver and 95% of bile acids are reabsorbed in the ileum. Later the bile acids are re-absorbed in liver, entering the enterohepatic circulation. GM converts primary bile salts to secondary bile salts by bile acid de-hydroxylation [109]. Bile acids can also result in the release of GLP-1 from enteroendocrine L cells via activation of Takeda G protein coupled receptor-5

Bile acids affect glucose homeostasis via activation of nuclear farnesoid X receptor (FXR) and the membrane-bound G protein coupled receptor, TGR5. These receptors are expressed in liver, ileum and pancreas [85]. Some bile acids act as agonists for FXR, and others are FXR antagonists [86–88]. Known FXR agonists are CDCA, lithocholic acid, deoxycholic acid, and cholic acid [89]. The antidiabetic effects exhibited by vertical sleeve gastrectomy, bariatric surgery, occurs through FXR [90]. Also, intestinal FXR agonist treatment can improve insulin sensitivity [91]. In the ileum, activation of FXR leads to the production of fibroblast growth factor-19, a hormone that affects glucose tolerance through mechanisms that are largely independent of insulin [92, 93]. Activation of TGR5 produces glucagon-like peptide-1 (GLP-1) from ileum improves both energy and glucose homeostasis [94]. Activation of FXR in pancreas regulates insulin transport and secretion [95], and protects the islets from lipotoxicity [96]. FXR activation in liver improves insulin sensitivity in T2D patients [97]. The GM can modulate the amount and type of secondary bile acids produced via FXR and TGR5 signaling. GM enzymes such as bile salt hydrolase for deconjugation, 7-alpha dehydroxylase for dihydroxylation and 7α-hydroxysteroid dehydrogenase for epimerization of bile acids are reduced in T2D patients compared to healthy controls [98]. Bile acid concentrations in the circulation show a diurnal pattern since they increase after food intake [99].

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The GM of T2D patients exhibit low population of Firmicutes and Clostridia and high ratio of Bacteroidetes:Firmicutes (**Table 1**) [112, 113]. However, the GM of T2DM and obese patients are not always identical because the GM of obese patients show decreased Bacteroidetes:Firmicutes ratio [113–115, 118]. GM of T2DM patients also show low population of butyrate-producing bacteria. Shortchain fatty acids (SCFAs) like butyrate, acetate, and propionate are fermented from dietary fiber in large intestine by GM. SCFAs regulate energy metabolism, immune responses and tumorigenesis in gut. Butyrate is the energy source for colonic epithelial cells. Butyrate perpetuates intestinal integrity and thereby avert translocation of Gram-negative intestinal bacteria across the lumen of the gut. This phenomenon ultimately leads to endotoxemia triggering a low-grade inflammation during T2D [15, 113, 115].

The major risk factors behind T2D are genetic predisposition, less physical activity, fetal programming, obesity and altered GM [114, 116]. Total weight of GM in the distal gut is about 1.5 kg and it is considered as a microbial organ. The GM consists of embers from Bacteria, Archaea, Eukarya and viruses, but a large part of the population includes anaerobic bacteria. 90% of the bacterial species present in gut are grouped into the two bacterial phyla Firmicutes (Gram-positive) and Bacteroidetes (Gram-negative) (**Table 1**) [15, 16]. An average adult fosters a minimum of 160 bacterial species and a set of genes in the GM is obligatory for proper functioning of the GM [15]. The GM gives protection from disease causing pathogens and facilitates the immune system. GM also help in production of vitamin K and many B-vitamins like folate, vitamin B12. Metagenomic studies about sequencing of T2D patients exhibit dysbiotic GM and less butyrate-producing bacteria (*Roseburia* species and *Faecalibacterium prausnitzii*). Metabolic disorders like obesity and impaired glucose metabolism are related with an altered ratio of Firmicutes and Bacteroidetes [118–120]. Populations of Proteobacteria and particularly *Escherichia coli* are also high in T2D patients (**Table 1**) [113, 121]. Gram-negative bacteria contribute to inflammatory lipopolysaccharides (LPS) stimulating pro-inflammation, during T2D and obesity. Oral administration of metformin, a widely used drug for T2D elevates populations of Enterobacteriaceae and lowers populations of *Clostridium* and *Eubacterium*. Metformin also increases the populations of *Akkermansia* sp. in high-fat diet-fed mice, hence improving glucose metabolism [122]. Oral administration of *Akkermansia muciniphila* also improves metabolic dysfunctions like endotoxemia and adipose tissue inflammation (**Table 1**) [122, 123]. Hence metformin can be used as a potent drug in improvising the GM content in T2D patients, managing glucose tolerance and inflammation.
