- Gut microbiota plays an essential role in health and diseases, a fact already established. With the explosive increase in obesity and its main complication, Type 2 Diabetes Mellitus (T2D), there is an immediate need for novel methods of diagnosis, prevention and treatment of these conditions. In the last one and half decades, the gut microbiota has been a focal point of study in obesity and T2D. Several studies have proposed using gut microbiota to predict, diagnose, prevent, and treat obesity and T2D. However, there is no consistency between studies except for few microbes. The U.S Food and Drug Administration (FDA) has not approved any microbe or microbial products for the treatment of T2D, though many are in the stages of clinical trials. Diet rich in processed fat and sugar, also known as the western diet (WD), is the main culprit of T2D. WD changes the metabolic phenotypes leading to obesity, insulin resistance, glucose intolerance and T2D. WD also changes gut microbiota composition, as evidenced by the difference between obese and lean individuals. So, there seems obvious crosstalk between diet, microbiota and metabolic complications. In this dissertation, using WD induced T2D mouse model, we established the interactions between WD, microbiota and host. We identified and validated gut microbes that mediate beneficial effects in systemic glucose homeostasis. We also determined the effects of the western diet that are dependent on microbiota and identified a gut microbe associated with WD, promoting insulin resistance via induction of a metallopeptidase in adipose tissue.
In the second chapter, we did a comprehensive review of current literature involving human subjects to identify the potential role of different bacterial taxa affecting glucose intolerance, insulin resistance and T2D. This systemic review showed a negative association of T2D with
genera of Bifidobacterium, Bacteroides, Faecalibacterium, Akkermansia and Roseburia and positive association with the genera of Ruminococcus, Fusobacterium and Blautia. We also discussed several molecular mechanisms of microbiota effects in the onset and progression of T2D.
In the third chapter, we used animal models, systems biology, & in-vitro systems to infer, validate and identify the potential probiotic microbiota. We also showed that these microbes improved glucose metabolism by promoting healthy hepatic mitochondria, hepatic beta-oxidation, and lipid composition. We used the western diet-induced mouse model of T2D as the in-vivo animal model. Using the data-driven approach called Transkingdom Network analysis, developed in Shulzhenko and Morgun labs, we modeled the host-microbiome interactions under WD. This network analysis inferred us about the microbes that can potentially contribute to the altered host metabolism due to WD. We identified two species of Lactobacillus as the beneficial microbes that improve glucose metabolism in mice. Similarly, one species of Rombutsia was identified and tested as a microbe that worsens the western diet's effect in mice. Data from humans also showed the concordant association between these microbes and obesity. Supplementation of Lactobacilli in WD-fed mice improved the fatty acid composition in the liver and systemic glucose metabolism, which led us to explore the effect of Lactobacilli in the liver. Gene expression and electron microscopy of the liver showed that Lactobacilli can act on hepatic mitochondria and improves hepatic beta-oxidation, lipid composition, and systemic glucose metabolism. We then performed a metabolomics study of serum collected from mono-colonized mice with Lactobacillus or germ-free mice. This analysis revealed glutathione as a major metabolome that mediates the beneficial effect of Lactobacillus. We also established an in-vitro system and validated that glutathione, indeed, upregulates the well-established genes associated with mitochondrial functions and homeostasis such as mt-Atp6, Ndufv1, Mfn1, Foxo3, Gabpa, Usp50, Ifitm3 & Rai12. Their expression was also upregulated in the liver of mice supplemented with Lactobacilli. Hence, this study identified probiotic strains that can prevent T2D and established a mechanistic insight into the mode of action. Overall, our studies on host-microbiota interactions in diet induced diabetic mouse model identified microbiota-mitochondria crosstalk as one of the mechanisms by which the commensal bacteria promote beneficial effects on the host. Our work
also identified a pathobiont, Rombutsia ilealis, that promotes glucose intolerance potentially via different mechanisms than Lactobacilli. These findings from our studies echoed the idea that targeted microbial therapies rather than attempting to restore the overall composition of microbiota could be an effective way to develop microbiota based therapeutics.