Gut Microbiota and Bariatric Surgery

*Natalia Bastón-Paz, Manuel Ponce-Alonso, José Avendaño, María Garriga and Rosa del Campo*

### **Abstract**

The gut microbiota comprise all the living organisms in our intestine. Microbiota has key roles in metabolic homeostasis, digestion and nutrient metabolism protection against pathogens or modulation of the immune system. Advances in techniques such as metagenomics or metabolomics have expanded our knowledge of the intestinal ecosystem. Beyond genetic, behavioral, or environmental factors, alterations of gut microbiota parameters such as composition, diversity, or metabolites including shortchain fatty acids, have shown to be associated with cardiovascular comorbidities. In this chapter, we described the role of the gut microbiota in obesity and type 2 diabetes pathophysiology, and the changes it undergoes during bariatric surgery, as well as explored the possibilities of modifying the microbiome to obtain potential clinical benefits.

**Keywords:** gut microbiota, obesity, type 2 diabetes, bariatric surgery, diet, probiotics, fecal microbiota transplant

## **1. Introduction**

The human organism is a complex biological system composed of cells belonging to three domains: Eukarya, Bacteria, and Archaea, in addition to viruses [1, 2]. The microbiome has been considered as the last human organ [1] and can be defined as the whole genomic and metabolomic content of the microbial community that coexists and interacts with our cells [3]. The gut microbiota, the most complex and abundant microbiome [4], is the focus of this chapter because of its direct relationship with obesity and bariatric surgery.

The gut microbiota has traditionally been studied by culturing, with the aim of identifying and characterizing single isolated microorganisms related to acute or chronic infections [5]. Although culturing techniques are improving with various strategies [6, 7], their resolution is insufficient because most bacteria are uncultivable. Today, microbiota studies are focusing on the overall ecosystem, not only individual microorganisms; and to address the real effect of microbiota colonization on human health over prolonged periods.

Knowledge of the human microbiome has exploded in the last two decades due to the development of genomic strategies based on marker genes such as 16S ribosomal

#### **Figure 1.**

 *Schematic representation of the usual workflow in the study of the bacterial composition in a sample by NGS of 16S rDNA amplicons.* 



#### **Table 1.**

*Summary of the most common techniques available for the study of the microbiome.*

DNA (rDNA), and massive sequencing techniques, also known as next-generation sequencing (NGS) (**Figure 1**). Multi-omics technologies, including metagenome, metatranscriptome, metaproteome, and metabolome approaches provide valuable information on microbial functions [8]. The high potential of combining various "omics" techniques to analyze host-microorganism interactions allows us to dissect the molecular mechanisms by which microbiomes influence human health. A summary of the characteristics of each technique is shown in **Table 1**. Bioinformatics analysis of these big data allows us to characterize the ecological biodiversity of a given microbial community and draw conclusions [9]. Nonetheless, this field of research is beyond the scope of this chapter.
