## 4 Discussion  
Obesity is a global health issue whose prevalence has been increasing rapidly. Severe differences in microbiota composition have been reported by several studies in obese people. Currently, the most common treatment for obesity is bariatric surgery. In this context, it has been described that the human gut microbiota composition and functionality perform a crucial role during obesity as well as in the recovery after medical intervention [30; 31; 32; 33; 34]. For example, [PERSON] et al. (2015) demonstrated that two types of bariatric surgery, namely Roux-en-Y gastric bypass and vertical banded gastroplasty, produced long-term alterations of the gut microbiome independently of BMI and that these alterations modulated host metabolism and fat mass deposition. However, the studies that have been carried out to understand how the microbiota is involved in the development and establishment of obesity have paid little attention to viruses. The virome is starting to be recognized as a study object inside of microbiome research, where the identification of the viral community composition can be associated with adverse outcomes for the human host, whereas other states may be characteristic of health [35]. Some studies support evidence that prokaryotic viruses can have an impact on human health by affecting the structure and function of the bacteria community and its symbiotic interaction with humans [36; 37]. Shotgun metagenomics provides a great quantity of data, which, however, are not always explored completely. Here, we used previously published sequences obtained from public databases from gut microbiome patients before and after undergoing gastric surgery to survey what occurred with the gut viral content. Our results show that during obesity, the alpha diversity index is lower compared with subjects that undergo obesity intervention, suggesting that the viral context turns out to be more diverse after treatment. Moreover, we identified some viral biomarkers in each condition, which can be associated with the bacterial changes reported in the works where the sequences used come from. To date, bioinformatics databases may have ignored a large proportion of unknown phages in the metagenomic data, limiting the amount of information obtained from these kinds of studies. In a recent article, the authors managed to assemble a Metagenomic Gut Virus catalog, which comprised 189,680 viral genomes by using 11,810 published human stool metagenomes. They found that over 75% of genomes represented dsDNA phages, and they also identified 54,118 candidate viral species, of which, 92% were not found on previous databases studied [20]. These findings suggest that there is a low representation  
Figure 2: Heatmap of the core virome in pre- (**A**) and post-obesity (**B**) groups. Taxa prevalence is denoted red for higher values and blue for lower values. of known viruses in public databases, which could generate biases during metagenomic studies, such as the one carried out in this work. Moreover, there is a great diversity of RNA viruses that are out of focus of shotgun DNA sequencing, pointing out the need to use metatranscriptomic analysis to survey the presence of RNA viruses in the gut microbiome. Given the above, optimized methods to characterize RNA-based viruses are needed to piece together the whole picture of the gut virome. In this sense, a recent study aimed to characterize gut virome alterations in both an individual with obesity and type 2 diabetes (T2 DM) and an individual without it [38]. The authors found that obese patients with T2 DM in particular, had a decreased gut viral richness and diversity when they were compared with lean control subjects. Moreover, they revealed a trans-kingdom correlation between phages (viruses) and bacteria in lean controls, but this correlation was weak in the case of obese patients [38]. The cited study points out that the virome may play an important role in the development of obesity and T2 DM.