• gut microbiota
• bariatric surgery
• severe obesity
• microbial gene richness

The gut microbiota is considered as one of the relevant contributors involved in the complex causes of obesity and related metabolic disorders. Indeed, a plethora of publications describing the composition of the microbiota in subjects suffering from overweight or obesity as well as type 2 diabetes and cardiometabolic diseases are available (for review1 2). Conversely, the characteristics of the microbiome in extreme forms of obesity (ie, severe and morbid Body Mass Index >35–40 kg/m²) has been barely described.

Bariatric surgery has dramatically increased worldwide and is now appearing as a solution to reduce cardiovascular risks and diabetes, thereby leading to consider this surgical procedure as a ‘metabolic surgery’.3 4 Interestingly, one of the key features of the microbiome characterising obesity is the so-called low microbial gene richness, which strongly correlates with metabolic disorders such as low-grade inflammation, insulin resistance, adipocyte size but also chance of success to respond to dietary intervention.5 6 However, we lack comprehensive data exploring the impact of bariatric surgery on severe obesity and the evolution of the microbiome (ie, composition, gene richness and activity) at several time points after the surgical intervention. Indeed, although some studies demonstrated changes in gut microbiota composition after bariatric surgery, most of the investigations were performed using different techniques (eg, qPCR, 16S rDNA sequencing) in a small cohort of subjects and only very few of them used whole metagenome sequencing.

In GUT, Aron-Wisnewski and colleagues have recently filled this gap …