Bariatric surgery (BS) is associated with changes in the composition of the gut microbiota; however, whether these changes mediate weight loss response and metabolic outcomes after surgery remains to be determined.


50 patients who underwent Roux-en-Y gastric bypass surgery or sleeve gastrectomy were enrolled in this pilot study. The gut microbiota composition was characterized before surgery and one and six months after surgery using whole genome sequencing. Mixed linear model and multivariate analysis were used to assess changes in the gut microbiota composition and functional gene pathways following surgery.


Multivariate analysis revealed that BS was associated with significant shifts in the composition of gut microbiota (p-value=0.01, r2=0.05), although most of these changes were driven by non-dominant gut bacterial phyla, including Verrucomicrobia, Fusobacteria, Actinobacteria, and Proteobacteria. At the genus level, Streptococcus, Klebsiella, and Rothia were significantly increased at one and six months post-surgery compared to baseline (p-value<0.0001, r2 = 0.21-0.37). Bifidobacterium and Faecalibacterium were significantly decreased at one month post-surgery (p-value<0.0001, r2 = 0.15-0.21) but tended to increase from one to six months following surgery. In parallel to compositional changes, microbial gene pathways involved in micronutrient and macronutrient metabolism were significantly changed after one and six months after surgery (p-value= 0.01, r2=0.07).


The preliminary data showed significant changes in the composition and function of the gut microbiome as early as one month after surgery. Additional patient recruitment is underway to determine if these changes in the gut microbiome could be predictive of weight outcomes after surgery.