Fecal Coprococcus, Hidden behind Abdominal Symptoms in Patients with Small Intestinal Bacterial Overgrowth

Abstract

Background: Small intestinal bacterial overgrowth (SIBO) is the presence of an abnormally excessive amount of bacterial colonization in the small bowel. Hydrogen and methane breath test has been widely applied as a non-invasive method for SIBO. However, the positive breath test representative of bacterial overgrowth could also be detected in asymptomatic individuals. Methods: To explore the relationship between clinical symptoms and gut dysbiosis, and find potential fecal biomarkers for SIBO, we compared the microbial profiles between SIBO subjects with positive breath test but without abdominal symptoms (PBT) and healthy controls (HC). Results: Fecal samples were collected from 63 SIBO who complained of diarrhea, distension, constipation or abdominal pain, 36 PBT and 55 HC. Increased taxonomic diversity and decreased functional diversity were consistent with the progression of SIBO. At the genus level, significantly decreased Bacteroidesand increased Coprococcus_2 were observed, and unique Butyrivibrio could ferment multiple carbohydrates producing hydrogen and hydrogen sulfide. There was a significant correlation between Coprococcus_2 and the severity of abdominal symptoms. Differently, The unique Veillonella, Escherichia-Shigella, Barnesiella and Tyzzerella_3 in PBT group were related to amino acid fermentation. Interestingly, the co-occurrence network density of PBT is the largest indicating a complicated interaction of genera. The Euclidean distance between paired networks using either the betweenness centrality or the degree distribution showed that PBT is closer to SIBO. Conclusions: Increased taxonomic diversity and decreased functional diversity were consistent with the progression of SIBO. Butyrivibrio and Coprococcus_2 abundance along with lower Bacteroides contributed to more noticeable discomfort of SIBO patients. The enriched Coprococcus may be one of the potential biomarkers of SIBO. Represented by Veillonella, asymptomatic PBT objects exhibited a different microbiome spectrum associated with the fermentation of amino acids and peptides rather than carbohydrates. The network of PBT was more stable which may play a protective role, but it deserved further attention in view of the shared essential “bridged” genera with SIBO.