Exploration of the distribution of intestinal bacteria in mice under normal and intestinal leakage (IBD) conditions.

Abstract

Introduction. The distribution of micro-organisms in healthy organisms remains a subject of debate. Emerging evidence revealed the colonization of microbial communities in multiple anatomical sites previously considered sterile under homeostatic conditions. However, the mechanistic relationship between compromised intestinal epithelial barrier integrity and subsequent translocation of gut-resident bacteria into systemic circulation has yet to be comprehensively elucidated.Hypothesis/Gap Statement. Under intestinal leakage, gut micro-organisms can break through the intestinal barrier and then translocate to other organs.Aim. This study investigates the distribution of micro-organisms in healthy organisms to determine whether gut bacteria translocate to sterile organs only under the condition of intestinal leakage using GFP-labelled Escherichia coli (GFP-E. coli) tracing.Methodology. Female C57BL/6 mice (5 weeks old) were administered either a glacial acetic acid enema [inflammatory bowel disease (IBD) group] or a sterile normal saline enema [normal control (NC) group]. All mice were subsequently gavaged with GFP-E. coli. HE staining and Alcian blue staining were performed to evaluate the colon injury. The expression levels of intestinal tight junction proteins (ZO-1 and occludin) were tested by reverse transcription quantitative PCR and immunofluorescence staining. The distribution of GFP-E. coli in multiple organs was assessed through bacterial culture, confocal microscopy and PCR.Results. In the IBD mice, mucopolysaccharide accumulation levels (P<0.01) and tight junction proteins ZO-1 (P<0.001) and occludin (P<0.01) in the colon were significantly decreased compared with the NC group. Bacterial culture showed that there was no GFP-E. coli in the blood, heart, liver, spleen, lungs, kidneys or oviducts of normal mice, while the number of GFP-E. coli colonies in the blood (219 c.f.u. ml^-1), liver (2.39×10⁵ c.f.u. ml^-1) and lungs (2.50×10⁸ c.f.u. ml^-1) of the IBD mice was significantly higher than that of the NC group. The confocal microscopy and PCR results also showed that the number of GFP-E. coli in the liver and lungs of the IBD group was significantly higher than that of the NC group (P<0.001).Conclusion. Healthy mice maintain a sterile microenvironment in the blood, heart, liver, spleen, lungs, kidneys and oviducts. However, compromised intestinal barrier integrity facilitates microbial translocation from the intestinal lumen into the blood, liver and lungs. This study advances our understanding of endogenous infections caused by IBD, demonstrating the crucial role of intestinal permeability in bacterial infections.