Results
Protective effect of the gut microbiota during APEC-inducedchicken colibacillosis
To study the role the gut microbiota played in chicken colibacillosis, we first applied the broad-spectrum antibiotic (Abx) to deplete the gut microbiota. Administration of Abx resulted in a considerable reduction of richness (Fig. 1A) and diversity (Fig. 1B) of gut microbiota in chickens. Furthermore, principle coordinate analysis (PCoA) plot based on Unweighted Unifrac revealed that there was clear separation of clusters between control group compared with Abx-treated group chicken (Fig. 1C). In addition, Venn diagrams showing numbers of genera in Abx-treated group were significantly reduced (Fig. 1D). These results suggested that treatment of Abx significantly changes the structure of gut microbiota, and reduced the gut microbiota diversity and richness in chicken.
Furthermore, chicken from control and gut microbiota-depleted groups were intranasally challenged with APEC to induce the model of colibacillosis. Infection of APEC results in severe pathological injury of lung (Fig. 2A), and colon, heart, liver, and spleen (see supplementary fig. S1A), increased the production of proinflammatory cytokines TNF-α, IL-1β, and IL-6 both at gene and protein levels (Fig. 2B-D and supplementary Fig. S1B-D), as well as increased the bacterial outgrowth in the lung (Fig. 1E). In addition, infection of APEC also increased the viscera index of lung, heart, liver, spleen, and kidney (see supplementary Table S1). However, these inflammatory injuries induced by APEC were more pronounced in gut microbiota-depleted chicken when compared to the infected with control chicken (Fig. 2A-E and supplementary Fig. S1A-D and Table S1). Moreover, the concentration of total proteins in bronchoalveolar lavage fluid (BALF) and the expression of tight junction protein of lung tissues were measured to evaluate pulmonary air-blood barrier permeability. Infection of APEC increased the levels of total protein in BALF, and the levels of total protein were higher in Abx+APEC group than APEC group (Fig. 2F). In addition, the expression of junction protein of air-blood barrier, including occludin, claudin-3, and ZO-1 were significantly reduced after infected with APEC, and these changes are more obviously in gut Abx+APEC group (Fig. 2G-I). There results suggested that depletion of gut microbiota increase the severity of colibacillosis and increased the permeability of air-blood barrier induced by APEC.