[Antibiotics exacerbate Chlamydia infection of the lung and other organs and influence the gut bacterial community composition in mice].

Objective: To investigate the effects of antibiotics on Chlamydia muridarum (CM) infection in the lung and other organs of mice, the immune response, and gut bacteria. Methods: C57BL/6J mice were randomly divided into antibiotic and control groups. The control group mice were given drinking water, while the antibiotic group mice were given an aqueous solution of vancomycin and gentamicin to drink for 2 weeks, in addition to intragastric administration of the solution twice. Both groups were intranasally infected with CM and the weight of the mice was monitored continuously. On the 10th day after infection, the mice were euthanized and the CM loads of several organs were analyzed. Immunofluorescence and pathological analyses of the lung tissue were performed. Serum titers of CM-specific IgG and subtypes thereof were evaluated by enzyme-linked immunosorbent assay (ELISA), and the levels of TNF-α, IFN-γ, IL-5, and IL-13 in the spleen were analyzed by flow cytometry. Mouse fecal samples were collected for qPCR analysis of the gut bacterial community composition. SPSS 26.0 software was used for data statistics and analysis of variance (ANOVA) was used for comparison between groups. Results: After intranasal infection with CM, the weight of the mice in the two groups decreased from the second day; that of the control group decreased rapidly and then recovered after the sixth day, while that of the antibiotic group decreased slowly and then recovered after the eighth day. The CM loads of the lung, heart, liver, spleen, and kidney in the antibiotic group were higher (F=11.06, F=20.87, P<0.01;F=7.09, F=8.07, F=5.66, P<0.05), whereas the CM loads in the lower gastrointestinal tract (jejunum, ileum, cecum, colon, and rectum) were lower (F=37.23, F=40.90, F=18.07, F=13.53, F=12.51, P<0.01) than in the control group. The CM numbers in the antibiotic group (8.94±4.13 infectious units [IFU]/low power field) were higher than the control group (3.73±1.49 IFU/low power field) (F=7.058, P<0.05), as determined by immunofluorescence. Pathology showed significant infiltration of inflammatory cells and greater pathological damage; the score of the antibiotic group (3.12±0.59) was higher than that of the control group (1.80±0.69) (F=10.47, P<0.05). Serum CM-specific IgG titers in the antibiotic group were lower (F=5.95, P<0.05), the TNF-α levels in CD4⁺and CD8⁺T cells in the spleen were higher (F=5.93, F=9.98, P<0.05), and the IL-13 levels were lower (F=5.97, P<0.05; F=11.70, P<0.01) than those in the control group. After antibiotic treatment, the proportion of Bacteroides in the gut bacterial population decreased significantly (F=97.57, P<0.01), whereas the proportion of Firmicutes increased (F=154.51, P<0.01). After CM infection, the proportions of Bacteroides, α-Proteobacteria, and Clostridium perfringens decreased in the control group (F=4.50, F=5.29, F=5.29, P<0.05), whereas that of Akkermansia increased (F=9.58, P<0.01). In the antibiotic group, the proportions of Bacteroides and Firmicutes decreased (F=4.85, P<0.05; F=25.35, P<0.01), whereas those of Verrucomycetes and Akkermansia increased (F=11.44, F=15.46, P<0.01). Conclusions: Antibiotics induced CM infection in the lung and other organs of the mice, aggravated pathological damage in the lungs, decreased serum IgG titers, increased the TNF-α levels and decreased the IL-13 levels of CD4⁺and CD8⁺T cells in the spleen, and caused changes in the composition of the gut bacterial community.