Objective:To study the effects of broad-spectrum antibiotics and induced antibiotic-resistant bacteria on the efficacy of 5-fluorouracil (5-FU) chemotherapy for mice with colon cancer and to investigate the underlying mechanisms associated with anti-tumor immune responses.Methods:BALB/c mice were subcutaneously injected with CT26 colon cancer cells and randomized into four groups: tumor-bearing control group, antibiotic group treated with ampicillin, streptomycin and colistin, 5-FU group and anitibiotic+ 5-FU group. Tumor volumes and body weights were measured and recorded. Seven days after the last 5-FU treatment, the percentages of splenic immune cell subpopulations and proliferated CD8 + T cells after co-culturing with CT26 were analyzed by flow cytometry. Gut microbiota composition was detected by 16S rRNA sequencing and the bacteria in mesenteric lymph nodes (mLN) were isolated and cultured. Bone marrow-derive macrophages were stimulated with identified bacteria and the expression of M1 and M2 polarization markers were assessed by quantitative PCR. The proliferation of CD8 + T cells co-cultured with bacteria-treated macrophages was analyzed by flow cytometry. In addition, tumor-bearing mice were treated with 5-FU and oral gavage of bacteria isolated from antibiotic+ 5-FU group or PBS. Tumor volumes, gut microbiota composition and the percentages of proliferated CD8 + T cells co-cultured with CT26 were assessed. Results:Tumor volumes were larger and body weights were lower in the antibiotic+ 5-FU group than in the 5-FU group. The percentages of CD4 + T cells, CD8 + T cells and neutrophils did not varied significantly after using antibiotics, however, the percentage of monocytes was increased in the antibiotic group. The percentage of proliferated tumor-specific CD8 + T cells in the antibiotic+ 5-FU group was decreased compared with that in the 5-FU group. Compared with the control group and 5-FU group, antibiotic usage was associated with the changes in gut microbiota composition with decreased α diversity indexes. Escherichia coli, Klebsiella pneumonia, and Proteus mirabilis were isolated from mLNs of the antibiotic group, 5-FU group and antibiotic+ 5-FU group, respectively. Bone marrow-derived macrophages stimulated with Proteus mirabilis expressed arginase at a high level, which was a M2 polarization marker of macrophage, and associated with the decreased percentage of proliferated CD8 + T cells after co-culturing. Bacteria of the genus Proteus were enriched in the gut microbiota of 5-FU-treated tumor-bearing mice with the oral gavage of Proteus mirabilis. Although no significant inhibitory effect on tumor growth was observed, the oral gavage of Proteus mirabilis was associated with the decreased percentage of proliferated tumor-specific CD8 + T cells in vitro. Conclusions:Broad-spectrum antibiotics inhibited the efficacy of chemotherapy and the proliferation of tumor-specific CD8 + T cells, in which antibiotic-resistant bacteria might be involved.