Background and Aim Chronic inflammation is a major contributor to the initiation and progression of cancers. Lactobacillus helveticus NS8, which was originally separated from fermented koumiss, exhibited anti‐inflammatory functions in our prior studies. In this study, NS8 was investigated for its potential to prevent colitis‐associated colorectal cancer (CAC).

Methods The protective effects of NS8 against CAC was explored by employing the azoxymethane plus dextran sodium sulfate‐induced carcinogenesis mouse model. The prevalences of T cells expressing specific inflammatory cytokines were measured by flow cytometry at the early stage of CAC. Inflammatory modulation by NS8 was also tested in the Caco2‐Raw264.7 cell co‐culture system. The alternations in the intestinal microbiota following the health–inflammation–cancer sequence were analyzed by 16S rDNA sequencing.

Results Oral intake of NS8 lactobacilli clearly reduced tumor number and the degree of hyperplasia. The increased proliferation of enterocytes at the early stage of CAC was significantly suppressed by NS8, while the level of apoptosis was elevated. The anticancer effects of NS8 were associated with its anti‐colitis outcomes before tumor formation. NS8 significantly suppressed the activation of NF‐κB and upregulated the anti‐inflammatory cytokine IL‐10. Further analysis revealed the marked downregulation of IL‐17‐producing T cells by NS8. Furthermore, NS8 modulated intestinal dysbiosis by promoting beneficial commensal microbes while suppressing cancer‐associated microbes. Notably, Bacteroides acidifaciens was the most sensitive commensal bacteria to NS8 intervention.