ObjectiveProbioticLactococcus lactisis known to confer health benefits to humans. Here, we aimed to investigate the role ofL. lactisin colorectal cancer (CRC).DesignL. lactisabundance was evaluated in patients with CRC (n=489) and healthy individuals (n=536).L. lactiswas isolated from healthy human stools with verification by whole genome sequencing. The effect ofL. lactison CRC tumourigenesis was assessed in transgenicApcMin/+mice and carcinogen-induced CRC mice. Faecal microbiota was profiled by metagenomic sequencing. Candidate proteins were characterised by nano liquid chromatography-mass spectrometry. Biological function ofL. lactisconditioned medium (HkyuLL 10-CM) and functional protein was studied in human CRC cells, patient-derived organoids and xenograft mice.ResultsFaecalL. lactiswas depleted in patients with CRC. A newL. lactisstrain was isolated from human stools and nomenclated asHkyuLL 10. HkyuLL 10supplementation suppressed CRC tumourigenesis inApcMin/+mice, and this tumour-suppressing effect was confirmed in mice with carcinogen-induced CRC. Microbiota profiling revealed probiotic enrichment includingLactobacillus johnsoniiinHkyuLL 10-treated mice.HkyuLL 10-CM significantly abrogated the growth of human CRC cells and patient-derived organoids. Such protective effect was attributed toHkyuLL 10-secreted proteins, and we identified that α-mannosidase was the functional protein. The antitumourigenic effect of α-mannosidase was demonstrated in human CRC cells and organoids, and its supplementation significantly reduced tumour growth in xenograft mice.ConclusionHkyuLL 10suppresses CRC tumourigenesis in mice through restoring gut microbiota and secreting functional protein α-mannosidase.HkyuLL 10administration may serve as a prophylactic measure against CRC.