Methods are provided herein for preventing, delaying the onset of or reducing the progression of colorectal tumorigenesis in a subject identified as at risk of colorectal tumorigenesis, comprising adjusting the composition of gut microbiota in the subject via administering to the subject a composition comprising Bacteroides bacteria or administering to the subject a therapeutically effective amount of a pharmaceutical composition comprising zwitterionic polysaccharide (ZPS). In another aspect, methods are provided for treating or ameliorating a colorectal cancer in a subject, comprising adjusting the composition of gut microbiota in the subject having the colorectal cancer. In a further aspect, methods are provided for relieving gastrointestinal (GI) distress of a subject having a colorectal condition, comprising: determining the colorectal condition of the subject； and relieving GI distress in the subject by adjusting the composition of gut microbiota in the subject.
