Inactivated Bacteroides Fragilis for Prevention of Chemotherapy-Induced Diarrhea

Inclusion Criteria: 1. Voluntarily sign the informed consent form, be able to comply with the protocol and have the capacity to complete relevant procedures. 2. Aged ≥ 18 years old (based on the date of signing the ICF), regardless of gender. 3. Patients with malignant tumors confirmed by pathology or cytology. 4. Patients scheduled to receive the following first-line treatments at the standard dose specified in the protocol: * Cohort 1: Patients with advanced colorectal cancer receiving the FOLFIRI regimen (irinotecan + fluorouracil + leucovorin); * Cohort 2: Patients with breast cancer receiving neratinib treatment. 5. ECOG performance status score of 0-1 at the start of the study. 6. Expected survival time ≥ 12 weeks. 7. Subjects of childbearing potential (including males and females) agree to adopt effective contraceptive measures approved by the investigators (e.g., intrauterine device, contraceptive pills or condoms) during the trial and within 3 months after the trial ends. Female subjects of childbearing potential must have a negative result in serum human chorionic gonadotropin (hCG) test. Exclusion Criteria: 1. History of altered bowel habits or abnormal stool characteristics prior to screening; or use of laxatives within 7 days prior to randomization; or use of antidiarrheal agents within 48 hours prior to randomization. 2. Complicated with diseases causing diarrhea, including but not limited to inflammatory bowel disease (e.g., ulcerative colitis and Crohn's disease), suspected or confirmed infectious diarrhea, irritable bowel syndrome, celiac disease, etc. 3. Receipt of any chemotherapeutic agents, cetuximab or PD-1 antibodies other than the chemotherapy/treatment regimen specified in the protocol during the trial. 4. Previous or concurrent abdominopelvic radiotherapy during the trial period. 5. Medical history of chronic use of laxatives (≥ 30 non-consecutive days). 6. Need for antibiotic treatment during the trial or long-term use of antibiotics prior to randomization. 7. Patients with stomas (temporary stomas should be closed for at least 6 months prior to randomization). 8. Presence of unhealed surgical wounds, ulcers or fractures. 9. Known history of human immunodeficiency virus (HIV) infection (positive HIV antibody test). 10. History of allergy or suspected allergy to antidiarrheal drugs (e.g., loperamide and its components). 11. Suspected or confirmed history of alcohol or drug abuse. 12. Pregnant or lactating women. 13. Concurrent participation in other clinical trials. 14. Other conditions deemed inappropriate for study participation by the investigator.