Colorectal cancer (CRC) is one of the most common and deadly cancers worldwide. About 1 in 4 people with CRC already have cancer spread (metastasis) when first diagnosed, and about half develop spread during their illness. Recent research shows that bacteria living in the gut and even within tumors might play an important role in how cancer spreads.

The goal of this study is to better understand how bacteria might influence the spread of colorectal cancer. The main questions the investigators aim to answer are:

Are there differences in bacteria between people whose cancer has spread and those whose cancer has not spread? Could certain bacteria help predict which cancers might spread?

To answer these questions, the investigators will:

Collect different types of samples from participants:

Tumor tissue Normal tissue near the tumor Tissue from where cancer has spread Stool samples before surgery Study the bacteria in these samples using advanced testing methods Compare bacterial patterns between different groups

People can take part in this study if they:

Are between 18 and 75 years old Have colorectal cancer confirmed by doctors Have not taken antibiotics recently Do not have immune system problems

This research may help us:

Understand why some colorectal cancers spread Find new ways to predict which cancers might spread Develop better treatments for colorectal cancer

Inclusion Criteria:

• Age between 18 and 75 years
• Pathologically confirmed colorectal cancer
• Clearly defined clinical staging: including imaging or pathologically confirmed metastatic colorectal cancer (stage M1) and colorectal cancer without distant metastasis (stage M0)
• Expected survival ≥ 3 months
• Voluntary participation and signed informed consent

Exclusion Criteria:

• Use of probiotics, antibiotics, or immunosuppressive agents within 1 month before surgery
• Preoperative complete intestinal obstruction or gastrointestinal perforation
• Intraoperative gastrointestinal perforation or tumor rupture
• Previous history of gastrointestinal surgery (excluding colorectal cancer surgery, appendectomy, and cholecystectomy) or concurrent severe gastrointestinal diseases such as inflammatory bowel disease
• Concurrent active systemic immune or infectious diseases, including severe allergies, rheumatoid arthritis, systemic lupus erythematosus, viral hepatitis, acquired immunodeficiency syndrome, etc.
• Concurrent unhealed primary malignant tumors
• Severe organ dysfunction or failure
• Other conditions deemed unsuitable for this study by the investigator

This study will enroll patients aged 18-75 years with pathologically confirmed colorectal cancer treated at the Sixth Affiliated Hospital of Sun Yat-sen University. Participants will be divided into two main groups: those with peritoneal metastasis (metastasis group) and those without distant metastasis (non-metastasis group). The study population will include both male and female patients who meet all inclusion criteria and none of the exclusion criteria. All participants must have clearly defined clinical staging and be scheduled for surgical treatment, with an expected survival of at least 3 months. Patients with recent use of antibiotics, probiotics, or immunosuppressants, those with intestinal obstruction or perforation, and those with other severe comorbidities will be excluded.