Infections remain a prevalent complication after major abdominal surgery. The common belief that most surgical site infections (SSIs) following elective surgery with modern antiseptic techniques are due to intraoperative contamination is still not confirmed. Therefore, alternative mechanisms for SSI development, such as the Trojan Horse theory-which suggests that pathogens from distant sites like the gastrointestinal tract may cause postoperative infections-should be explored. This study aims to analyze the preoperative microbiome of surgical patients' gut and oral cavities and assess whether microorganisms found there are present at the infection site. Additionally, this study will investigate a panel of biomarkers for predicting postoperative infections.

Infections remain a significant concern following major abdominal surgery. The prevailing notion that most surgical site infections (SSIs) after elective surgery with modern antiseptic techniques are solely caused by intraoperative contamination remains unconfirmed. Therefore, alternative mechanisms for SSI development, such as the Trojan Horse theory-which suggests that pathogens from distant sites like the gastrointestinal tract may contribute to postoperative infections-require thorough investigation. This longitudinal observational study aims to either support or challenge the Trojan Horse theory.

This study will enroll patients undergoing major abdominal surgery for confirmed or suspected cancer. Biological samples from stool, the oral cavity, and infection sites will be collected for sequencing and microbiome analysis to evaluate the presence of pathogens potentially responsible for postoperative infections originating from the gastrointestinal tract. Additionally, blood samples will be collected to identify predictive biomarkers associated with the development of postoperative infections.

Inclusion Criteria:

1. The patient is scheduled to undergo a major resection-type abdominal surgery due to cancer of the esophagus, stomach, pancreas, liver, bile ducts, colon or rectum.
2. Patient is willing to participate.
3. Age ≥ 18 years.

Exclusion Criteria:

1. Pregnancy.
2. Previous surgical resection of the digestive tract, excluding appendectomy and/or cholecystectomy.
3. Anticipated operation with preventive ileostomy.
4. The operation is planned to be performed as a matter of urgency.
5. Antibiotic therapy ≤1 month. before surgery.
6. Chronic inflammatory bowel disease (non-specific ulcerative colitis, Crohn's disease) or radiation or other colitis of origin.
7. During the last year, the patient suffered from Cl. difficile colitis or was Cl. difficile carrier, had salmonellosis or others intestinal infections.
8. During the last year, the patient used (> 3 months) pre-/pro-/(syn)biotics.
9. During the last year, the patient has been taking proton pump inhibitors continuously (> 6 months).

Patients scheduled to undergo major abdominal surgery due to the cancer of the esophagus, stomach, pancreas, liver, bile ducts, colon, or rectum due to cancer will be eligible to participate.