Rectal Cancer Surgery Rapidly Reshapes the Gut Microbiome : BJS Open | Jun 2026

Introduction:

The gut microbiome plays a fundamental role in maintaining intestinal homeostasis through its effects on metabolism, immune regulation, and resistance to pathogenic colonization. Increasing evidence suggests that disruptions in microbial composition may influence surgical recovery, infectious complications, and long-term outcomes in colorectal cancer. However, the immediate effects of rectal cancer surgery and perioperative interventions on the gut microbiome remain poorly characterized.

Problem Statement:

Patients undergoing rectal cancer surgery are exposed to multiple factors that can alter microbial ecology, including bowel preparation, antibiotics, selective digestive decontamination, dietary changes, and ileostomy formation. Understanding how these interventions affect the microbiome is essential for developing strategies that may improve postoperative recovery and reduce complications. However, the relative contribution of these perioperative factors has remained unclear.

Summary:

This analysis from the IMARI trial provides important insights into the early microbiome changes occurring after rectal cancer surgery. The investigators demonstrated that surgery itself is the dominant driver of postoperative microbial disruption, leading to a marked reduction in microbial diversity and a substantial shift in overall microbial composition. These changes were characterized by an increase in facultative anaerobic organisms, particularly Enterococcus and Klebsiella, alongside a decline in beneficial obligate anaerobic bacteria, including several members of the Firmicutes phylum. Ileostomy formation emerged as an additional major determinant of microbiome alteration, exerting a greater influence on microbial composition than selective digestive decontamination. While selective digestive decontamination affected specific bacterial taxa, its overall impact on microbial diversity was comparatively modest. These findings highlight the profound ecological consequences of rectal cancer surgery and suggest that postoperative microbial imbalance may represent a potentially modifiable factor influencing surgical outcomes. The study lays the foundation for future investigations exploring whether microbiome-directed interventions, including optimized antibiotic stewardship, nutritional strategies, probiotics, or targeted microbial therapies, can improve recovery and clinical outcomes in patients undergoing rectal cancer surgery.