# The perioperative microbiome of patients undergoing rectal cancer surgery: A pilot study

## TL;DR

This pilot study explores how the gut microbiome changes in rectal cancer patients before, during, and after surgery, highlighting shifts in bacterial composition and diversity.

## Contribution

The study is one of the first to examine the perioperative microbiome dynamics in rectal cancer surgery patients using multiple sample types and time points.

## Key findings

- Postoperative samples showed increased Enterococcus and Streptococcus compared to preoperative and intraoperative samples.
- Postoperative samples had significantly lower alpha diversity than preoperative and intraoperative samples.
- Preoperative oral antibiotics altered intraoperative microbiome composition and reduced postoperative diversity.

## Abstract

The gut microbiome plays a crucial role in health and disease, and its involvement in postoperative complications like anastomotic leakage (AL) is of growing interest. Despite substantial preclinical evidence linking microbiome alterations to surgical outcomes, human studies are scarce, particularly those exploring the perioperative dynamics of the gut microbiome beyond a single time point. This descriptive, hypothesis‐generating pilot study aims to elucidate the perioperative changes in the faecal microbiome of patients undergoing rectal cancer surgery.

Seventeen patients from Amsterdam University Medical Centers participated in the IMARI‐study and the IntAct‐trial between April 2020 and April 2022. All patients in these studies underwent rectal resection for malignancy with a primary anastomosis, with or without a diverting ileostomy. Samples collected included preoperative stool, intraoperative anastomotic colonic tissue and swab and postoperative stool. Bacterial DNA was extracted and analysed using 16S rRNA gene sequencing.

An increase in Enterococcus and Streptococcus was observed postoperatively compared to preoperative and intraoperative samples. Postoperative samples showed a significant decrease in alpha diversity compared to preoperative and intraoperative samples. Beta diversity analysis revealed distinct clustering of postoperative stool and ileostomy samples. Preoperative oral antibiotics significantly altered the intraoperative microbiome composition and reduced postoperative alpha diversity.

This pilot study reveals significant perioperative shifts in the gut microbiome of rectal cancer patients. These findings underscore the importance of considering microbiome dynamics perioperatively when designing and interpreting studies that correlate the microbiome with clinical outcomes. However, the conclusions should be viewed as preliminary and require confirmation in larger studies, including causal relation, to postoperative outcomes.