100 patients undergoing elective left colonic or rectal resection with a primary anastomosis will be recruited pre-operatively throughout the study period. This study will not affect or delay the intended treatment for study participants.

Patients will undergo serial endoscopic examination of the anastomosis post-operatively. Blood, urine, stool, and mucosal biopsies will be serially collected.

This interventional arm will demonstrate the safety of re-introducing ileostomy effluent into the downstream (distal) limb of an ileostomy. The preliminary data will enable exploration of the association between microbiome and post-operative function and enable adequate powering of future interventional studies.

A subgroup of 20 patients undergoing a resection with a covering ileostomy will be recruited to the intervention arm. Complete healing of the colorectal anastomosis will first be confirmed by water-soluble contrast enema 8 weeks post-operatively (this is standard practice). Patients will be taught how to inject the output from the proximal ileostomy limb into the distal limb (this connects to the colon and thus the colorectal anastomosis) daily until the ileostomy closure date.

Patients will undergo water soluble contrast enema to confirm anastomotic healing 8 weeks post operatively as per standard practice at Imperial.

Patients will be educated in how to reintroduce ileostomy output to the distal (downstream) loop of their ileostomy. They will be asked to reintroduce 50ml of ileostomy output using a conventional long-tipped bladder syringe daily until surgery to close the stoma.

Healing of the colorectal anastomosis when assessed endoscopically - with no visible defects in the anastomosis.

Anastomotic leak detected either by radiological imaging (CT scanning) or a direct return to theatre

Changes in microbial abundance and diversity at the anastomosis. The nature of any change is as yet not investigated and unpredictable - this is an observational outcome measure.