Background: The dysbiosis is defined as a disturbed symbiotic relationship between microbiota and the host and can cause a pro-inflammatory imbalance impairing the healing process at anastomotic level. The aim of this study is to detect, in fecal samples collected in the preoperative time, a peculiar microbiota composition that could predict the onset of colorectal anastomotic leakage.

Materials and methods: We compared gut microbiota of healthy patients (Group A) and patients with colorectal cancer eligible for surgery (Group B). Group B was divided into patients who developed anastomotic leak (Group BL) and patients who had uneventful recovery (Group BNL). Stool samples were collected before surgery and after neoadjuvant treatment.

Results: We analyzed stool samples from 48 patients, 27 belonging to Group A and 21 to Group B. In Group B, five patients developed anastomotic leakage (Group BL). Compared to healthy subjects, Group B showed a moderate increase of Bacteroidetes and Proteobacteria, a moderate reduction of Firmicutes and Actinobacteria, and a statistically significant reduction of Faecalibacterium prausnitzii. Group BL patients showed an array of bacterial species which promoted dysbiosis, such as Acinetobacter lwoffii and Hafnia alvei. Group BNL patients showed that bacterial species like Faecalibacterium prausnitzii and Barnesiella intestinihominis have a protective function.

Conclusions: The bacterial flora in subjects with colorectal cancer is statistically different compared to healthy patients. The presence of preoperative aggressive bacteria and the lack of protective strains has strengthened the hypothesis that a peculiar microbiota composition could represent a risk factor for the occurrence of anastomotic leakage.

Keywords: Anastomotic leakage; Colorectal cancer; Gut microbiota; Microbiome.