Table 1.

Clinical investigations on the gut microbiota and anastomotic leak (AL).

Reference	Year	No. of cases	Sample type	Time point	Method	Design of the study	Main findings
Mizuta et al. (2016)	2016	60	Stool	Before surgery and 1 week after surgery	16S rRNA sequencing	Patients undergoing colorectal resection were randomized to two groups before resection. One group received a probiotic supplement (Bifidobacterium longum BB536), preoperatively for 7–14 days and postoperatively for 14 days, while the other group received no intervention as a control. Postoperative infectious complications were the primary endpoint.	The proportions of fecal bacteria changed significantly in both groups. Actinobacteria increased in the probiotic group, Bacteroidetes and Proteobacteria increased in the control group, and Firmicutes decreased in both groups. Four patients in the control group, but none in the probiotic group, experienced postoperative anastomotic leakage (p = 0.10).
van Praagh et al. (2016)	2016	16 (AL = 8)	Anastomosis site tissue	During surgery	16S rRNA sequencing	Eight patients who developed AL requiring reintervention and eight matched controls without AL were compared.	Lachnospiraceaeis is higher, while the microbial diversity levels were lower in AL patients.
van Praagh et al. (2019)	2019	123 (AL = 29)	Anastomosis site tissue	During surgery	16S rRNA sequencing	Twenty-nine patients who developed AL were matched by sex, age, and preoperative chemotherapy and radiotherapy with 94 patients who did not.	In non-C-seal patients, AL development was related to low microbial diversity and high abundance of Bacteroidaceae and Lachnospiraceae. In C-seal patients, where the AL rates were slightly higher (25% vs. 17%), association with the gut microbiota composition was hardly detectable. A few opportunistic pathogenic taxa were associated with AL in C-seal patients, especially Prevotella oralis.
Shogan et al. (Shogan, et al., 2015)	2015	11 (AL = 1)	Distal and proximal end swabs	During surgery	16S rRNA sequencing	Patients undergoing colon surgery consented to participate in the study. When the operating surgeon removed the colon sample, the distal and proximal ends were immediately swabbed for 16S rRNA analysis and aerobic culture.	Disturbed microbial community structure and membership distribution in anastomotic tissues among the 11 patients. One patient who received reoperation for AL showed a ratio of Proteobacteria to Bacteroidetes of 3:1, indicative of a highly imbalanced microbiota.
Komen et al. (2014)	2014	243 (AL = 19)	Abdominal drain fluid	Postoperative days 1–5	RT-PCR for specific microbes	Patients enrolled in a multicenter prospective observational study underwent left-sided colorectal resection for malignant and benign tumors. In all patients, an intra-abdominal drain was placed during the operation. The quantitative results of the RT-PCR on days 2–5 were compared to those of day 1 to detect changes.	Increased Escherichia coli concentration was found in AL patients on days 4 and 5. For Enterococcus faecalis, this result was found for days 2–4, with the highest on day 3.
Palmisano et al. (2020)	2020	48 (AL = 5)	Stool samples	Before surgery and after neoadjuvant treatment	16S rRNA sequencing	Colorectal cancer patients were divided into an anastomotic leak group and an uneventful recovery group.	AL patients showed increased Acinetobacter lwoffii and Hafnia alvei, an array of bacterial species that promoted dysbiosis. Non-AL patients showed increased Faecalibacterium prausnitzii and Barnesiella intestinihominis, which have a protective function.
Mima et al. (2020)	2020	256	Fresh frozen tissues of colorectal cancer	During surgery	RT-PCR for specific microbes	This retrospective case–control study included colorectal cancer patients who underwent elective colorectal resection.	Patients with high Bifidobacterium levels are at high risk of anastomotic leakage.