Comparison 1. MBP+oAB vs. MBP.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
1.1 SSI	16	3917	Risk Ratio (M‐H, Random, 95% CI)	0.56 [0.42, 0.74]
1.2 Incisional SSI	10	3054	Risk Ratio (M‐H, Random, 95% CI)	0.47 [0.33, 0.66]
1.3 Organ/space SSI	10	3054	Risk Ratio (M‐H, Random, 95% CI)	0.65 [0.44, 0.98]
1.4 Anastomotic leakage	10	2356	Risk Ratio (M‐H, Random, 95% CI)	0.60 [0.36, 0.99]
1.5 Mortality	3	639	Risk Ratio (M‐H, Random, 95% CI)	0.87 [0.27, 2.82]
1.6 Mild postoperative complications according to Clavien‐Dindo(I + II)	3	695	Risk Ratio (M‐H, Random, 95% CI)	0.76 [0.29, 2.00]
1.7 Severe postoperative complications according to Clavien‐Dindo (III + IV)	3	695	Risk Ratio (M‐H, Random, 95% CI)	1.00 [0.59, 1.70]
1.8 Incidence of postoperative ileus	6	2013	Risk Ratio (M‐H, Random, 95% CI)	0.89 [0.59, 1.32]
1.9 Length of hospital stay	3	621	Mean Difference (IV, Random, 95% CI)	‐0.19 [‐1.81, 1.44]
1.10 Side effects of Intervention ‐ Nausea/Vometing	3	545	Risk Ratio (M‐H, Random, 95% CI)	2.22 [1.33, 3.72]
1.11 Side effects of Intervention ‐ Abdominal pain	3	545	Risk Ratio (M‐H, Random, 95% CI)	1.79 [0.67, 4.82]
1.12 C. difficile‐related diarrhoea	3	1547	Risk Ratio (M‐H, Random, 95% CI)	0.89 [0.24, 3.34]
1.13 SSI_Subgroup analysis regarding surgery indication	16	3915	Risk Ratio (M‐H, Random, 95% CI)	0.56 [0.42, 0.74]
1.13.1 Only malignant surgical indications	9	2160	Risk Ratio (M‐H, Random, 95% CI)	0.68 [0.48, 0.96]
1.13.2 Only benign surgical indications	2	380	Risk Ratio (M‐H, Random, 95% CI)	0.45 [0.17, 1.22]
1.13.3 Both malignant and benign surgical indications	5	1375	Risk Ratio (M‐H, Random, 95% CI)	0.43 [0.26, 0.71]
1.14 SSI_Subgroup analysis regarding the type of surgery	14	3577	Risk Ratio (M‐H, Random, 95% CI)	0.59 [0.44, 0.80]
1.14.1 Colon and rectum resections	13	3461	Risk Ratio (M‐H, Random, 95% CI)	0.62 [0.46, 0.83]
1.14.3 Rectum resections only	1	116	Risk Ratio (M‐H, Random, 95% CI)	0.16 [0.04, 0.67]
1.15 SSI_Subgroup analysis regarding the surgical approach	13	3075	Risk Ratio (M‐H, Random, 95% CI)	0.58 [0.41, 0.82]
1.15.1 Minimally invasive surgical approach	3	1062	Risk Ratio (M‐H, Random, 95% CI)	0.71 [0.46, 1.10]
1.15.2 Open surgical approach	5	684	Risk Ratio (M‐H, Random, 95% CI)	0.74 [0.39, 1.40]
1.15.3 Both open and minimally invasive surgical approach	5	1329	Risk Ratio (M‐H, Random, 95% CI)	0.41 [0.24, 0.72]
1.16 SSI_Subgroup analysis regarding the duration of mechanical bowel preparation	15	3871	Risk Ratio (M‐H, Random, 95% CI)	0.56 [0.42, 0.74]
1.16.1 Bowel preparation on the day before surgery	13	3547	Risk Ratio (M‐H, Random, 95% CI)	0.56 [0.40, 0.77]
1.16.2 Bowel preparation over several days before the operation	2	324	Risk Ratio (M‐H, Random, 95% CI)	0.54 [0.30, 0.99]
1.17 SSI_Subgroup analysis regarding the agent combination of oral antibiotics	16	3915	Risk Ratio (M‐H, Random, 95% CI)	0.56 [0.42, 0.74]
1.17.1 Combination of metronidazole and an aminoglycoside (neomycin or kanamycin)	9	2717	Risk Ratio (M‐H, Random, 95% CI)	0.55 [0.39, 0.78]
1.17.2 Combination of erythromycin and an aminoglycoside (neomycin or kanamycin)	3	757	Risk Ratio (M‐H, Random, 95% CI)	0.62 [0.40, 0.94]
1.17.3 Other oral antibiotic combinations	4	441	Risk Ratio (M‐H, Random, 95% CI)	0.46 [0.13, 1.59]
1.18 SSI_Subgroup analysis regarding the duration of intravenous antibiotic prophylaxis	16	3915	Risk Ratio (M‐H, Random, 95% CI)	0.56 [0.42, 0.74]
1.18.1 Only perioperative intravenous antibiotic prophylaxis	9	2290	Risk Ratio (M‐H, Random, 95% CI)	0.51 [0.36, 0.72]
1.18.2 Continuation of perioperative intravenous antibiotic prophylaxis for 24 hours	3	824	Risk Ratio (M‐H, Random, 95% CI)	0.46 [0.19, 1.09]
1.18.3 Continuation of perioperative intravenous antibiotic prophylaxis beyond 24 hours	4	801	Risk Ratio (M‐H, Random, 95% CI)	0.76 [0.44, 1.30]