Outcomes	Arm 1		Arm 2
	Event or mean ± SD	Total	Event or mean ± SD	Total
Guida 2017	Synbiotic		Placebo
GI function: change in microbiotacomposition (decrease in total plasma p‐cresol from baseline) Measured by high‐performance liquid chromatography, median (IQR) Time: 15 and 30 days post‐operation	Baseline: 3.0 (1.6 to 4.5)   15 days post‐operation: 1.79 (0.7 to 2.43); decrease by 40%   30 days post‐operation: 2.3 (0.9 to 2.72); decrease by 33%   P < 0.01 versus baseline; P < 0.01 versus placebo	22	Baseline: 4.2 (2.4 to 5.8)   15 days post‐operation: 3.82 (0.72 to 6.0); remained stable   30 days post‐operation: 4.4 (3.0 to 6.4); remained stable	12
GI function: faecal characteristics (instrument not reported) Time: at 30 days post‐operation	"No changes observed"	22	"No changes observed"	12
Adverse events (not described) Time: at 30 days post‐operation	GI side effects: 0 Borborygmus: 13 Abdominal pain: "virtually absent" "Well tolerated"	22	Abdominal pain: "virtually absent" "Well tolerated"	12
Organ function measures: kidney function (eGFR mL/min) Time: at 30 days post‐operation	Baseline: 50.6 ± 17.6 30 days post‐operation: 53.5 ± 16.0   "No significant change"	22	Baseline: 58.5 ± 24.0 30 days post‐operation: 57.3 ± 22.1   "No significant change"	12
Organ function measures: kidney function (albumin mg/dL) Time: at 30 days post‐operation	Baseline: 4.53 ± 0.3 30 days post‐operation: 4.35 ± 0.3   "No significant change"	22	Baseline: 4.60 ± 0.4 30 days post‐operation: 4.35 ± 0.3   "No significant change"	12
Liu 2015f	Probiotics		Placebo
GI function: post transplant GI side effects Time: "within 1 week postop"	17.24% "Significantly higher than control group (P < 0.05)"	Not reported	42.31%	Not reported
Organ function measures: serum total bilirubin Time: postoperative day 8	"Significantly lower than control group (P < 0.05)"	Not reported	Not reported	Not reported
Organ function measures: ALT Time: postoperative day 8	"Significantly lower than control group (P < 0.05)"	Not reported	Not reported	Not reported
Organ function measures: serum albumin and prealbumin Time: postoperative day 5	"Significantly higher than control group (P < 0.05)"	Not reported	Not reported	Not reported
Organ function measures: serum albumin and prealbumin Time: postoperative day 8	"Significantly higher than control group (P < 0.05)"	Not reported	Not reported	Not reported
Infection: post‐transplant infection incidence Time: "within 1 week postop"	41.38% "Significantly higher than control group (P < 0.05)"	Not reported	69.23%	Not reported
Orr 2016	Probiotics		Placebo
BP: mean change in BP Time: at 24 weeks	"No significant difference between treatment arms"	15	"No significant difference between treatment arms"	15
Organ function measures: liver (mean change in Fibroscan CAP (dB/m), change in fatty liver) Time: at 24 weeks		15		15
CVD (mean change in lipids) Time: at 24 weeks		15		15
Rayes 2002	Synbiotics		Prebiotics
GI function: time to first bowel movement postoperative (days) Time: postoperative period	2.2 (SD not reported)	31	2.4 (SD not reported)	32
Graft health: acute liver rejection Time: postoperative period	10 (2 muromonab‐CD3 therapy)	31	15 (3 muromonab‐CD3 therapy)	32
Adverse events: number reporting abdominal side effects (distension, cramps, diarrhoea) Time: postoperative period	6	31	11	32
Serious adverse events: number reporting non‐infectious complications Time: postoperative period	Total: 16 Acute rejections: 10 (2 muromonab‐CD3 therapy) Rate of kidney insufficiencies requiring HD: 2 Relaparotomy: 4 (haemorrhage or biliary leak)	31	Total: 19 Acute rejections: 15 (3 muromonab‐CD3 therapy) Rate of kidney insufficiencies requiring HD: 4 Relaparotomy: 2 (haemorrhage or arterial stenosis)	32
Death: perioperative deaths Time: postoperative period	0	31	0	32
Organ function measures: kidney function (serum albumin) Time: postoperative period	No numerical data available	31	No numerical data available	32
Organ function measures: kidney function (SCr (mg/dL)) Time: post‐operation day 5	1.1 ± 0.09	31	Not reported	32
Organ function measures: kidney function (SCr (mg/dL)) Time: post‐operation day 10	1.3 ± 0.1	31	Not reported	32
Organ function measures: kidney function (BUN)	No numerical data available	31	No numerical data available	32
Use of immunosuppressants (postoperative immunosuppression) Time: post‐operation day 1	CSA: 16 TAC: 15 Prednisolone: 31	31	CSA: 13 TAC: 19 Prednisolone: 31	32
Infection: number with postoperative infections Time: postoperative period	4	31	11	32
Infection: total number of postoperative infections Time: postoperative period	4	31	17	32
Infection: type of postoperative infections Time: postoperative period	Cholangitis: 2 Pneumonia: 1 Sepsis: 0 UTI: 0 Wound infection: 0 Others: 1	31	Cholangitis: 8 Pneumonia: 4 Sepsis: 0 UTI: 3 Wound infection: 0 Others: 2	32
Infection: type of postoperative infections isolated bacteria Time: postoperative period	Enterococci: 1 E. coli: 0 Staphylococci: 1 Klebsiella: 0 None: 2	31	Enterococci: 8 E. coli: 1 Staphylococci: 3 Klebsiella: 1 None: 5	32
Infection: mean cumulative length of antibiotic therapy (days) Time: postoperative period	7 ± 7	31	12 ± 18	32
Other: changes in leukocyte count Time: postoperative period	"Lower in (synbiotics) group than in the other groups, but the difference was not statistically significant"	31	Not reported	32
Other: cellular immune variables: CD4/CD8 Time: postoperative period	"Course of the CD4/CD8 ratio was higher in (synbiotics) group but the difference was not statistically significant (P =  0.06)"	31	Not reported	32
Rayes 2005	Synbiotics		Prebiotics
Graft health: incidence of acute rejection	6	33	7	33
Graft health: initial non‐function of liver followed by re‐transplantation	0	33	1	33
Adverse events: number reporting diarrhoea Time: at 30 days post‐operation	Diarrhoea: 3 Abdominal cramps: 5 Abdominal distension and cramps: 0	33	Diarrhoea: 4 Abdominal cramps: 0 Abdominal distension and cramps: 3	33
Serious adverse events: number reporting non‐infectious complications Time: post‐operation	Total: 12 Biliary tract stenosis or fistulas treated endoscopically with stents: 4 Lienalis‐steal syndrome requiring intervention with angiography: 4 Abdominal haemorrhage requiring relaparotomy: 2 AKI: 2	33	Total: 4 Abdominal haemorrhage requiring relaparotomy: 2 AKI: 1 Initial non‐function of the liver followed by re‐transplantation: 1	33
Death: perioperative death Time: at 30 days post‐operation	0	33	0	33
Use of immunosuppressants: routine immunosuppression (triple regimen of prednisolone and TAC or CSA with induction therapy with an IL‐2 antibody) Time: at 30 days post‐operation	"No differences"	33	"No differences"	33
Infection: length of antibiotic therapy without prophylaxis (days) Time: at 30 days post‐operation	0.1 ± 0.1 (P < 0.05) "Significantly shorter compared to prebiotics"	33	3.8 ± 0.9	33
Infection: incidence (no. patients with an infection) Time: at 30 days post‐operation	1 (P < 0.05)	33	16	33
Infection: total types of infection (not per patient) Time: at 30 days post‐operation	Urinary tract: 1 Wound: 0 Pneumonia: 0 Cholangitis: 0   Isolated bacteria ‐ E. faecalis/faecium: 1 ‐ E. coli: 0 ‐ Enterobacter cloacae: 0 ‐ Pseudomonas aeruginosa: 0 ‐ Staphylococcus aureus: 0	‐‐	Urinary tract: 12 Wound: 1 Pneumonia: 1 Cholangitis: 2   Isolated bacteria ‐ E. faecalis/faecium: 11 ‐ E. coli: 3 ‐ Enterobacter cloacae: 2 ‐ Pseudomonas aeruginosa: 2 ‐ Staphylococcus aureus: 1	‐‐
Infection: number days with fever (< 38.5ºC) Time: during 30 days post‐operation	1	33	22	33
Footnotes: AKI: acute kidney injury; ALT: alanine aminotransferase; BP: blood pressure; BUN: blood urea nitrogen; CAP: Controlled Attenuated Parameter; CSA: cyclosporin; CVD: cardiovascular disease; eGFR: estimated glomerular filtration rate; GI: gastrointestinal; HD: haemodialysis; IL‐2: interleukin‐2; IQR: interquartile range; SCr: serum creatinine; TAC: tacrolimus; UTI: urinary tract infection