*Purpose: Although recent evidence shows that modifications of gut microbiota by antibiotics (Abx) influences liver allograft function, its compositional role in orthotopic liver transplantation (OLT) remains unknown. Here we aimed to determine how recipient Abx pretreatment affects hepatic ischemia-reperfusion injury (IRI) and OLT outcomes.

*Methods: C57BL/6 recipient mice were pretreated with amoxicillin (Amx) or metronidazole (Mtz) prior to transplantation of Balb/c livers subjected to ex-vivo hepatic cold storage (18 h/4C). Groups of C57BL/6 mice were fed Amx or Mtz (days -11 to -1) and then conditioned with or without fecal microbiota transfer

(FMT) by gavage from naïve C57BL/6 mice (200 mg/ml; day -1 and 0) prior to OLT. Genomic DNA of fecal samples was then isolated and subjected to microbiome 16S rRNA gene sequencing.

*Results: Taxonomic classification revealed significant differences between the Amx-treated and untreated groups in the family-, genus, and species-level analyses. Faith’s Phylogenetic Diversity index, Shannon Index, and Chao1 Index all revealed significant declines in taxon richness, species diversity, and overall abundance after Amx-treatment alone (p<0.0001 in each index). By contrast, FMT re-established the microbiota to Sham and OLT-levels, suggesting that normal commensal flora is antagonistic to liver IRI-resistance. The bacterial phyla Verrucomicrobia and Firmicutes showed the largest decline of representation in the Amx-treated microbiota compared to Sham-treated mice. Remarkably, Mtz pretreated OLT recipients experienced severe IRI, with increased sALT (8853±353.5 vs. 5486±1345 U/L, p<0.05), augmented Suzuki’s score, TUNEL+ cells, and graft-infiltrating neutrophils/macrophages, and decreased hepatic EP4 expression vs. controls. However, adjunctive FMT in Mtz-treated OLT recipients mitigated IR-stress and restored hepatocellular function to control levels (sALT: 4301±2145U/L, ns vs. control, p<0.05 vs. Mtz group).

*Conclusions: This study documents the striking benefits of microbiota modulation in a clinically relevant mouse OLT model; and identifies a specific microbiota genus profile as a target for novel therapeutic manipulation in IRI-OLT.

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