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Background & Aims: Rifaximin-alpha is efficacious for the prevention of recurrent hepatic encephalopathy (HE), but its mechanism of action remains unclear. We postulated that rifaximin-alpha reduces gut microbiota-derived endotoxemia and systemic inflammation, a known driver of HE. Methods: In a placebo-controlled, double-blind, mechanistic study, 38 patients with cirrhosis and HE were randomised 1:1 to receive either rifaximin-alpha (550 mg BID) or placebo for 90 days. Primary outcome: 50% reduction in neutrophil oxidative burst (OB) at 30 days. Secondary outcomes: changes in psychometric hepatic encephalopathy score (PHES) and neurocognitive functioning, shotgun metagenomic sequencing of saliva and faeces, plasma and faecal metabolic profiling, whole blood bacterial DNA quantification, neutrophil toll-like receptor (TLR)-2/4/9 expression and plasma/faecal cytokine analysis. Results: Patients were well-matched: median MELD (11 rifaximin-alpha vs. 10 placebo). Rifaximin-alpha did not lead to a 50% reduction in spontaneous neutrophil OB at 30 days compared to baseline (p = 0.48). However, HE grade normalised (p = 0.014) and PHES improved (p = 0.009) after 30 days on rifaximin-alpha. Rifaximin-alpha reduced circulating neutrophil TLR-4 expression on day 30 (p = 0.021) and plasma tumour necrosis factor-alpha (TNF-alpha) (p <0.001). Rifaximin-a suppressed oralisation of the gut, reducing levels of mucin-degrading sialidase-rich species, Streptococcus spp, Veillonella atypica and parvula, Akkermansia and Hungatella. Rifaximin-alpha promoted a TNF-alpha-and interleukin17E-enriched intestinal microenvironment, augmenting antibacterial responses to invading pathobionts and promoting gut barrier repair. Those on rifaximin-alpha were less likely to develop infection (odds ratio 0.21; 95% CI 0.05-0.96). Conclusion: Rifaximin-alpha led to resolution of overt and covert HE, reduced the likelihood of infection, reduced oralisation of the gut and attenuated systemic inflamm, QC 20220304