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The gut microbiota and the liver. Pathophysiological and clinical implications

Open AccessPublished:November 26, 2012DOI:https://doi.org/10.1016/j.jhep.2012.11.023

      Keywords

      Introduction

      The term microbiota is used to describe the complete population of microorganisms that populate a certain location, such as the gut, and is preferred to the term flora as the former incorporates not just bacteria but also archaea, viruses, and other microorganisms, such as protozoa. Though the potential role of the microbiota (through such concepts as “the putrefactive principle associated with faeces” and “intestinal toxins”) in the pathogenesis of systemic disorders has been recognized since antiquity, a firm scientific basis for a role for the gut microbiome in liver disease did not emerge until the middle of the last century with the recognition of the relationship between hepatic coma and the absorption of nitrogenous substances from the intestine [
      • Phillips G.B.
      • Schwartz R.
      • Gabuzda Jr., G.J.
      • Davidson C.S.
      The syndrome of impending hepatic coma in patients with cirrhosis of the liver given certain nitrogenous substances.
      ]. This was followed by the description of abundant coliforms in the small intestine of cirrhotics [
      • Martini G.A.
      • Phear E.A.
      • Ruebner B.
      • Sherlock S.
      The bacterial content of the small intestine in normal and cirrhotic subjects: relation to methionine toxicity.
      ] and the role of bacteria was clinched by trials demonstrating that antibiotics led to clinical improvement in hepatic encephalopathy (HE) [
      • Phear E.A.
      • Ruebner B.
      • Sherlock S.
      • Summerskill W.H.
      Methionine toxicity in liver disease and its prevention by chlortetracycline.
      ]. Subsequently, these same gut-derived bacteria were implicated in another complication of chronic liver disease and portal hypertension, spontaneous bacterial peritonitis. Most recently, more credence has been given to a suggestion that has lingered in the background for decades, namely, that the gut microbiota might play a role in the pathogenesis or progression of certain liver diseases, including alcoholic liver disease [
      • Szabo G.
      • Bala S.
      • Petrasek J.
      • Gattu A.
      Gut-liver axis and sensing microbes.
      ], non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steato-hepatitis (NASH) [
      • Abu-Shanab A.
      • Quigley E.M.
      The role of the gut microbiota in nonalcoholic fatty liver disease.
      ], total parenteral nutrition (TPN)/intestinal failure-related liver disease (IFALD) [
      • Quigley E.M.M.
      • Marsh M.N.
      • Shaffer J.L.
      • Markin R.S.
      Hepatobiliary complications of total parenteral nutrition.
      ], and primary sclerosing cholangitis (PSC) [
      • Terjung B.
      • Spengler U.
      Atypical p-ANCA in PSC and AIH: a hint toward a “leaky gut”?.
      ], either through the direct effects of bacteria or their products, via inflammatory mediators such as tumor necrosis factor α (TNF), whose release had been triggered by constituents of the microbiota, or, as in the case of primary sclerosing cholangitis (PSC), through cross-reactivity between microbial antigens and human tissue components (e.g., atypical anti-nuclear cytoplasmic antibodies (p-ANCA), in PSC, recognize both tubulin beta isoform 5 in human neutrophils, and the bacterial cell division protein FtsZ) [
      • Terjung B.
      • Söhne J.
      • Lechtenberg B.
      • Gottwein J.
      • Muennich M.
      • Herzog V.
      • et al.
      P-ANCAs in autoimmune liver disorders recognise human beta-tubulin isotype 5 and cross-react with microbial protein FtsZ.
      ]. Indeed, inflammatory mediators have also been implicated in the development and maintenance of the hyperdynamic circulation that is a feature of portal hypertension [
      • Quigley E.M.M.
      Gastrointestinal dysfunction in liver disease – gut–liver interactions revisited.
      ], in impairing liver function and contributing to haemostatic failure [
      • Thalheimer U.
      • Triantos C.K.
      • Samonakis D.N.
      • Patch D.
      • Burroughs A.K.
      Infection, coagulation and variceal bleeding in cirrhosis.
      ]. It is in these contexts that modulation of the microbiota has emerged as a potential therapeutic strategy in the management of liver disease.

      The microbiota in the pathogenesis of liver disease and its complications

      The human intestinal microbiota consists of trillions of microorganisms including 150–200 prevalent and ∼1000 less common bacterial species, harbouring over 100-fold more genes than those present in the human genome [
      • Turnbaugh P.J.
      • Ley R.E.
      • Mahowald M.A.
      • Magrini V.
      • Mardis E.R.
      • Gordon J.I.
      An obesity-associated gut microbiome with increased capacity for energy harvest.
      ,
      • Qin J.
      • Li R.
      • Raes J.
      • Arumugam M.
      • Burgdorf K.S.
      • Manichanh C.
      • et al.
      A human gut microbial gene catalogue established by metagenomic sequencing.
      ,
      • Clemente J.C.
      • Ursell L.K.
      • Parfrey L.W.
      • Knight R.
      The impact of the gut microbiota on human health: an integrative view.
      ]. This microbiota consists predominantly of bacteria, but also contains archaea, protozoa, and viruses; all have co-evolved with the human host. The microbiota performs vital functions essential to health maintenance, including food processing, digestion of complex indigestible polysaccharides and synthesis of vitamins [
      • Backhed F.
      Programming of host metabolism by the gut microbiota.
      ,
      • Marques T.M.
      • Wall R.
      • Ross R.P.
      • Fitzgerald G.F.
      • Ryan C.A.
      • Stanton C.
      Programming infant gut microbiota: influence of dietary and environmental factors.
      ]. Furthermore, it secretes a range of bioactive metabolites with diverse functions, ranging from inhibition of pathogens, metabolism of toxic compounds to modulation of host metabolism [
      • Marques T.M.
      • Wall R.
      • Ross R.P.
      • Fitzgerald G.F.
      • Ryan C.A.
      • Stanton C.
      Programming infant gut microbiota: influence of dietary and environmental factors.
      ]. A perturbed microbiota has been implicated in an ever-increasing list of disorders in humans, from necrotizing enterocolitis (NEC) in infants, to obesity, diabetes, metabolic syndrome, irritable bowel syndrome (IBS), and inflammatory bowel disease (IBD) in adults [
      • Kosloske A.M.
      Pathogenesis and prevention of necrotizing enterocolitis- a hypothesis based on personal observation and a review of the literature.
      ,
      • Ley R.E.
      • Turnbaugh P.J.
      • Klein S.
      • Gordon J.I.
      Microbial ecology – human gut microbes associated with obesity.
      ,
      • Kassinen A.
      • Krogius-Kurikka L.
      • Makivuokko H.
      • Rinttila T.
      • Paulin L.
      • Corander J.
      • et al.
      The fecal microbiota of irritable bowel syndrome patients differs significantly from that of healthy subjects.
      ,
      • Peterson D.A.
      • Frank D.N.
      • Pace N.R.
      • Gordon J.I.
      Metagenomic approaches for defining the pathogenesis of inflammatory bowel diseases.
      ,
      • Flint H.J.
      • O’Toole P.W.
      • Walker A.W.
      Special issue: the human intestinal microbiota.
      ,
      • Jeffery I.B.
      • Quigley E.M.
      • Ohman L.
      • Simrén M.
      • O’Toole P.W.
      The microbiota link to Irritable Bowel Syndrome: an emerging story.
      ]. However, little is known about the physiological impact the microbiota has on host health, an area which has recently been described as “one of the hottest areas in medicine” [
      • Shanahan F.
      The gut microbiota in 2011: translating the microbiota to medicine.
      ].
      Table 1 lists a number of mechanisms that might underpin the role of the microbiota in the pathogenesis of liver disease and/or its complications.
      Table 1The microbiota in the pathogenesis of liver disease and its complications. Possible factors.
      NAFLD, non-alcoholic fatty liver disease; NASH, non-alcoholic steatohepatitis; IFALD, intestinal failure-associated liver disease; SBP, spontaneous bacterial peritonitis; TMA, trimethylamine; TMAO, trimethylamine oxide; PSC, primary sclerosing cholangitis; TNFα, tumor necrosis factor alpha.

      Small intestinal bacterial overgrowth (SIBO)

      Among the various potential contributions of the microbiota to liver disease, small intestinal bacterial overgrowth (SIBO) has been one of the most extensively studied. Indeed, an altered gut microbiota was first noted by Hoefert in chronic liver disease over 80 years ago [
      • Hoefert B.
      Über die bakterienbefunde im duodenalsaft von gesunden und kranken.
      ]; since then SIBO has been documented to be common in liver disease, to correlate with its severity and to be linked to minimal and overt encephalopathy and an increased risk for SBP through translocation across the gut wall [
      • Nieuwenhuijs V.B.
      • Verheem A.
      • van Duijvenbode-Beumer H.
      • Visser M.R.
      • Verhoef J.
      • Gooszen H.G.
      • et al.
      The role of interdigestive small bowel motility in the regulation of gut microflora, bacterial overgrowth, and bacterial translocation in rats.
      ,
      • Husebye E.
      Gastrointestinal motility disorders and bacterial overgrowth.
      ,
      • Yang C.Y.
      • Chang C.S.
      • Chen G.H.
      Small-intestinal bacterial overgrowth in patients with liver cirrhosis, diagnosed with glucose H2 or CH4 breath tests.
      ,
      • Abu Shanab A.
      • Scully P.
      • Crosbie O.
      • Buckley M.
      • O’Mahony L.
      • Shanahan F.
      • et al.
      Small intestinal bacterial overgrowth in non-alcoholic steato-hepatitis; association with toll-like receptor 4 expression and plasma levels of interleukin 8.
      ,
      • Gupta A.
      • Dhiman R.K.
      • Kumari S.
      • Rana S.
      • Agarwal R.
      • Duseja Chawla Y.
      Role of small intestinal bacterial overgrowth and delayed gastrointestinal transit time in cirrhotic patients with minimal hepatic encephalopathy.
      ]. Moreover, factors predisposing to SIBO such as altered small bowel motility, increased intestinal permeability, and delayed gut transit have been reported in patients with liver cirrhosis [
      • Quigley E.M.M.
      The liver and gastrointestinal disease.
      ]. Alterations in gastrointestinal motility in cirrhosis have been variably ascribed to the effects of autonomic dysfunction, altered levels of circulating neuropeptides and the effects of inflammatory mediators on gut muscle and nerve [
      • Quigley E.M.M.
      Gastrointestinal dysfunction in liver disease – gut–liver interactions revisited.
      ,
      • Quigley E.M.M.
      The liver and gastrointestinal disease.
      ]. With regard to intestinal permeability, changes in intestinal tight junctional proteins have been described in cirrhosis and, though the precise pathophysiology of barrier dysfunction is unclear, roles for metabolites of alcohol and/or pro-inflammatory cytokines have been postulated [
      • Szabo G.
      • Bala S.
      • Petrasek J.
      • Gattu A.
      Gut-liver axis and sensing microbes.
      ,
      • Abu-Shanab A.
      • Quigley E.M.
      The role of the gut microbiota in nonalcoholic fatty liver disease.
      ,
      • Terjung B.
      • Spengler U.
      Atypical p-ANCA in PSC and AIH: a hint toward a “leaky gut”?.
      ,
      • Teltschik Z.
      • Wiest R.
      • Beisner J.
      • Nuding S.
      • Hofmann C.
      • Schoelmerich J.
      • et al.
      Intestinal bacterial translocation in rats with cirrhosis is related to compromised Paneth cell antimicrobial host defense.
      ,
      • Purohit V.
      • Bode J.C.
      • Bode C.
      • Brenner D.A.
      • Choudhry M.A.
      • Hamilton F.
      • et al.
      Alcohol, intestinal bacterial growth, intestinal permeability to endotoxin, and medical consequences: summary of a symposium.
      ]. Impaired antimicrobial defence mechanisms may further contribute to the development of bacterial translocation in portal hypertension and cirrhosis [
      • Teltschik Z.
      • Wiest R.
      • Beisner J.
      • Nuding S.
      • Hofmann C.
      • Schoelmerich J.
      • et al.
      Intestinal bacterial translocation in rats with cirrhosis is related to compromised Paneth cell antimicrobial host defense.
      ]. Taken together, these observations have led to the development of a schema which links intestinal motility, stasis, increased intestinal permeability and translocation of bacteria and endotoxin to the development of many of the complications of liver disease, as well as in the progression of liver disease, per se (Fig. 1).
      Figure thumbnail gr1
      Fig. 1A schema to explain potential roles of the small intestinal bacterial overgrowth (SIBO) in liver disease and its complications. Impaired motility promotes stasis; quantitative and qualitative changes in the small intestinal microbiota ensue leading to the development of small intestinal bacterial overgrowth (SIBO). Alterations in the integrity of the intestinal barrier result in enhanced permeability and facilitate the translocation of bacteria or their products into the portal circulation. Changes in the enteric microbiota also lead to the enhanced release of pro-inflammatory cytokines which enhance translocation and exert a variety of effects on liver structure and function. If portal-systemic shunting is present, access for the products of translocation and cytokines to the systemic circulation is enhanced.
      A more fundamental role for SIBO has been proposed in both alcoholic liver disease (through the generation of acetaldehyde) [
      • Purohit V.
      • Bode J.C.
      • Bode C.
      • Brenner D.A.
      • Choudhry M.A.
      • Hamilton F.
      • et al.
      Alcohol, intestinal bacterial growth, intestinal permeability to endotoxin, and medical consequences: summary of a symposium.
      ] and NAFLD (by promoting both steatosis and inflammation) [
      • Abu-Shanab A.
      • Quigley E.M.
      The role of the gut microbiota in nonalcoholic fatty liver disease.
      ]. The potential of microbes of enteric origin to induce a progressive and even fatal steato-hepatitis had been recognized some years ago in relation to the liver injury that complicated jejuno-ileal bypass operations for morbid obesity; indeed, that procedure has provided a valuable experimental model for exploring the impact of the microbiota in liver disease.

      Quantitative or qualitative changes in the microbiota

      The advent of molecular techniques based on sequence divergences of the small subunit ribosomal ribonucleic acid (16S rRNA) of bacteria [
      • Fraher M.H.
      • O’Toole P.W.
      • Quigley E.M.M.
      Techniques used to characterise the intestinal microbiota: a guide for the clinician.
      ] now permits the complete delineation of the constituents of the microbiota in health, thereby, revealing the composition of the human gut microbiota from early infancy [
      • Palmer C.
      • Bik E.M.
      • DiGiulio D.B.
      • Relman D.A.
      • Brown P.O.
      Development of the human infant intestinal microbiota.
      ] through to old age [
      • Claesson M.J.
      • Cusack S.
      • O’Sullivan O.
      • Greene-Diniz R.
      • de Weerd H.
      • Flannery E.
      • et al.
      Composition, variability, and temporal stability of the intestinal microbiota of the elderly.
      ,
      • Claesson M.J.
      • Jeffery I.B.
      • Conde S.
      • Power S.E.
      • O’Connor E.M.
      • Cusack S.
      • et al.
      Gut microbiota composition correlates with diet and health in the elderly.
      ]. Following birth, the human intestine is rapidly colonized and factors known to influence colonization include gestational age, mode of delivery (vaginal birth vs. assisted delivery), diet (breast milk vs. formula), level of sanitation, and exposure to antibiotics [
      • Marques T.M.
      • Wall R.
      • Ross R.P.
      • Fitzgerald G.F.
      • Ryan C.A.
      • Stanton C.
      Programming infant gut microbiota: influence of dietary and environmental factors.
      ,
      • Fouhy F.
      • Ross R.P.
      • Fitzgerald G.F.
      • Stanton C.
      • Cotter P.D.
      Composition of the early intestinal microbiota: knowledge, knowledge gaps and the use of high-throughput sequencing to address these gaps.
      ]. The intestinal microbiota of newborn infants is characterized by low diversity and a relative dominance of the phyla Proteobacteria and Actinobacteria; thereafter, the microbiota becomes more diverse with the emergence of the dominance of Firmicutes and Bacteroidetes, which characterises the adult microbiota [
      • Qin J.
      • Li R.
      • Raes J.
      • Arumugam M.
      • Burgdorf K.S.
      • Manichanh C.
      • et al.
      A human gut microbial gene catalogue established by metagenomic sequencing.
      ,
      • Backhed F.
      Programming of host metabolism by the gut microbiota.
      ,
      • Eckburg P.B.
      • Bik E.M.
      • Bernstein C.N.
      • Purdom E.
      • Dethlefsen L.
      • Sargent M.
      • et al.
      Diversity of the human intestinal microbial flora.
      ]. By the end of the first year of life, infants possess a microbial profile distinct for each infant; by 2.5 years of age, the microbiota fully resembles that of an adult in terms of composition [
      • Palmer C.
      • Bik E.M.
      • DiGiulio D.B.
      • Relman D.A.
      • Brown P.O.
      Development of the human infant intestinal microbiota.
      ,
      • Koenig J.E.
      • Spor A.
      • Scalfone N.
      • Fricker A.D.
      • Stombaugh J.
      • Knight R.
      • et al.
      Succession of microbial consortia in the developing infant gut microbiome.
      ]. More recently, three different ‘enterotypes’ have been described in the adult human microbiome [
      • Arumugam M.
      • Raes J.
      • Pelletier E.
      • Le Paslier D.
      • Yamada T.
      • Mende D.R.
      • et al.
      Enterotypes of the human gut microbiome.
      ] These distinct ‘enterotypes’ are dominated by Prevotella, Ruminococcus, and Bacteroides, respectively, and their appearance appears to be independent of sex, age, nationality and body mass index (BMI). Once the microbiota has reached maturity, it is thought to remain stable until old age when changes are seen, possibly related to alterations in digestive physiology and diet [
      • Clemente J.C.
      • Ursell L.K.
      • Parfrey L.W.
      • Knight R.
      The impact of the gut microbiota on human health: an integrative view.
      ,
      • Mariat D.
      • Firmesse O.
      • Levenez F.
      • Guimaraes V.D.
      • Sokol H.
      • Dore J.
      • et al.
      The Firmicutes/Bacteroidetes ratio of the human microbiota changes with age.
      ,
      • O’Toole P.W.
      • Claesson M.J.
      Gut microbiota: changes throughout the lifespan from infancy to elderly.
      ]. Indeed, Claesson et al. were able to identify clear correlations, in the elderly, not only between the composition of the gut microbiota and diet, but also with health status [
      • Claesson M.J.
      • Jeffery I.B.
      • Conde S.
      • Power S.E.
      • O’Connor E.M.
      • Cusack S.
      • et al.
      Gut microbiota composition correlates with diet and health in the elderly.
      ].
      These molecular approaches have, to some extent, been directed at the investigation of the microbiota in liver disease. Chen et al., for example, studied 36 patients with cirrhosis and 24 healthy controls using 454 pyrosequencing of the 16S ribosomal RNA variable region 3 (V3) followed by real-time quantitative polymerase chain reaction (qPCR) and found that, in cirrhosis, Bacteroidetes were reduced, Proteobacteria and Fusobacteria increased, Enterobacteriaceae, Veillonellaceae, Streptococcaceae increased, and Lachnospiraceae reduced [
      • Chen Y.
      • Yang F.
      • Lu H.
      • Wang B.
      • Chen Y.
      • Lei D.
      • et al.
      Characterization of fecal microbial communities in patients with liver cirrhosis.
      ]. Some of these changes correlated with clinical parameters such as the Child-Turcotte-Pugh score. Several of these findings have been confirmed by Bajaj et al. who went on to show that Veillonellaceae were more abundant among cirrhotics with hepatic encephalopathy (HE) than those without [
      • Bajaj J.S.
      • Ridlon J.M.
      • Hylemon P.B.
      • Thacker L.R.
      • Heuman D.M.
      • Smith S.
      • et al.
      Linkage of gut microbiome with cognition in hepatic encephalopathy.
      ]. As an extension of the evolving concept of the microbiota–gut–brain-axis, which includes the suggestion that the microbiota can influence cognition and behaviour, these authors also described an association between certain bacterial families, cognition and inflammation in HE. The finding of an increased abundance of members of the family Enterobacteriaceae in cirrhosis by these and other authors [
      • Lu H.
      • Wu Z.
      • Xu W.
      • Yang J.
      • Chen Y.
      • Li L.
      Intestinal microbiota was assessed in cirrhotic patients with hepatitis B virus infection. Intestinal microbiota of HBV cirrhotic patients.
      ] is of interest given that this family includes such important Gram-negative pathogens as Salmonella, Shigella, Yersinia pestis, Klebsiella and, especially, Escherichia coli, a common isolate in SBP and bacteremia in chronic liver disease. Others have documented, not only a decrease in total bifidobacterial counts, but also further variations in specific Bifidobacterium species (e.g., Bifidobacterium dentium increased; Bifidobacterium longum decreased) in chronic liver disease [
      • Xu M.
      • Wang B.
      • Fu Y.
      • Chen Y.
      • Yang F.
      • Lu H.
      • et al.
      Changes of fecal Bifidobacterial species in adult patients with hepatitis B-Virus-induced chronic liver diseases.
      ].

      Interactions with diet and nutrients

      Several mechanisms have been identified relevant to the involvement of the microbiota in the pathogenesis of NAFLD/NASH. In particular, a role for the microbiota and its interaction with diet in the pathogenesis of obesity per se has been extensively investigated [
      • Bäckhed F.
      • Ley R.E.
      • Sonnenburg J.L.
      • Peterson D.A.
      • Gordon J.I.
      Host-bacterial mutualism in the human intestine.
      ,
      • Ley R.E.
      • Bäckhed F.
      • Turnbaugh P.
      • Lozupone C.A.
      • Knight R.D.
      • Gordon J.I.
      Obesity alters gut microbial ecology.
      ], and pertinent findings include the ability of Gram-negative anaerobes, such as Bacteriodes thetaiotamicron, to cleave most glycosidic linkages, degrade plant polysaccharides and, thereby, supply the host with 10–15% of its calorific requirement [
      • Turnbaugh P.J.
      • Ley R.E.
      • Mahowald M.A.
      • Magrini V.
      • Mardis E.R.
      • Gordon J.I.
      An obesity-associated gut microbiome with increased capacity for energy harvest.
      ,
      • Ley R.E.
      • Turnbaugh P.J.
      • Klein S.
      • Gordon J.I.
      Microbial ecology – human gut microbes associated with obesity.
      ,
      • Bäckhed F.
      • Ley R.E.
      • Sonnenburg J.L.
      • Peterson D.A.
      • Gordon J.I.
      Host-bacterial mutualism in the human intestine.
      ,
      • Ley R.E.
      • Bäckhed F.
      • Turnbaugh P.
      • Lozupone C.A.
      • Knight R.D.
      • Gordon J.I.
      Obesity alters gut microbial ecology.
      ]. The microbiota of obese individuals, as well as the caecal microbiota of ob/ob mice, are more efficient at the extraction of energy from the diet and in the production of short chain fatty acids (SCFAs) [
      • Turnbaugh P.J.
      • Ley R.E.
      • Mahowald M.A.
      • Magrini V.
      • Mardis E.R.
      • Gordon J.I.
      An obesity-associated gut microbiome with increased capacity for energy harvest.
      ,
      • Schwiertz A.
      • Taras D.
      • Schafer K.
      • Beijer S.
      • Bos N.A.
      • Donus C.
      • et al.
      Microbiota and SCFA in lean and overweight healthy subject.
      ]. Furthermore, the microbiota has been shown to stimulate hepatic triglyceride production through suppression of the lipoprotein lipase (LPL) inhibitor, fasting-induced adipose factor (Fiaf; also known as angiopoietin-like 4), thereby leading to continued expression of LPL, a key regulator of fatty acid release from triglycerides in the liver [
      • Bäckhed F.
      • Ding H.
      • Wang T.
      • Hooper L.V.
      • Koh G.Y.
      • Nagy A.
      • et al.
      The gut microbiota as an environmental factor that regulates fat storage.
      ]. The gut microbiota can also modulate systemic lipid metabolism through modification of bile acid metabolic patterns, impacting directly on the emulsification and absorption properties of bile acids and thus, indirectly, on the storage of fatty acids in the liver. The microbiota has also been implicated in the development of insulin resistance [
      • Bäckhed F.
      • Ding H.
      • Wang T.
      • Hooper L.V.
      • Koh G.Y.
      • Nagy A.
      • et al.
      The gut microbiota as an environmental factor that regulates fat storage.
      ], a fundamental abnormality in the metabolic syndrome, by affecting energy balance, glucose metabolism, and the low-grade inflammatory state that has been associated with obesity and related metabolic disorders. Its role in choline metabolism [
      • Dumas M.E.
      • Barton R.H.
      • Toye A.
      • Cloarec O.
      • Blancher C.
      • Rothwell A.
      • et al.
      Metabolic profiling reveals a contribution of gut microbiota to fatty liver phenotype in insulin-resistant mice.
      ,
      • Wang Z.
      • Klipfell E.
      • Bennett B.J.
      • Koeth R.
      • Levison B.S.
      • Dugar B.
      • et al.
      Gut flora metabolism of phosphatidylcholine promotes cardiovascular disease.
      ,
      • Rak K.
      • Rader D.J.
      Cardiovascular disease: the diet-microbe morbid union.
      ], as well as in activation of pro-inflammatory cytokines (e.g., TNFα), appears relevant to the development of NAFLD and progression to NASH. Most recently, studies in experimental models have shown that defective/deficient inflammasome sensing and related dysbiosis result in an abnormal accumulation of bacterial products in the portal circulation and promote progression of NAFLD/NASH [
      • Henao-Mejia J.
      • Elinav E.
      • Jin C.
      • Hao L.
      • Mehal W.Z.
      • Strowig T.
      • et al.
      Inflammasome-mediated dysbiosis regulates progression of NAFLD and obesity.
      ].

      Interactions with alcohol

      Alcohol-related liver injury may also involve the active participation of the microbiota through a number of mechanisms [
      • Purohit V.
      • Bode J.C.
      • Bode C.
      • Brenner D.A.
      • Choudhry M.A.
      • Hamilton F.
      • et al.
      Alcohol, intestinal bacterial growth, intestinal permeability to endotoxin, and medical consequences: summary of a symposium.
      ]. Firstly, alcohol promotes the growth of Gram-negative bacteria in the gut with the consequent production of endotoxin. Secondly, Gram-negative bacteria metabolize alcohol to acetaldehyde which, in turn, increases intestinal permeability. Other effects of alcohol conspire to increase permeability, promote transfer of endotoxin across the intestinal epithelium and impair the host immune response [
      • Purohit V.
      • Bode J.C.
      • Bode C.
      • Brenner D.A.
      • Choudhry M.A.
      • Hamilton F.
      • et al.
      Alcohol, intestinal bacterial growth, intestinal permeability to endotoxin, and medical consequences: summary of a symposium.
      ]. Protein adducts formed from metabolites of alcohol have also been shown to combine with LPS to induce liver injury [
      • Schaffert C.S.
      • Duryee M.J.
      • Hunter C.D.
      • Hamilton 3rd, B.C.
      • DeVeney A.L.
      • Huerter M.M.
      • et al.
      Alcohol metabolites and lipopolysaccharide: roles in the development and/or progression of alcoholic liver disease.
      ].

      Therapeutic impact of modulating the microbiota

      Antibiotics

      Infectious complications. Reflecting the involvement of the microbiota in the genesis of the infectious complications of liver disease [
      • Almeida J.
      • Galhenage S.
      • Yu J.
      • Kurtovic J.
      • Riordan S.M.
      Gut flora and bacterial translocation in chronic liver disease.
      ], the fundamental role of antibiotic therapy directed at Gram-negative enteric bacteria, in the management and prevention of infectious complications of liver disease and portal hypertension and chronic liver disease, such as SBP and bacteraemia, is well established [
      • Cohen M.J.
      • Sahar T.
      • Benenson S.
      • Elinav E.
      • Brezis M.
      • Soares-Weiser K.
      Antibiotic prophylaxis for spontaneous bacterial peritonitis in cirrhotic patients with ascites, without gastro-intestinal bleeding.
      ]. Indeed, antibiotics have also been advocated to both prevent infectious complications and reduce re-bleeding rates among patients with variceal haemorrhage [
      • Thalheimer U.
      • Triantos C.K.
      • Samonakis D.N.
      • Patch D.
      • Burroughs A.K.
      Infection, coagulation and variceal bleeding in cirrhosis.
      ,
      • Cohen M.J.
      • Sahar T.
      • Benenson S.
      • Elinav E.
      • Brezis M.
      • Soares-Weiser K.
      Antibiotic prophylaxis for spontaneous bacterial peritonitis in cirrhotic patients with ascites, without gastro-intestinal bleeding.
      ,
      • Chavez-Tapia N.C.
      • Barrientos-Gutierrez T.
      • Tellez-Avila F.I.
      • Soares-Weiser K.
      • Uribe M.
      Antibiotic prophylaxis for cirrhotic patients with upper gastrointestinal bleeding.
      ]. Based on the aforementioned role of endotoxemia resulting from bacterial translocation in the pathogenesis of the hyperdynamic circulation that characterizes portal hypertension (through the induction of nitric oxide [NO] synthase), selective intestinal decontamination has also been proposed as a strategy in the management of the circulatory complications of cirrhosis [
      • Rasaratnam B.
      • Connelly N.
      • Chin-Dusting J.
      Nitric oxide and the hyperdynamic circulation in cirrhosis: is there a role for selective intestinal decontamination?.
      ].

      Hepatic encephalopathy

      The recent studies, indicating the efficacy of the poorly absorbed antibiotic with activity against enteric organisms, rifaximin, in overt [
      • Bass N.M.
      • Mullen K.D.
      • Sanyal A.
      • Poordad F.
      • Neff G.
      • Leevy C.B.
      • et al.
      Rifaximin treatment in hepatic encephalopathy.
      ] and sub-clinical [
      • Sidhu S.S.
      • Goyal O.
      • Mishra B.P.
      • Sood A.
      • Chhina R.S.
      • Soni R.K.
      Rifaximin improves psychometric performance and health-related quality of life in patients with minimal hepatic encephalopathy (the RIME Trial).
      ] HE, have bolstered the importance of the role of these organisms in the pathogenesis of this common complication of liver disease; the precise nature of the changes in the composition and/or metabolic activity of the microbiota that generate these benefits remains to be determined.

      Intestinal failure-associated liver disease (IFALD)

      The term intestinal failure-associated liver disease has replaced that of total parenteral nutrition (TPN)-associated liver disease to describe alterations in liver morphology and function that occur among subjects with intestinal failure who may or may not also be on TPN. The etiology of intestinal failure-associated liver disease (IFALD) is multifactorial, with primary contributors including prematurity (in infants), sepsis, nutritional deficiencies or hepatotoxic effects of TPN infusates, and lack of enterally stimulated bile flow [
      • Carter B.A.
      • Karpen S.J.
      Intestinal failure-associated liver disease: management and treatment strategies past, present, and future.
      ]. Though SIBO may be an important feature in intestinal failure of various aetiologies and has been implicated in the pathogenesis of IFALD, there is currently insufficient evidence to support the use of antibiotics, such as metronidazole, in the prevention of IFALD [
      • Barclay A.R.
      • Beattie L.M.
      • Weaver L.T.
      • Wilson D.C.
      Systematic review: medical and nutritional interventions for the management of intestinal failure and its resultant complications in children.
      ].

      Probiotics and prebiotics

      Experimental

      NAFLD: In a pioneering study in the ob/ob mouse (an animal model of NAFLD), Li et al. compared the probiotic cocktail VSL#3 to an anti-TNFα antibody. Though, in comparison to the anti-TNFα antibody, VSL#3 had less effect on steatosis, its impact on inflammation and liver injury was similar, yet did not appear to be exerted through a reduction in hepatic expression of TNFα [
      • Li Z.
      • Yang S.
      • Lin H.
      • Huang J.
      • Watkins P.A.
      • Moser A.B.
      • et al.
      Probiotics and antibodies to TNF inhibit inflammatory activity and improve nonalcoholic fatty liver disease.
      ]. In a model that is more relevant to humans, diet-induced obesity, Ma et al. showed that the same probiotic preparation could prevent the development of steatosis through a natural killer T-cell (NKT)-dependent mechanism [
      • Ma X.
      • Hua J.
      • Li Z.
      Probiotics improve high fat diet-induced hepatic steatosis and insulin resistance by increasing hepatic NKT cells.
      ]; in contrast, other workers found that VSL#3, while attenuating fibrosis, had no effect on either steatosis or steatohepatitis in NASH induced by a methionine-choline-deficient diet [
      • Velayudham A.
      • Dolganiuc A.
      • Ellis M.
      • Petrasek J.
      • Kodys K.
      • Mandrekar P.
      • et al.
      VSL#3 probiotic treatment attenuates fibrosis without changes in steatohepatitis in a diet-induced nonalcoholic steatohepatitis model in mice.
      ]. Based on their studies, Xu et al. concluded that B. longum was more effective than Lactobacillus acidophilus in NAFLD and that these beneficial effects were related to a modification of the gut microbiota [
      • Xu R.Y.
      • Wan Y.P.
      • Fang Q.Y.
      • Lu W.
      • Cai W.
      Supplementation with probiotics modifies gut flora and attenuates liver fat accumulation in rat nonalcoholic fatty liver disease model.
      ]. Our own studies have shown that the modulation of the enteric microbiota can influence the fatty acid composition of host tissues, including the liver, adipose tissue, and brain [
      • Wall R.
      • Ross R.P.
      • Shanahan F.
      • O’Mahony L.
      • O’Mahony C.
      • Coakley M.
      • et al.
      Metabolic activity of the enteric microbiota influences the fatty acid composition of murine and porcine liver and adipose tissues.
      ,
      • Wall R.
      • Marques T.M.
      • O’Sullivan O.
      • Ross R.P.
      • Shanahan F.
      • Quigley E.M.
      • et al.
      Contrasting effects of Bifidobacterium breve DPC 6330 and Bifidobacterium breve NCIMB 702258 on fatty acid metabolism and gut microbiota composition.
      ]. Thus, we observed a significant elevation in c9, t11 conjugated linoleic acid (CLA) in the livers of mice and pigs, after feeding with linoleic acid in combination with the probiotic organism Bifidobacterium breve NCIMB 702258. CLA is a microbial metabolite that has been associated with the alleviation of NAFLD [
      • Nagao K.
      • Inoue N.
      • Wang Y.M.
      • Shirouchi B.
      • Yanagita T.
      Dietary conjugated linoleic acid alleviates nonalcoholic fatty liver disease in Zucker (fa/fa) rats.
      ]. Indeed, it is noteworthy that the VSL#3 probiotic cocktail has been shown to produce CLA in vitro, and the c9, t11 CLA isomer, in particular [
      • Ewaschuk J.B.
      • Walter J.W.
      • Diaz H.
      • Madsen K.L.
      Bioproduction of conjugated linoleic acid by probiotic bacteria occurs in vitro and in vivo in mice.
      ] Prebiotic preparations have also been shown to ameliorate liver inflammation in obese mice through a glucagon-like peptide-2 (GLP-2)-dependent effect on the gut barrier [
      • Cani P.D.
      • Possemiers S.
      • Van den Wiele T.
      • Guiot Y.
      • Everard A.
      • Rottier O.
      • et al.
      Changes in gut microbiota control inflammation in obese mice through a mechanism involving GLP-2-driven improvement of gut permeability.
      ] and hold promise for the management of NAFLD and related disorders [
      • Parnell J.A.
      • Raman M.
      • Rioux K.P.
      • Reimer R.A.
      The potential role of prebiotic fibre for treatment and management of non-alcoholic fatty liver disease and associated obesity and insulin resistance.
      ].
      Alcoholic liver disease: In an animal model of alcoholic liver disease, Forsyth et al. found that Lactobacillus rhamnosus GG (LGG) could reduce liver fat content, necro-inflammatory score and myleoperoxidase expression. Interestingly, these beneficial effects correlated with a reduction in, or normalization of, intestinal permeability [
      • Forsyth C.B.
      • Farhadi A.
      • Jakate S.M.
      • Tang Y.
      • Shaikh M.
      • Keshavarzian A.
      Lactobacillus GG treatment ameliorates alcohol-induced intestinal oxidative stress, gut leakiness, and liver injury in a rat model of alcoholic steatohepatitis.
      ]. Indeed, the concept that a significant component of the beneficial effect of probiotics in alcohol-related liver injury is exerted at the level of the intestinal barrier is supported by other studies [
      • Wang Y.
      • Liu Y.
      • Sidhu A.
      • Ma Z.
      • McClain C.
      • Feng W.
      Lactobacillus rhamnosus GG culture supernatant ameliorates acute alcohol-induced intestinal permeability and liver injury.
      ], as well as by a considerable literature attesting to important effects of probiotics on gut tight junctional and transcellular permeability in diverse disease models. Indeed, Wang et al. have provided a molecular basis for the action of the probiotic L. rhamnosus GG, one of the most widely studied organisms in this context, by demonstrating that the restoration of barrier function, in alcohol-induced liver injury, was mediated through the recovery of hypoxia-inducible factor (HIF) and trefoil factor in gut epithelial cells [
      • Wang Y.
      • Kirpich I.
      • Liu Y.
      • Ma Z.
      • Barve S.
      • McClain C.J.
      • et al.
      Lactobacillus rhamnosus GG treatment potentiates intestinal hypoxia-inducible factor, promotes intestinal integrity and ameliorates alcohol-induced liver injury.
      ].
      Complications of liver disease: A variety of probiotic preparations have been studied in relation to the various complications of liver disease. While Lactobacillus-fermented milk supplements failed to reduce bacterial translocation in an experimental model of prehepatic portal hypertension [
      • Wiest R.
      • Chen F.
      • Cadelina G.
      • Groszmann R.J.
      • Garcia-Tsao G.
      Effect of Lactobacillus-fermented diets on bacterial translocation and intestinal flora in experimental prehepatic portal hypertension.
      ], in contrast, in a model of acute liver injury, Adawi et al. found that rectal administration of a number of Lactobacillus species, with or without arginine supplementation, led to a reduction in bacterial translocation. In this same model, Lactobacillus plantarum alone abrogated the severity of the liver injury [
      • Adawi D.
      • Kasravi F.B.
      • Molin G.
      • Jeppsson B.
      Effect of Lactobacillus supplementation with and without arginine on liver damage and bacterial translocation in an acute liver injury model in the rat.
      ]. In an animal model of minimal HE, Jia and Zhang showed that a probiotic cocktail was as effective as lactulose in reducing HE and blood ammonia levels as well as decreasing endotoxaemia, liver injury and inflammation [
      • Jia L.
      • Zhang M.H.
      Comparison of probiotics and lactulose in the treatment of minimal hepatic encephalopathy in rats.
      ].
      Effects on acute liver injury and chronic liver disease: Probiotics have been shown to reduce the impact of a variety of forms of acute liver injury [
      • Rishi P.
      • Bharrhan S.
      • Singh G.
      • Kaur I.P.
      Effect of Lactobacillus plantarum and l-arginine against endotoxin-induced liver injury in a rat model.
      ,
      • Sharma S.
      • Chaturvedi J.
      • Chaudhari B.P.
      • Singh R.L.
      • Kakkar P.
      Probiotic Enterococcus lactis IITRHR1 protects against acetaminophen-induced hepatotoxicity.
      ,
      • Li Y.T.
      • Wang L.
      • Chen Y.
      • Chen Y.B.
      • Wang H.Y.
      • Wu Z.W.
      • et al.
      Effects of gut microflora on hepatic damage after acute liver injury in rats.
      ,
      • Ewaschuk J.
      • Endersby R.
      • Thiel D.
      • Diaz H.
      • Backer J.
      • Ma M.
      • et al.
      Probiotic bacteria prevent hepatic damage and maintain colonic barrier function in a mouse model of sepsis.
      ,
      • Osman N.
      • Adawi D.
      • Ahrne S.
      • Jeppsson B.
      • Molin G.
      Endotoxin- and d-galactosamine-induced liver injury improved by the administration of Lactobacillus, Bifidobacterium and blueberry.
      ], as well as the severity of more chronic forms of liver disease, such as that related to total parenteral nutrition (TPN) [
      • Wu J.
      • Wang X.
      • Cai W.
      • Hong L.
      • Tang Q.
      Bifidobacterium adolescentis supplementation ameliorates parenteral nutrition-induced liver injury in infant rabbits.
      ].

      Clinical

      Given all that has been reviewed above, it should come as no surprise that here is an ever-increasing interest in the potential therapeutic value of probiotics in a variety of contexts in relation to the liver and its diseases [
      • Carter B.A.
      • Karpen S.J.
      Intestinal failure-associated liver disease: management and treatment strategies past, present, and future.
      ,
      • Lata J.
      • Jurankova J.
      • Kopacova M.
      • Vitek P.
      Probiotics in hepatology.
      ,
      • Gratz S.W.
      • Mykkanen H.
      • El-Nezami H.S.
      Probiotics and gut health: a special focus on liver diseases.
      ]. These include: HE, translocation and infectious complications, NAFLD/NASH and the prophylaxis of infections after liver transplantation [
      • Gratz S.W.
      • Mykkanen H.
      • El-Nezami H.S.
      Probiotics and gut health: a special focus on liver diseases.
      ]. In attempting to assess the relevant literature, the reviewer is confronted, not only by the issues that bedevil the interpretation of animal studies (varying probiotic strains and preparations, different animal models and outcomes), but also by limitations in study size and design, as well as variations in probiotic dose, formulation, route and duration of therapy. There is also limited data on the microbiological impact of probiotics on the abnormal microbiota that has been described in cirrhosis and other liver diseases [
      • Zhao H.Y.
      • Wang H.J.
      • Lu Z.
      • Xu S.Z.
      Intestinal microflora in patients with liver cirrhosis.
      ]. While probiotics are generally assumed to be safe, there have been reservations regarding their use in immunosuppressed individuals; whether these preparations offer a greater risk of initiating septic complications among individuals with liver disease, given the presence of both portal-systemic shunting and impaired immune function (be it related to underlying disease or iatrogenic), is unclear and safety assessments have been limited [
      • Gratz S.W.
      • Mykkanen H.
      • El-Nezami H.S.
      Probiotics and gut health: a special focus on liver diseases.
      ].
      NAFLD/NASH: A Cochrane Collaboration Systematic Review published in 2007, while failing to identify any randomized clinical trials, concluded that preliminary data from two pilot non-randomised studies suggested that probiotics may be well tolerated, improve conventional liver function tests and decrease markers of lipid peroxidation in NAFLD/NASH [
      • Lirussi F.
      • Mastropasqua E.
      • Orando S.
      • Orlando R.
      Probiotics for non-alcoholic fatty liver disease and/or steatohepatitis.
      ]. Their review included one study employing L. Bulgaricus, which produced a reduction in transaminases [
      • Aller R.
      • De Luis D.A.
      • Izaola O.
      • Conde R.
      • Gonzalez Sagrado M.
      • Primo D.
      • et al.
      Effect of a probiotic on liver aminotransferases in nonalcoholic fatty liver disease patients: a double blind randomized clinical trial.
      ], and another with L. rhamnosus GG, which resulted in lower levels of alanine transaminase [
      • Vajro P.
      • Mandato C.
      • Licenziati M.R.
      • Franzese A.
      • Vitale D.F.
      • Lenta S.
      • et al.
      Effects of Lactobacillus rhamnosus strain GG in pediatric obesity-related liver disease.
      ].
      Alcoholic liver disease: Kirpich et al. randomized 66 Russian males with alcoholic psychosis to 5 days of therapy with a combination of Bifidobacterium bifidum and L. plantarum or standard therapy. At baseline, alcoholics had fewer bifidobacteria, lactobacilli and enterococci; however, after 5 days of probiotic therapy, bifidobacteria, and lactobacilli increased and, when checked 7 days after the initiation of probiotic therapy, alanine transaminase levels had declined significantly in the probiotic but not in the control group [
      • Kirpich I.A.
      • Solovieva N.V.
      • Leikhter S.N.
      • Shidakova N.A.
      • Lebedeva O.V.
      • Sidorov P.I.
      • et al.
      Probiotics restore bowel flora and improve liver enzymes in human alcohol-induced liver injury: a pilot study.
      ].
      Complications of liver disease: Despite the presence, in the literature, of several individual trials attesting to the benefits of various probiotic preparations in HE and minimal/subclinical HE [
      • Liu Q.
      • Duan Z.P.
      • Ha D.K.
      • Bengmark S.
      • Kurtovic J.
      • Riordan S.M.
      Synbiotic modulation of gut flora: effect on minimal hepatic encephalopathy in patients with cirrhosis.
      ,
      • Malaguarnera M.
      • Greco F.
      • Barone G.
      • Gargante M.P.
      • Malaguarnera G.
      • Toscano M.A.
      Bifidobacterium longum with fructo-oligosaccheride (FOS) treatment in minimal hepatic encephalopathy: a randomized, double-blind, placebo-controlled study.
      ,
      • Malaguarnera M.
      • Gargante M.P.
      • Malaguarnera G.
      • Salmeri M.
      • Mastrojeni S.
      • Rampello L.
      • et al.
      Bifidobacterium combined with fructo-oligosaccheride versus lactulose in the treatment of patients with hepatic encephalopathy.
      ,
      • Mittal V.V.
      • Sharma B.C.
      • Sharma P.
      • Sarin S.K.
      A randomized controlled trial comparing lactulose, probiotics and l-ornithine l-aspartate in treatment of minimal hepatic encephalopathy.
      ], a recent Cochrane Systematic Review concluded that, while probiotics appeared to exert a significant effect on blood ammonia levels, problems with trial interpretation (related, in turn, to shortcomings in study design) did not allow them to conclude that probiotics were effective against clinically meaningful end points [
      • McGee R.G.
      • Bakens A.
      • Wiley K.
      • Riordan S.M.
      • Webster A.C.
      Probiotics for patients with encephalopthay.
      ]. It should be noted, however, that a separate systematic review that focused exclusively on minimal HE concluded that prebiotics, probiotics, and synbiotics were effective for this indication, but that lactulose (itself a prebiotic) was superior to both probiotics and synbiotics [
      • Shukla S.
      • Shukla A.
      • Mehboob S.
      • Guha S.
      Meta-analysis: the effects of gut flora modulation using prebiotics, probiotics and synbiotics on minimal hepatic encephalopathy.
      ]. Similar issues limit one’s ability to draw conclusions and generate recommendations regarding the role of probiotics in preventing infectious complications of liver disease. While reported effects of probiotics on modulating the microbiota [
      • Gratz S.W.
      • Mykkanen H.
      • El-Nezami H.S.
      Probiotics and gut health: a special focus on liver diseases.
      ,
      • Lata J.
      • Novotny I.
      • Pribramska V.
      • Jurankova J.
      • Fric P.
      • Kroupa R.
      • et al.
      The effect of probiotics on gut flora, level of endotoxin and Child-Pugh score in cirrhotic patients: results of a double-blind randomized study.
      ], reducing levels of endotoxemia [
      • Lata J.
      • Novotny I.
      • Pribramska V.
      • Jurankova J.
      • Fric P.
      • Kroupa R.
      • et al.
      The effect of probiotics on gut flora, level of endotoxin and Child-Pugh score in cirrhotic patients: results of a double-blind randomized study.
      ], normalizing impaired neutrophil function [
      • Stadlbauer V.
      • Mookerjee R.P.
      • Hodges S.
      • Wright G.A.
      • Davies N.A.
      • Jalan R.
      Effect of probiotic treatment on deranged neutrophil function and cytokine responses in patients with compensated cirrhosis.
      ] and reducing levels of pro-inflammatory cytokines [
      • Loguerico C.
      • Federico A.
      • Tuccillo C.
      • Terracciano F.
      • D’Auria M.V.
      • De Simone C.
      • et al.
      Beneficial effects of a probiotic VSL#3 on parameters of liver dysfunction in chronic liver diseases.
      ] could well translate into clinical benefit, this has yet to be demonstrated. The efficacy of lactulose, a prebiotic, and antibiotics, such as neomycin, paromomycin, and rifaximin, in HE, has been well documented. A recent meta-analysis suggested that the efficacy of rifaximin was similar to that of lactulose and older antibiotic regimes but associated with less adverse events [
      • Eltawil K.M.
      • Laryea M.
      • Peltekian K.
      • Molinari M.
      Rifaximin vs. conventional oral therapy for hepatic encephalopathy: a meta-analysis.
      ]. There have been limited studies of probiotics in other complications; the probiotic cocktail VSL#3 had no effect on portal pressure in either compensated or very early decompensated cirrhosis [
      • Tandon P.
      • Moncrief K.
      • Madsen K.
      • Arrieta M.C.
      • Owen R.J.
      • Bain V.G.
      • et al.
      Effects of probiotic therapy on portal pressure in patients with cirrhosis: a pilot study.
      ].
      Effects on acute liver injury and chronic liver disease: Data on the impact of probiotics on the evolution or natural history of liver disease, per se, is very limited. Though uncontrolled studies suggest that probiotics may reduce SIBO in IFALD, there is, as yet, no evidence that their administration can prevent the development or progression of this form of liver disease [
      • Carter B.A.
      • Karpen S.J.
      Intestinal failure-associated liver disease: management and treatment strategies past, present, and future.
      ]. Similarly, small studies in compensated cirrhosis [
      • Pereg D.
      • Kotliroff A.
      • Gadoth N.
      • Hadary R.
      • Lishner M.
      • Kitay-Cohen Y.
      Probiotics for patients with compensated liver cirrhosis: a double-blind placebo-controlled study.
      ] and PSC [
      • Vleggaar F.P.
      • Monkelbaan J.F.
      • van Erpecum K.J.
      Probiotics in primary sclerosing cholangitis: a randomized placebo-controlled crossover pilot study.
      ] showed no impact from the relatively short-term (3–6 months) administration of probiotic on common laboratory parameters. While probiotic supplementation has been shown to reduce a biomarker of risk for hepatocellular cancer [
      • El-Nezami H.S.
      • Polychronaki N.N.
      • Ma J.
      • Zhu H.
      • Ling W.
      • Salminen E.K.
      • et al.
      Probiotic supplementation reduces a biomarker for increased risk of liver cancer in young men from Southern China.
      ], the actual clinical impact of this approach is unknown.

      Conclusions and future studies

      A central role for the microbiota in the precipitation of infectious and non-infectious complications of liver disease has been established and evidence for a more fundamental role in the aetiology of certain liver diseases, such as NAFLD and NASH, continues to accumulate. However, the description of the microbiota and its metabolic [
      • Nicholson J.K.
      • Holmes E.
      • Kinross J.
      • Burcelin R.
      • Gibson G.
      • Jia W.
      • et al.
      Host–gut microbiota metabolic interactions.
      ] and immunological [
      • Seki E.
      • Schnabl B.
      Role of innate immunity and the microbiota in liver fibrosis: crosstalk between the liver and gut.
      ] functions in various forms and stages of liver disease is still far from complete, but should be nigh given the widespread availability of high-throughput sequencing and metabolomic technologies. While high quality clinical evidence supports the use of antibiotic therapy, presumably as a modulator of the microbiota, in the management of HE, SBP, and other infectious complications, how these interventions impact on the microbiota and microbiota-host interactions has not been clearly defined. While probiotics hold promise in the management of liver disease and their potential is supported by a considerable volume of laboratory work [
      • Cesaro C.
      • Tiso A.
      • Del Prete A.
      • Cariello R.
      • Tuccillo C.
      • Cotticelli G.
      • et al.
      Gut microbiota and probiotics in chronic liver diseases.
      ], high-quality clinical evidence is scanty and its paucity, together with all of the quality control, strain selection and dose optimization issues that have been an unfortunate feature of this field, precludes, for the moment, recommendations on the use of probiotics, in general, and specific strains, in particular, in clinical practice. However, there is a sufficiently strong rationale for the use of strategies that involve the modulation of the microbiota in the management of liver diseases, such as NAFLD [
      • Blanco C.
      • Loguercio C.
      • Machado M.V.
      • Cortez-Pinto H.
      Gut microbiota and nonalcoholic fatty liver disease.
      ,
      • Compare D.
      • Coccoli P.
      • Rocco A.
      • Nardone O.M.
      • De Maria S.
      • Cartenì M.
      • et al.
      Gut–liver axis: the impact of gut microbiota on non alcoholic fatty liver disease.
      ,
      • Frazier T.H.
      • DiBaise J.K.
      • McClain C.J.
      Gut microbiota, intestinal permeability, obesity-induced inflammation, and liver injury.
      ], and their complications to suggest that these approaches are deserving of further exploration.

      Conflict of interest

      The authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.

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