Hyperammonemia is a common complication of acute and chronic liver diseases. Often accompanied with side effects, therapeutic interventions such as antibiotics or lactulose are generally targeted to decrease the intestinal production and absorption of ammonia. In this study, we aimed to modulate hyperammonemia in three rodent models by administration of wild-type Lactobacillus plantarum, a genetically engineered ammonia hyperconsuming strain, and a strain deficient for the ammonia transporter. Wild-type and metabolically engineered L. plantarum strains were administered in ornithine transcarbamoylase-deficient Sparse-fur mice, a model of constitutive hyperammonemia, in a carbon tetrachloride rat model of chronic liver insufficiency and in a thioacetamide-induced acute liver failure mice model. Constitutive hyperammonemia in Sparse-fur mice and hyperammonemia in a rat model of chronic hepatic insufficiency were efficiently decreased by Lactobacillus administration. In a murine thioacetamide-induced model of acute liver failure, administration of probiotics significantly increased survival and decreased blood and fecal ammonia. The ammonia hyperconsuming strain exhibited a beneficial effect at a lower dose than its wild-type counterpart. Improved survival in the acute liver failure mice model was associated with lower blood ammonia levels but also with a decrease of astrocyte swelling in the brain cortex. Modulation of ammonia was abolished after administration of the strain deficient in the ammonium transporter. Intestinal pH was clearly lowered for all strains and no changes in gut flora were observed. Conclusion: Hyperammonemia in constitutive model or after acute or chronic induced liver failure can be controlled by the administration of L. plantarum with a significant effect on survival. The mechanism involved in this ammonia decrease implicates direct ammonia consumption in the gut. (HEPATOLOGY 2008;48:1184-1192 H yperammonemia (HA) is a well-known complication of acute and chronic liver diseases and plays a central role in the pathogenesis of hepatic encephalopathy (HE). [1][2][3][4][5] This neurological dysfunction results, at least in part, from an increase in plasma ammonia level and the severity of the symptoms correlates with blood ammonia level. 6-9 Animal models used in studying hyperammonemic disorders are multiple: fulminant hepatic failure, 10 chronic liver failure, 11 or urea cycle deficiency. 12 Ammonia is produced by many tissues but its major external source results from deaminase and urease activities of the gut flora. Although its absorption occurs through the intestinal epithelium, ammonia is carried by the portal vein into the liver where it is metabolized into urea. 13 Classical treatments of HE, except through liver transplantation and liver replacement therapies, consist in decreasing the ammonia production of urease-positive bacteria by antibiotics or decreasing the ammonia absorption into the gut by acidifying the colon content with nonabsorbable sugars such as lactulose. 4,[13][14][1...