Initiation of Protein Synthesis by a Labeled Derivative of theLactobacillus caseiDN-114 001 Strain during Transit from the Stomach to the Cecum in Mice Harboring Human Microbiota

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Abstract
Although studies on the survival of bacteria in the digestive tract have been reported in the literature, little data are available on the physiological adaptation of probiotics to the digestive environment. In previous work, a transcriptional fusion system (i.e., luciferase genes under the control of a deregulated promoter) was used to demonstrate that a derivative of theLactobacillus caseiDN-114 001 strain, ingested in a fermented milk and thus exhibiting initially a very weak metabolic activity, synthesized proteins de novo after its transit in the digestive tract of mice harboring human microbiota (known as human-microbiota-associated mice). With the same genetic system and animal model, we here investigate for the first time the ability ofL. caseito reinitiate synthesis in the different digestive tract compartments. In this study, most ingestedL. caseicells transited from the stomach to the duodenum-jejunum within 1 h postingestion. No luciferase activity was observed in these digestive tract compartments after the first hour. At later times, the bulk of bacteria had transited to the ileum and the cecum. Luciferase synthesis was detected between 1.5 and 2.0 h postingestion at the ileal level and from 1.5 h to at least 6.0 h postingestion in the cecum, where the activity remained at a maximum level. These results demonstrate that ingestedL. casei(derivative of the DN-114 001 strain) administered via a fermented milk has already reinitiated protein synthesis when it reaches the ileal and cecal compartments.