Influence du microbiote intestinal sur le métabolisme et la virulence des Escherichia coli entérohémorragiques

Enterohemorrhagic E. coli (EHEC) represent an important public threat especially in developed countries. EHEC are human pathogens involved in food- and water-borne outbreaks and the cause of hemorrhagic colitis and the hemolytic and uremic syndrome that is deadly to young children. The serotype O157:H7 contains the most virulent strain of EHEC and bovines are their natural reservoir. To date, no cure exists to avoid symptoms of an EHEC infection. Consequently, it is of interest to enhance our knowledge about mechanisms used by the pathogen to regulate virulence and to efficiently colonize the intestinal tract.  The adaptive response of EHEC O157:H7 strain EDL933 to the metabolic activity of the human intestinal microbiota was first analysed at the transcriptional level. The strain EDL933 was grown in the cecal content of germ-free rats (GFC medium) and that of rats associated with the human intestinal microbiota (HMC medium). The HMC medium is an in vivo microbiota-conditioned medium. An important shift in the metabolic profile of EDL933 was observed in HMC as compared to GFC, with a decreased expression of genes involved in the glycolysis pathways and an increased expression of anaplerosis genes. These results, together with a targeted metabolomics analysis, revealed the nutrient starvation encounters by the pathogen in HMC and the metabolic strategies used to adapt the intestinal microbiota. Moreover, the expression of several virulence genes including those of the locus of enterocyte effacement (LEE) encoding for the type III secretion system was decreased, indicating the ability of the intestinal microbiota to decrease virulence expression of pathogens.  The influence of several intestinal compounds present in the cecal contents of rats was then tested for their ability to modulate the virulence gene expression of EDL933. Results demonstrated that the two compounds, N-acetylneuraminic acid (Neu5Ac) and N-acetylglucosamine (GlcNAc) down-regulate the expression of the LEE genes. This repression was dependant of the protein NagC which is the sensor of intracellular GlcNAc-6-P and the regulator of the GlcNAc and the galactose catabolism in E. coli. NagC is a transcriptional regulator that is inactivated in presence of GlcNAc-6-P derived from the Neu5Ac catabolism and the GlcNAc transport. This work led to the identification of NagC as an activator of the LEE genes and the discovery of a new mechanism involved in the coordination of virulence and metabolism in EHEC O157:H7. The concentration of Neu5Ac and GlcNAc was enhance in vivo by the human intestinal symbiont Bacteroides thetaiotaomicron indicating the ability of some species of the microbiota to release compounds that repress virulence of pathogens.  This work led to the identification of the metabolic adaptive responses of EHEC O157:H7 in the intestinal milieu conditioned by the intestinal microbiota and the discovery of a new mechanism used to coordinate metabolism and virulence. These data could contribute to the development of news approaches leading to decrease EHEC infections.

Keyphrases: Enterohemorrhagic Escherichia coli, Intestinal microbiota, Metabolism, Virulence, Regulation