الفهرس 
Investigating the role of L-rhamnose utilization in metabolism and virulence of pathogenic Escherichia coli
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Abstract
The human gastrointestinal tract is inhabited by trillions of microbes, which remarkably affect human physiology and homeostasis. The host and its microbiota exchange chemical signaling and share different nutrients; invading pathogens compete for these resources as well. Escherichia coli is a common resident of intestine, but some strains are highly pathogenic, e.g., E. coli O157:H7, which causes hemorrhagic colitis, bloody diarrhea, and hemolytic uremic syndrome (HUS). Many gut bacteria, including E. coli, can degrade multiple polysaccharides, including mucin. Recent evidence indicates that enteropathogens are equipped with a large set of specific metabolic pathways to survive nutritional limitations in the in vivo environment, thus increasing bacterial fitness during infection. L-rhamnose, a hexose deoxy sugar, is one of the components of the carbohydrate mucins of the mucosal glycoconjugates. Although L-rhamnose degradation pathway is encoded by the genomes of many enteropathogens (e.g., Enterococcus faecalis, Listeria monocytogenes, Clostridium difficile, Salmonella enterica, Shigella species, and many E. coli strains) a role of this metabolic pathway in vivo remains to be elucidated. In this study, we investigated the function of two key genes in the L-rhamnose utilization operon, rhaD and rhaR. Bioinformatics subystems analysis and PCR screening indicated the co-occurrence of both genes in screened genomes and clinical isolates, respectively. Precisely deleting each gene from E. coli O157:H7 led to deficient bacterial growth in L-rhamnose as a sole carbon source. The inducing effect of L-rhamnose was studied at the transcriptional level by quantitative reverse-transcription PCR, which also demonstrated the activating effect of rhaR on the key rhaA and rhaD genes. Finally, the impact of rhaR and rhaD on E. coli O157:H7 virulence was studied in a mouse model of gastrointestinal infection. Histopathological examination of the kidneys and colons of CD1 mice infected by wildtype (WT) or mutant strains (ΔrhaR and ΔrhaD) revealed that rhaR deletion had a greater impact on pathogenesis and kidney damage, and a higher level of attachment to epithelial cells than rhaD deletion. Daily fecal bacterial counts, during seven days of infection, showed similar results, as ΔrhaR was more predominant in animal feces than ΔrhaD or WT, confirming the potential virulence attenuating activity of RhaR. Future studies will address the causal and mechanistic basis of this observation.