Carbohydrate utilization by enterohaemorrhagic Escherichia coli O157:H7 in bovine intestinal content.

The bovine gastrointestinal (GI) tract is the main reservoir for enterohaemorrhagic Escherichia coli (EHEC) responsible for food-borne infections. Characterization of nutrients preferentially used by EHEC in the bovine intestine would help to develop ecological strategies to reduce EHEC carriage. However, the carbon sources that support the growth of EHEC in the bovine intestine are poorly documented. In this study, a very low concentration of glucose, the most abundant monomer included in the cattle dietary polysaccharides, was detected in bovine small intestine contents (BSIC) collected from healthy cows at the slaughterhouse. Six carbohydrates reported to be included in the mucus layer covering the enterocytes [galactose, N-acetyl-glucosamine (GlcNAc), N-acetyl- galactosamine (GalNAc), fucose, mannose and N-acetyl neuraminic acid (Neu5Ac)] have been quantified for the first time in BSIC and accounted for a total concentration of 4.2 mM carbohydrates. The genes required for enzymatic degradation of the six mucus-derived carbohydrates are highly expressed during the exponential growth of the EHEC strain O157:H7 EDL933 in BSIC and are more strongly induced in EHEC than in bovine commensal E. coli. In addition, EDL933 consumed the free monosaccharides present in the BSIC more rapidly than the resident microbiota and commensal E. coli, indicating a competitive ability of EHEC to catabolize mucus-derived carbohydrates in the bovine gut. Mutations of EDL933 genes required for the catabolism of each of these sugars have been constructed, and growth competitions of the mutants with the wild-type strain clearly demonstrated that mannose, GlcNAc, Neu5Ac and galactose catabolism confers a high competitive growth advantage to EHEC in BSIC and probably represents an ecological niche for EHEC strains in the bovine small intestine. The utilization of these mucus-derived monosaccharides by EDL933 is apparently required for rapid growth of EHEC in BSIC, and for maintaining a competitive growth rate as compared with that of commensal E. coli. The results suggest a strategy for O157:H7 E. coli survival in the bovine intestine, whereby EHEC rapidly consumes mucus-derived carbohydrates that are poorly consumed by bacteria belonging to the resident intestinal microbiota, including commensal E. coli.

Escherichia coli O157:H7 and other E. coli strains share physiological properties associated with intestinal colonization.

Escherichia coli isolates (72 commensal and 10 O157:H7 isolates) were compared with regard to physiological and growth parameters related to their ability to survive and persist in the gastrointestinal tract and found to be similar. We propose that nonhuman hosts in E. coli O157:H7 strains function similarly to other E. coli strains in regard to attributes relevant to gastrointestinal colonization.

The Life of Commensal Escherichia coli in the Mammalian Intestine.

In this chapter we review the literature with respect to what is known about how Escherichia coli colonizesthe mammalian intestine. We begin with a brief discussion of the mammalian large intestine, the major site that commensal strains of E. coli colonize. Next, evidence is discussed showing that, in order to colonize, E. coli must be able to penetrate and grow in the mucus layer of the large intestine. This is followed by discussions of colonization resistance, i.e., factors that are involved in the ability of a complete microbiota (microflora) to resist colonization by an invading bacterium, the advantages and disadvantages of the in vivo colonization models used in colonization research, the initiation and maintenance stages of E. coli colonization, and the rate of E. coli growth in the intestine. The next two sections of the chapter discuss the role of motility in colonization and how adhesion to mucosal receptors aids or inhibits penetration of the intestinal mucus layer and thereby either promotes or prevents E. coli colonization. Finally, the contribution of nutrition to the ability of E. coli to colonize is discussed based on the surprising finding that different nutrients are used by E. coli MG1655, a commensal strain, and by E. coli EDL933, an enterohemorrhagic strain, to colonize the intestine.