Colonization, translocation and immune responses in a gnotobiotic rat model

Abstract

Certain strains of E. coli may persist for extended periods of time in the intestine (resident), whereas others are rapidly replaced (transient). We have developed a gnotobiotic rat model for the study of which bacterial traits determine the capacity to colonize in the intestine and to translocate over the gut epithelium, and whether the immune response to gut-colonizing E. coli is influenced by the simultaneous presence of other members of the normal intestinal microbiota, such as lactobacilli and peptostreptococci. Germfree rats were colonized with a mixture of two isogenic E. coli strains differing only in a certain property. The level of each of the isogenic strains was measured in the intestinal lumen and in the mesenteric lymph nodes (translocated bacteria). Using this approach, we could demonstrate that P fimbriae and the K5 capsule were traits enhancing the colonizing capacity of E. coli in the gut. These traits did not promote translocation, other than secondarily via increasing the bacterial population level, which is a factor known to promote translocation. For translocation, transient human E. coli strains expressing type 1 fimbriae, but no other adhesins, seemed the most efficient. Resistance to phagocytosis or lysis in fresh serum did not appear to be of importance for the capacity to translocate. Translocation of both E. coli and L. acidophilus was reduced by the concomittant presence of the obligate Gram positive anaerobe, Peptostreptococcus, despite the fact that the intestinal population numbers of the two bacteria were not affected. Significantly increased levels of anti-E. coli antibodies were also observed in the rats colonized by all three bacteria. These antibodies could not be absorbed by lactobacilli or peptostreptococci, and thus did not represent cross-reactive antibodies. Peptostreptococcus-colonized rats also showed increased level of total serum IgM. Immune effects of the probiotic bacterium L. plantarum were tested in the gnotobiotic rat model. Rats colonized with L. plantarum, in addition to E. coli, showed lower counts of intestinal E. coli, decreased proliferative spleen cell response in the early phase of colonization, an increased influx of CD25 (IL-2 receptor) positive cells in the lamina propria, and increased level of serum IgA, as compared with rats colonized with E. coli alone. CD25 positive cells have been implicated in the maintenance of tolerance to dietary antigens and autoantigens. The gnotobiotic rat model was found to be stable and reproducible, and to provide a versatile model for the establishment of competition of bacteria reflecting events in the human intestinal tract.