Enterohepatic Species

Most of the members of the genus do not normally colonize the gastric mucosa, but instead thrive in the mucosal surfaces of the intestinal tract and/or the liver of humans, other mammals, and birds. These enterohepatic species have features of ultrastructure and physiology in common with and the other gastric species, and have been the subject of several recent reviews (35, 41, 108). Because the enterohepatic species were first recognized in laboratory rodents, in which they are highly prevalent in most inbred strains and outbred stocks, they have been considered a component of the resident microbiota, or "normal flora." It is now clear that some of the enterohepatic species, and perhaps all, have the ability to cause disease in normal, immunocompetent rodents. A growing number of enterohepatic species have also been reported to be associated with gastroenteritis, hepatitis, and other disease states in humans and in other animal species. The significance of the enterohepatic species in human disease and the true prevalence of these organisms in human populations remain to be determined. What follows is a survey of this emerging group of organisms. Early studies characterizing the resident microbiota in the gut of laboratory rodents led to the discovery of a diverse population of spiral-shaped bacteria uniquely adapted to thrive in the mucosal surfaces of the intestine. These early studies, which used transmission electron microscopy rather than culture and isolation, described two morphologic types of organisms, both of which are now known to be enterohepatic species. Members of the first group superficially resemble species but are longer and have a single polar flagellum at each end. Representative organisms are shown in Fig. 1 and are listed in Table 1 as having no periplasmic fibers. Members of the second group have periplasmic fibers that wrap helically around the body of the bacterium as well as bipolar tufts of sheathed flagella. Representative organisms are shown in Fig. 2 and are listed in Table 1 as having periplasmic fibers. In studies that characterized the patterns of bacterial colonization of the large intestine of laboratory rodents, Davis et al. identified both morphologic types of organisms in the mucus of the cecum and colon (18, 19). The bacteria could be found during the first week of life, and they remained on the surface of the intestinal epithelium and packed deep in the crypts throughout the life of the animals. Perhaps because their ultrastructure is less remarkable, the early literature contains fewer reports of the simple spiral-shaped organisms than of the organisms with periplasmic fibers. Nonetheless, the simple spiral-shaped organisms have been isolated from a variety of mammals, including humans, pigs, dogs, cats, mice, rats, hamsters, gerbils, and several wild and domestic species of birds. The distinction between these organisms and the organisms with periplasmic fibers has a morphologic basis only. No comparable phylogenetic dichotomy has been recognized. On the other hand, the presence of periplasmic fibers has facilitated the recognition of members of the second group of enterohepatic species in a variety of locations. Spiral-shaped bacteria with periplasmic fibers were observed free in the cytoplasm of enterocytes as well as deeper in the lamina propria of mice following treatment with nitrogen mustard (53). Such treatment results in a generalized loss of epithelial integrity, but it is interesting to note that the enterohepatic species were the only organisms found to invade under these conditions. The abundance of these organisms in the mucus deep in the crypts of the ileum and their proximity to the apical surface of the epithelial cells lining the crypts may account, at least in part, for these observations. Erlandsen and Chase exploited the ultrastructural characteristics of these organisms to ascertain the fate of bacteria following phagocytosis from the crypts by differentiated enterocytes in the ileum of untreated rats (26). Davis et al. also noted the occasional penetration of enterohepatic species into the epithelium of the rat cecum (19). More recently, invasion into the lamina propria of the cecum by enterohepatic species in mice following challenge with the spirochete has been reported (58). The significance of cell entry and/or tissue invasion by enterohepatic species and the conditions under which these events take place have not been fully elucidated. Tissue invasion may be a prerequisite for or a consequence of -associated disease in the gastrointestinal tract. It may also play a role in bacterial translocation to the liver and/or into systemic circulation, either as a primary event or secondary to other disease states. The fact that many investigators have encountered enterohepatic species with periplasmic fibers in the gastrointestinal tract of laboratory rodents no doubt reflects the frequency with which these animals are used in biomedical research. Bacteria with the same morphology have also been isolated from the gastrointestinal tract of many mammal species, including humans, monkeys, sheep, pigs, dogs, cats, mice, rats, hamsters, and gerbils. The complete range of host species from which these organisms can be isolated is not known. It may be that such bacteria can flourish wherever a mucus-rich interface between epithelial cells and the lumen of an alimentary tract is found. Certainly the observation of bacteria that appear morphologically indistinguishable from enterohepatic species in the hind-gut of , the American cockroach (4), suggests that the distribution of these microbes is wide indeed.