No abstract

Colonization of the gastrointestinal tract by bacteria of the normal flora was followed by bacteriological and special histological techniques in mice from several colonies. These histological techniques were designed to preserve the intimate associations that become established between particular strains of microorganisms and the epithelium of the mucosa of certain areas of the gut. The findings were as follows: 1. The various strains of bacteria of the normal flora became established in the different areas of the guts of infant mice according to a definite time sequence. 2. The first types of bacteria that could be cultured from the gut were lactobacilli and Group N streptococci. Within the first day after birth, these bacteria colonized the entire digestive tract and formed layers on the stratified squamous epithelium of the nonsecreting portion of the stomach and of the distal esophagus. 3. The bacterial types that appeared next were coliforms and enterococci. From about the 9th to the 18th day after birth, these bacteria could be cultured in extremely high numbers from the cecum and the colon. Histological sections of those organs taken during the first 2 or 3 days of that interval revealed microcolonies of Gram-positive cocci in pairs and tiny Gram-negative rods embedded in the mucous layer of the epithelium. The microcolonies were well separated from the mixture of digesta and bacteria that occupied the center of the lumen; they may have consisted of the coliforms and enterococci mentioned above; but this possibility remains to be proved. 4. Histological sections also revealed that, at about the 12th day after birth, long, thin Gram-variable rods with tapering ends were present, side by side, with the small Gram-negative rods and Gram-positive cocci in the mucous layer. By the 15th day after birth, the fusiform bacteria formed thick layers in the mucus, and seemed to be the only bacteria remaining in that location. It has not yet been possible to enumerate these tapered rods by culture methods, but as judged by visual appearances in the histological sections, they seemed to outnumber all other bacteria in the cecum and the colon by a factor of as much as 1000. It must be stressed that these bacterial layers are readily disrupted and even washed away by conventional histological techniques; their discovery was largely due to the use of the special histological techniques described in the text. The bacteriological and histological findings described here constitute further evidence for the hypothesis that symbiotic associations exist between microorganisms and animals, and that a very large percentage of the bacteria in the gastrointestinal tract constitutes a true autochthonous flora. The constant occurrence of several distinct associations of bacteria with the special histological structures of the animal host renders obsolete the notion that the intestine constitutes a chemostat in which the bacterial populations are randomly mixed. For a full understanding of the ecology of the normal microflora, it is necessary to think of body surfaces as distinct microenvironments in which virtually pure cultures of a few species of microorganisms interact with their host and the adjacent microbial populations. Experiments based on this hypothesis are admittedly difficult to design, but on the other hand studies based on the assumption that microorganisms exist as mixtures in the gastrointestinal tract will be only of limited value and may often be misleading.

Aerobic and anaerobic cultural techniques and histological methods were used in a study of the effects of environmental and dietary stress on the indigenous microbiota of the gastrointestinal tract of mice. Mice previously inoculated with on July 15, 2020 by guest http://iai.asm.org/ Downloaded from on July 15, 2020 by guest http://iai.asm.org/ Downloaded from

Some indigenous microorganisms localize on epithelial surfaces in various areas of the digestive tracts of animals. One of these, a segmented, filamentous microbe, localizes on the epithelium in the small bowels of mice and rats. These filamentous microbes colonize mice at weaning time and persist in adult animals for at least 2 months. Results of the study of light and electron micrographs suggest that the microorganisms are procaryotic, and that they interact with small bowel epithelial cells to form an attachment site. This site consists of modified epithelial cell membrane and apical cytoplasm adjacent to the attached bacterium. The microbe fills the site with part of its first segment. This segment has a nipple-like appendage on the end inserted into the epithelial cell. The other segments, which compose the rest of the filament, are usually separated by septa. Many of the individual segments contain intrasegmental bodies that appear to be procaryotic cells. Some of these intrasegmental bodies are similar in morphology to the first segment of each filament inserted into an epithelial cell. These intracellular bodies may be components in the life cycle of the microorganism. The organism has not yet been cultured in recognizable form. Therefore, such a hypothesis cannot be proved as yet, nor can the microbe be classified with certainty. Because it localizes in an epithelial habitat in the small bowel, however, it may be a particularly important microbial type in the gastrointestinal ecosystem of laboratory rodents. Adult murine gastrointestinal tracts contain many types of indigenous microorganisms living in relatively stable communities localized in specific regions of the tract (5, 7, 10, 15, 16, 18). The various microbial types in these communities colonize the tracts of neonatal mice in a characteristic and reproducible succession. The succession in suckling mice has been reported for lactic acid bacteria, coliforms, enterococci, bacteroides, fusiform, and spiral-shaped microbes. The habitats of these microorganisms in suckling and adult mice have been described (4, 7, 17, 19). Another microbial type can be found on the epithelial surfaces of villi in the small intestines of adult rats (13, 14), mice (11; C. P. Davis, S. Erlandsen, and D. C. Savage, Abstr. Annu. Meet. Amer. Soc. Microbiol. 1973, p. 57), and chickens (8). This organism has never been identified with certainty. Moreover, little is known about its morphology, habitat, and ecology. In this report, we detail the ultrastructure, habitat, succession, and attachment to epithe-lial cells of these segmented, filamentous microbes in the small bowels of laboratory rodents. MATERIALS AND METHODS Animals. Specific pathogen-free male young adult rats were purchased from four different suppliers

Bacteria of numerous species isolated from the human gastrointestinal tract express bile salt hydrolase (BSH) activity. How this activity contributes to functions of the microorganisms in the gastrointestinal tract is not known. We tested the hypothesis that a BSH protects the cells that produce it from the toxicity of conjugated bile salts. Forty-nine strains of numerous Lactobacillus spp. were assayed to determine their capacities to express BSH activities (taurodeoxycholic acid [TDCA] hydrolase and taurocholic acid [TCA] hydrolase activities) and their capacities to resist the toxicity of a conjugated bile acid (TDCA). Thirty of these strains had been isolated from the human intestine, 15 had been recovered from dairy products, and 4 had originated from other sources. Twenty-six of the strains expressed both TDCA hydrolase and TCA hydrolase activities. One strain that expressed TDCA hydrolase activity did not express TCA hydrolase activity. Conversely, in one strain for which the assay for TDCA hydrolase activity gave a negative result there was evidence of TCA hydrolase activity. Twenty-five of the strains were found to resist the toxicity of TDCA. Fourteen of these strains were of human origin, nine were from dairy products, and two were from other sources. Of the 26 strains expressing both TDCA hydrolase and TCA hydrolase activities, 15 were resistant to TDCA toxicity, 6 were susceptible, and 5 gave inconclusive results. Of the 17 strains that gave negative results for either of the enzymes, 7 were resistant to the toxicity, 9 were susceptible, and 1 gave inconclusive results. These findings do not support the hypothesis tested. They suggest, however, that BSH activity is important at some level for lactobacillus colonization of the human intestine.