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Spirochetes are structurally unique microorganisms found in the gastrointestinal tracts of most mammals. In an attempt to determine the ecological status of these bacteria, enumeration and distribution of morphologically distinct spirochetes were studied in the tracts of conventional laboratory rats. Five different types were seen to colonize infant rats between 19 and 26 days of age and subsequently to form stable communities in all 15 adults examined. Two types were found predominantly in lumen contents of the large bowel. The other three were consistently seen in the mucous blanket, attached to enterocyte surfaces or deep in the glands of the cecum and proximal colon. One type inhabiting the mucosal environment was also seen to pass into and through epithelial cells with no detectable host response. We conclude that spirochetes fulfill all the criteria for autochthonicity to the rat gastrointestinal tract.

Mucin histochemistry on sections of colon from germ-free and conventional mouse pups showed that all goblet cell mucins were sulphated at birth. During the first two weeks of post natal development, the pattern of mucin production in the ascending colon changed to a distribution of non-sulphated mucins towards the apical zone of the crypts and sulphated sialomucins basally. In conventional animals during the third postnatal week when the complex micro-flora of the colon was becoming established, the typical adult mucin distribution pattern developed, with sulphated mucins now confined to the upper third of the crypt. However, in the absence of a colonizing micro-flora crypt mucins become more and more sulphated until at weaning, most goblet cells of the ascending colon were producing fully or partially sulphated mucins, except for one or two cells at the very base of the crypt.

Electron microscopy revealed that 80% of captured vervet monkeys, held in quarantine for experimental use, showed extensive proliferation of spiral-shaped bacteria on the mucosal epithelium of the large bowel. A consortium, consisting of a predominant spirillum together with a spirochaete, was usually seen as a lawn covering the colonic epithelium. Sparsely populated areas showed preferential colonization of the tubular glands. Pathological changes were minimal, being confined to the microvillus border, and affected animals showed no evidence of distress. These findings are compared with those of a similar condition known as "intestinal spirochaetosis" reported in other primates, including man.

During enterohepatic circulation conjugated steroids are hydrolyzed by intestinal bacteria, further metabolized mainly by anaerobic bacteria, reabsorbed, reconjugated and delivered to the blood for renal excretion. 21-Dehydroxlylation of tetrahydrodeoxycorticosterone (THCOD) to pregnanolone is performed exclusively by Eubacterium lentum, a normal inhabitant of the intestinal flora. The concentration of E. lentum in feces can be determined from the highest dilution of fresh voided stool capable of 21-dehydroxylating THDOC. Such studies revealed that the concentration increases from 106/g wet feces during the first year of life to 108/g feces in the 7th and 8th decade. Colonization appears to be independent of diet since the organisms are present in babies whether breast-fed or on formula, in subjects on a “Western diet” and in Africans consuming a diet low in meat and fat.