Antibiotic Experiments with Conventional Animals

Norman (3) demonstrated that the types of bile acids found in normal rat bile were not the same as those which were excreted in the feces. However, when the rats were fed high levels of antibiotics, the fecal bile acids were excreted essentially unchanged from the biliary bile acids (4). The intestinal bacteria were responsible for the hydrolysis of the biliary taurine-conjugated bile acids to the free bile acids found in the feces. Norman also showed that the dehydroxylation of cholic acid to deoxycholic acid could be prevented by Inhibiting the intestinal bacteria. The total amount of fecal bile acid excreted by conventional chicks has been found to be significantly lowered (5) by incorporation of an antibiotic into the diet. 

Cholesterol Feeding and Liver and Serum Cholesterol Levels of Germfree and Conventional Animals 

The intestinal microflora exerts a protective effect in several species 

Similar data have been reported for liver cholesterol levels in rats (7). Germfree and conventional rats fed a cholesterol-free diet had similar liver cholesterol levels however, as the amount of cholesterol in the diet was increased, the liver cholesterol levels of the germfree rats rose three times higher than did those of the comparable conventional rats. In these rats, the serum cholesterol levels did not change with microbiological status or dietary cholesterol. 

Large changes in serum cholesterol levels have been shown to occur when gnotobiotic swine were fed a cholesterol-containing diet. Gnotobiotic swine fed a cholesterol-containing diet had serum cholesterol levels of about 1000 mg %, but when they were removed from isolators and raised in a nonsterile environment, the serum cholesterol levels fell to 100 mg % within 3 weeks (8). The fall in serum cholesterol could be correlated with the development of an intestinal microflora which metabolized the primary bile acids to secondary bile acids. Similar swine fed an identical but cholesterol-free diet had serum cholesterol levels of about 100 mg % during both periods. 

---