Effects of Aureomycin on bile acids in rats.

Primary BA's secreted into the duodenum are extensively altered by bacteria in the large intestine. Such changes are not found in GF animals. As a result, GF rats reabsorb BA much more efficiently than do CV controls, and BA and cholesterol pools are higher in GF than CV rats. This indicates the importance of the intestinal flora in the homeostasis of cholesterol metabolism. Antibiotics can affect the extent to which BA's are altered by bacteria. In some cases, the antibiotic treatment also affects cholesterol levels in serum or liver. We have found that treatment of CV rats for only 5 days with low levels of Aureomycin (0.85 micron) led to a predominance of omega-MC over HDC in the feces at 10 days after withdrawal of the antibiotic. This reduced the usual HDC/omega-MC ratio from approximately 2.0 to 0.9 or less. These rats were also found to have liver cholesterol levels modestly elevated over those of controls. In other experiments the decrease in hyodeoxycholate/omega-muricholate ratio was found to persist for at least 90 days after discontinuation of treatment. Later experiments carried out with Aureomycin and with penicillin revealed the possible existence of a resistance factor to Aureomycin. A significant lowering of the hyodeoxycholate/omega-muricholate ratio was now found only with antibiotic concentrations 10 to 100 times greater than those used previously. Possible implications of the persistence of antibiotic effects, as measured by changes in fecal BA's, include effects on vitamin metabolism, colon cancer, and cholesterol metabolism.