Role of the Cecum in Maintaining Δ5-Steroid- and Fatty Acid-reducing Activity of the Rat Intestinal Microflora

Abstract

In conventional rats fed a 9% corn oil diet, 54.3% of the fecal C₁₈-fatty acids were saturated, as compared to only 22.9% in germfree rats. The fecal C₁₈-fatty acids of conventional rats consisted of 54.3% stearic acid, 36% octadecenoic acid, and 9.7% octadecadienoic acid. In addition, conventional rats excreted at least six unidentified “bacterial” fatty acids. Germfree rats did not excrete “bacterial” fatty acids, and the fecal C₁₈-fatty acids consisted of 22.9% stearic acid, 37.1% octadecenoic acid and 40% octadecadienoic acid. Cecectomy had no influence on the fecal C₁₈-fatty acids of germfree rats. In conventional rats, however, cecectomy resulted in almost complete disappearance of the “bacterial” fatty acids, and in reversion of the ratio of saturated to unsaturated C₁₈-fatty acids. These results suggested that microbial transformation of oleic acid into stearic acid was completely inhibited after cecectomy, whereas transformation of linoleic acid into octadecenoic acid was only partially inhibited. Evidence supporting this hypothesis was obtained from experiments in vitro. When incubated anaerobically, suspensions of fecal microorganisms from unoperated conventional rats converted oleic and linoleic acid into stearic acid. Under similar conditions, fecal suspensions of cecectomized rats converted linoleic acid into an octadecenoic acid, but failed to transform oleic acid into stearic acid. In conventional rats, or in gnotobiotic rats associated with a Clostridium sp. plus Eubacterium 21,408 (a Δ⁵-steroid-reducing microorganism), cholesterol was extensively hydrogenated into coprostanol. Production of coprostanol was abolished within 2 days after cecectomy. Eubacterium 21,408 was unable to develop in the intestine of cecectomized rats. Disappearance of Eubacterium 21,408 after cecectomy could not be related to modification of the pH or E_h values of the colonic contents. The available evidence suggests that the cecum of rodents is involved in maintaining normal microecology of the intestine.