METABOLISM OF TOXIC PYRROLIZIDINE ALKALOIDS FROM TANSY RAGWORT (SENECIO-JACOBAEA) IN OVINE RUMINAL FLUID UNDER ANAEROBIC CONDITIONS

The ability of ovine ruminal fluid to metabolize pyrrolizidine alkaloid (PA) from Senecio jacobaea under anaerobic conditions was evaluated. Four fistulated sheep fed PA served as individual sources of ruminal fluid, which was incubated in a defined minimal salts medium under two different anaerobic conditions, denitrifying and methanogenic. Anaerobic cultures amended with ovine ruminal fluids (20%), PA (100-mu-g/ml), and a defined minimal salts medium were monitored for a period of several days. These cultures revealed that while PA was not depleted in sterile, autoclaved controls or under denitrifying conditions, it was metabolized during periods of active methanogenesis under methanogenic conditions. In addition, samples of ruminal fluid were separated by differential centrifugation under anaerobic conditions, and the resultant supernatants were tested for their ability to metabolize PA as compared with those of the respective uncentrifuged control fluids. Uncentrifuged controls exhibited a PA depletion rate of -4.04 +/- 0.17-mu-g of PA per ml per h. Supernatants 1 (centrifuged at 41 x g for 2 min), 2 (centrifuged at 166 x g for 5 min), and 3 (centrifuged at 1,500 x g for 10 min) exhibited significantly slower depletion rates, with slopes of data representing -1.64 +/- 0.16, -1.44 +/-0.16, and -1.48 +/- 0.16-mu-g of PA metabolized per ml per h, respectively, demonstrating no statistically significant difference among the supernatant cultures. Microscopic evaluations revealed that protozoa were present in the control whole ruminal fluid and to a lesser extent in supernatant 1, while supernatants 2 and 3 contained only bacteria. Supernatant 4 (centrifuged at 1,900 x g for 20 min), from which all visible microflora were removed, exhibited no PA depletion during the course of incubation. The microbial population common to all supernatants and the control ruminal fluid were the small bacteria. These data suggest that the metabolism of PA can be mediated by a consortium (or single isolate) of small bacteria in the ovine rumen.