Genetically modified rumen bacteria protect sheep from fluoroacetate poisoning

In two separate experiments, test sheep were inoculated with a mixture of four genetically modified strains of Butyrivibrio fibrisolvens, which attained a combined population level of 1 06-10’ cells per ml in each animal. The bacteria had been modified to express the enzyme fluoroacetate dehalogenase. Uninoculated sheep were used as controls. Experiment I included two Merino/ Border Leicester cross-bred sheep in each group, and Experiment II included three pure-bred Merinos in each group. Sheep were fed incremental doses of fluoroacetate, and their condition and behaviour was monitored continuously. In both experiments, the control sheep showed dramatic symptoms of fluoroacetate poisoning, judged by behaviour and heart-rate, while the inoculated sheep remained comparatively unaffected. At higher doses of poison, inoculated sheep showed symptoms of intoxication. In Experiment I, control sheep exhibited hyper+xcitability as the first major symptom of poisoning, accompanied by compulsive eating. Vigorous movement ofthe control sheep broke loose the ECG monitoring cables, preventing comparison of heart-rate between groups. Both control sheep died from acute symptoms of fluoroacetate poisoning. The test sheep T2 with lo6 modified bacteria/ml of rurnen contents did not exhibit hyper-excitability or compulsive eating, but died of pulmonary oedema five hours after the control sheep. Sheep Tl, with 10’ bacteria/ml of rumen contents, survived without showing major symptoms of toxicity. In Experiment II, all three of the control sheep showed severe effects of poisoning that differed markedly from those seen in Experiment I. Activity was reduced to occasional periods of standing, and the animals ceased eating and drinking. ECG measurements showed frequent periods of extremely high heart rate (140 300 bpm) from 32 h to 72 h. The test sheep continued to eat and drink normally throughout the first 36 hours of the experiment, reducing intake by approximately 30% during the 36 72 hour period. Heart rates in the test sheep were occasionally raised to 80 100 bpm, during the later parts of the experiment. In Experiment II, none of the animals showed hyper-excitability at any stage of the experiment. The different responses of the animals to fluoroacetate poisoning appeared to be characteristic of the different breeds. The results showed clearly that rumen bacteria capable of detoxifying the poison fluoroacetate, significantly reduced the effect of this poison upon the host animals.