IDENTITY AND INTERACTIONS OF RUMEN MICROBES ASSOCIATED WITH DIETARY SULFATE-INDUCED POLIOENCEPHALOMALACIA IN CATTLE

To study their role in sulfate reduction, anaerobic bacteria were cult ured from numen fluid samples of cattle fed high-carbohydrate, short-f iber diets with and without added sulfate. The steers fed the diet wit h added sulfate developed polioencephalomalacia. Microbiological metho ds included colony-type profiles, molybdate sensitivity, presence of d esulfoviridin, sulfate reduction rates of pure and mixed cultures, and incubation time effects on sulfate reduction. Colony-type profiles in dicated decreased diversity, but no relative change in numbers of sulf ate-reducing bacteria in rumen fluid from cattle fed diets with and wi thout added sulfate. Thirteen bacterial isolates were selected for fur ther study on the basis of colony type, sulfate-reducing activity, and growth in lactate, sulfate, and yeast extract media. Seven of the iso lates had Desulfovibrio-like characteristics (ie, they were gram-negat ive, motile rods that reduced sulfate, were inhibited by molybdate, an d contained the pigment desulfoviridin). The remaining 6 isolates were gram-negative, nonmotile rods. Four of these released sulfide from cy steine, and 2 generated only limited amounts of sulfide from sulfate o r cysteine. The 7 sulfate-reducing isolates generated sulfide in rumen fluid broth medium at greater rates than those observed in fresh nume n fluid. Sulfate reduction could be sustained in cultures for prolonge d incubation times if the gas phase containing hydrogen sulfide was re placed at frequent intervals. Variations in the amount of sulfate redu ced by the pure cultures were most pronounced at short incubation time s. Sulfate reduction was not inhibited in mixed cultures of sulfate-re ducing and nonsulfate-reducing bacteria.