Variations in the ability of ruminal Gram-negative Prevotella species to resist monensin

Gram-negative, ruminal Prevotella strains (n = 15) differed greatly in thei r sensitivity to the feed additive monensin. Strains that were repeatedly t ransferred with sublethal doses tolerated more monensin than those that wer e unadapted, but growth experiments indicated that the sensitivity range wa s as great as 2000-fold. Prevotella bryantii B(1)4 grew with monensin conce ntrations as high as 20 mu M, but P. ruminicola H15a, D31d, 20-63, E40a, an d D42f never initiated growth if monensin was greater than 0.01 mu M. Washe d cell, preparations that were energized with glucose lost intracellular po tassium when monensin was added, and potassium depletion could also be used as an index of monensin sensitivity. Adapted cells of P. bryantii B(1)4 ha d a half-maximal potassium depletion constant (K-d) of 3.2 mu M, but the Kd values of P. ruminicola strains H15a, D31d, 20-63, E40a, and D42f were les s than 0.04 mu M Maximal potassium depletion (K-max) Values range from 90% to 40%, and monensin-adapted cells always had lower K-max values than unada pted cells. A linear regression of log K-d/K-max versus percentage decrease in optical density divided by monensin concentration had an r(2) of 0.75, and this regression indicated that potassium depletion from washed cells cl osely correlated with growth inhibition. P. bryantii B(1)4 had a K-d/K-max ratio that was sevenfold greater than other Prevotella strains, and this re sult indicated that P. bryantii may be unusual in its ability to grow with very high concentrations of monensin.