Variations in the Ability of Ruminal Gram-Negative Prevotella Species to Resist Monensin

Abstract.

Gram-negative, ruminal Prevotella strains (n = 15) differed greatly in their sensitivity to the feed additive monensin. Strains that were repeatedly transferred with sublethal doses tolerated more monensin than those that were unadapted, but growth experiments indicated that the sensitivity range was as great as 2000-fold. Prevotella bryantii B₁4 grew with monensin concentrations as high as 20 μM, but P. ruminicola H15a, D31d, 20-63, E40a, and D42f never initiated growth if monensin was greater than 0.01 μM. Washed cell preparations that were energized with glucose lost intracellular potassium when monensin was added, and potassium depletion could also be used as an index of monensin sensitivity. Adapted cells of P. bryantii B₁4 had a half-maximal potassium depletion constant (K _d) of 3.2 μM, but the K _d values of P. ruminicola strains H15a, D31d, 20-63, E40a, and D42f were less than 0.04 μM. Maximal potassium depletion (K _max) values range from 90% to 40%, and monensin-adapted cells always had lower K _max values than unadapted cells. A linear regression of log K _d/K _max versus percentage decrease in optical density divided by monensin concentration had an r² of 0.75, and this regression indicated that potassium depletion from washed cells closely correlated with growth inhibition. P. bryantii B₁4 had a K _d/K _max ratio that was sevenfold greater than other Prevotella strains, and this result indicated that P. bryantii may be unusual in its ability to grow with very high concentrations of monensin.