THE EFFECT OF TETRONASIN AND MONENSIN ON FERMENTATION, MICROBIAL NUMBERS AND THE DEVELOPMENT OF IONOPHORE-RESISTANT BACTERIA IN THE RUMEN

The Gram-negative rumen bacteria Fibrobacter succinogenes S85, Prevote lla ruminicola M384 and Veillonella parvula L59 were grown in media co ntaining successively increasing concentrations of the ionophores, mon ensin and tetronasin. All three species became more resistant to the i onophore with which they were grown. Increased resistance to one ionop hore caused increased resistance to the other, and cross-resistance to another ionophore-lasalocid-and an antibiotic-avoparcin. Recovery of tetronasin-resistant bacteria from the rumen of monensin-fed sheep inc reased and vice versa, indicating that similar cross-resistance occurr ed in vivo.