The isolation of tetracycline-resistant strains of strictly anaerobic bacteria from the rumen

Tetracycline resistant (Tc^R) strains of three of the major species of strictly anaerobic rumen bacteria Megasphaera elsdenii, Selenomonas ruminantium and Butyrivibrio fibrisolvens , were recovered with an isolation medium containing 20 μg/ml tetracycline. Only two of 14 strains of these species from other sources, isolated without antibiotic selection, showed tetracycline resistance. Evidence was found for the presence of plasmids in two tetracycline-resistant strains of M. elsdenii , and in some strains of S. ruminantium.