Effect of monensin and lasalocid-sodium on the growth of methanogenic and rumen saccharolytic bacteria.

It is thought that monensin increases the efficiency of feed utilization by cattle by altering the rumen fermentation. We studied the effect of monensin and the related ionophore antibiotic lasalocid-sodium (Hoffman-LaRoche) on the growth of methanogenic and rumen saccharolytic bacteria in a complex medium containing rumen fluid. Ruminococcus albus, Ruminococcus flavefaciens, and Butyrivibrio fibrisolvens were inhibited by 2.5 mug of monensin or lasalocid per ml. Growth of Bacteroides succinogenes and Bacteroides ruminicola was delayed by 2.5 mug of monensin or lasalocid per ml. Populations of B. succinogenes and B. ruminicola that were resistant to 20 mug of either drug per ml were rapidly selected by growth in the presence of each drug at 5.0 mug/ml. Selenomonas ruminantium was insensitive to 40 mug of monensin or lasalocid per ml. Either antibiotic (10 mug/ml) inhibited Methanobacterium MOH, Methanobacterium formicicum, and Methanosarcina barkeri MS. Methanobacterium ruminantium PS was insensitive to 40 mug of monensin or 20 mug of lasalocid per ml. The methanogenic strain 442 was insensitive to 40 mug of monensin but sensitive to 10 mug of lasalocid per ml. The results suggest that monensin or lasalocid acts in the rumen by selecting for succinate-forming Bacteroides and for S. ruminantium, a propionate producer that decarboxylates succinate to propionate. The selection could lead to an increase in rumen propionate formation. Selection against H(2) and formate producers, e.g. R. albus, R. flavefaciens, and B. fibrisolvens, could lead to a depression of methane production in the rumen.