The ability of "low G plus C gram-positive" ruminal bacteria to resist monensin and counteract potassium depletion

Gram-negative ruminal bacteria with an outer membrane are generally more re sistant to the feed additive, monensin, than Gram-positive species, but som e bacteria can adapt and increase their resistance. 16S rRNA sequencing ind icates that a variety of ruminal bacteria are found in the "low G + C Gram- positive group," but some of these bacteria are monensin resistant and were previously described as Gram-negative species (e.g., Selenomonas ruminanti um and Megasphaera elsdenii). The activity of monensin can be assayed by it s ability to cause potassium loss, and results indicated that the amount of monensin needed to catalyze half maximal potassium depletion (K-d) from lo w G + C gram-positive ruminal bacteria varied by as much as 130-fold. The K -d values for Butyrivibrio fibrisolvens 49, Streptococcus bovis JB1, Clostr idium aminophilum F. S. ruminantium H4, and M. elsdenii B159 were 10, 65, 1 00, 1020, and 1330 nM monensin, respectively. B. fibrisolvens was very sens itive to monensin, and ii: did not adapt. S. bovis and C. aminophilum cultu res that were transferred repeatedly with sub-lethal doses of monensin had higher K-d values than unadapted cultures, but the K-d was always less than 800 nM. S. ruminantium and M. elsdenii cells were highly resistant (K-d > 1000 nM), and this resistance could be explained by the ability of these lo w G + C Cram-positive bacteria to synthesize outer membranes.