BINDING OF RADIOLABELED MONENSIN AND LASALOCID TO RUMINAL MICROORGANISMS AND FEED

Gram-negative, ionophore-resistant ruminal bacteria and Gram-positive, ionophore-sensitive species bound similar amounts of [C-14]lasalocid, but neither group bound large amounts of [C-14]monensin. Membrane ves icles also bound more lasalocid than monensin (P < .05). The binding w as first-order at low cell or vesicle concentrations and saturable at high cell or vesicle densities. Streptococcus bovis was inhibited by b oth monensin and lasalocid (5 mu M), but cells that were re-incubated in medium lacking ionophore grew rapidly. Lasalocid-treated cells grew very slowly when they were, resuspended in fresh medium. Based on the se results, it seemed that lasalocid had a higher affinity for bacteri al membranes than monensin. Mixed bacteria, however, bound nearly equa l amounts of [C-14]inonensin and [C-14]lasalocid (P > .05). Monensin b inding was greatly reduced when the mixed ruminal bacteria were pretre ated with Tris+EDTA (P < .05), but Tris+EDTA did not affect the bindin g of lasalocid. Mixed ruminal protozoa always took up more lasalocid t han monensin (P < .05), but feed particles bound equal amounts of [C-1 4]lasalocid and [C-14]monensin (P > .05). Based on the binding capacit y of mixed ruminal bacteria, ruminal protozoa, and feed particles, the re would be little free ionophore in ruminal fluid.