SENSITIVITY OF STREPTOCOCCUS-BOVIS TO VARIOUS ANTIBIOTICS

Ruminal lactic acidosis may be initiated by a proliferation of S. bovis following the overeating of readily fermentable carbohydrate. Those antibiotics that are most active against this microbe were identified for later evaluation in vivo as a prophylactic treatment for lactic acidosis. A sensitive microbiological assay for activity against S. bovis was used to evaluate available antibiotics. A procedure for a large capacity disc assay is described. The assay produces a linear relationship between the log of the concentration of the antibiotic and the diameter of the inhibition zone squared. The assay is sensitive to the reference antibiotic, thiopeptin, at 0.3 .mu.g/ml and provides an assay range from 0.3 to 5.0 .mu.g/ml. Of the antibiotics evaluated, the penicillins, including penicillin G and ampicillin, and the S-containing peptide antibiotics, including thiopeptin, sulfomycin, thiostrepton, siomycin, sporangiomycin and taitomycin, were the most active. Although the penicillins were the most active in vitro, previous studies demonstrated that the rumen contains an active penicillinase which destroys penicillin and renders it relatively ineffective in vivo. The S-containing peptide antibiotics, especially thiopeptin, sulfomycin and thiostrepton, are highly active against S. bovis and may be excellent candidates for the control of ruminal S. bovis.