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Sensitivity of Streptococcus Bovis to Various Antibiotics

Merck Institute for Therapeutic Research¹, Rahway, NJ 07065

Abstract

Ruminal lactic acidosis is thought to be initiated by a proliferation of Streptococcus bovis following the overeating of readily fermentable carbohydrate. The purpose of this work was to identify those antibiotics that are most active against this microbe for later evaluation in vivo as a prophylactic treatment for lactic acidosis.

Studies were conducted to develop a sensitive microbiological assay for activity against Streptococcus bovis and to use this assay 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 zone of inihibition squared. The assay is sensitive to the reference antibiotic, thiopeptin, at .3 µg/ml and provides an assay range from .3 to 5.0 µg/ml. Of the antibiotics evaluated, the penicillins, including penicillin G and ampicillin, and the sulfur-containing peptide antibiotics, including thiopeptin, sulfo-mycin, thiostrepton, siomycin, sporangiomycin and taitomycin, were the most active.

Although the penicillins were the most active in vitro, previous work by others has demonstrated that the rumen contains an active penicillinase which destroys penicillin and renders it relatively ineffective in vivo. The sulfur-containing peptide antibiotics, especially thiopeptin, sulfomycin and thiostrepton, are highly active against Streptococcus bovis and appear to be excellent candidates for the control of ruminal Streptococcus bovis.

¹ Department of Animal Nutrition and Physiology.

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