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Most observations of the effect of antibiotics on the rumen microbial population were casual observations subsequent to incorporation of antibiotics in the animal feed, and only a few studies (2) were conducted in vitro. S. M. Finegold, L. A. Siewart, and W. L. Hewitt, (Bacteriol. Proc., p. 59, 1957) devised a medium containing neomycin (100 to 200 ,ug/ml) and vancomycin (7.5 Ag/ml) to isolate anaerobic Bacteroides from the human oral cavity. We, therefore, attempted to study the effect of these antibiotics on rumen microorganisms with the objective of selective isolation of rumen Bacteroides bacteria. Pure cultures which represent important cellulolytic or amylolytic bacteria in the rumen were grown anaerobically in 8-ml quantities of the general-purpose medium (1). This medium contained 0.1% (w/v) glucose, 0.1% (w/v) cellobiose, 0.2% (w/v) cellulose (Whatman no. 1 slurry), 0.5% Trypticase (BBL), minerals, 0.1 % (v/v) of each of isovaleric, n-valeric, isobutyric, and 2-methyl butyric acids, carbonatebicarbonate buffer, and cysteine hydrochloride. To each tube was added 1 ml of the sterile antibiotic solution and 1.0 ml of the inoculum. In control tubes, the antibiotic solution was replaced by 1.0 ml of sterile water. Inocula, at an optical density between 0.6 to 1.0, were taken from cultures that had been grown in the same medium for 18 hr. Growth was measured by determination of optical density both initially and after incubation for 18 hr at 39 C. Viable counts were made by diluting each inoculum in buffered mineral solution containing cysteine hydrochloride (1) and then inoculating culture tubes, each containing 9 ml of the general-purpose medium plus 2% agar. When neomycin was added, its final concentration was 100 ,ug/ml. The roll-tube technique (3) was used for inoculation. The tubes were incubated for 10 days at 39 C and the colonies were counted.

Effect of neomycin and vancomycin on growth of some rumen bacteria.