Antibiotics in Rumen Digestion and Synthesis III. The Effect of Aureomycin on Rumen Microorganisms, with Special Reference to the Streptococci and Coliform Groups,

The use of some antibiotics and feed supplements containing antibiotics to stimulate growth in certain farm mammals has been established, but the mode of action of these selective or inhibitory chemical agents is relatively unknown insofar as their effect on the microorganisms in the bovine rumen is concerned. Bartley et al.(2) could not find a consistent difference between rumen microorganisms in control and aureomycin-fed calves by using a direct microscopic method.Neumann et al. (22) observed that the total bacterial counts in the rumen ingesta of aureomycin-fed heifers were about the same as in control heifers, although the types of microorganisms were less diverse when aureomycin was fed.This observation suggested that aureomycin can inhibit some of the naturally-occurring rumen flora.In an in vitro study of cellulose digestion with tureen microorganisms, Wasserman et al. (31) found that low concentrations of penicillin and streptomycin stimulated the cellulolytic microorganisms and neomycin was stimulatory in all concentrations used, whereas chloromycetin inhibited the eellulolytic microorganisms.A corresponding increase in total bacterial count was observed also as cellulolytic activity increased.Other investigators have found that the composition of the r~tion has a definite influence on the number, size, and types of rumen microorganisms (1,4,5, 8,9,10,11,12,19,24,25,26). A high grain ration has been shown to decrease the microorganisms which are characteristically associated with a relatively high roughage ration (24,25,26).Burroughs et al. (10) suggested that the mineral constituents in roughage make an important contribution to the metabolism of the cellulolytic microorganisms.Bentley et al. (4) indicated that the low phosphorus content of poor quality hay was a limiting factor in cellulose digestion.The total bacterial count was decreased markedly when starch was added to a roughage ration, but the count increased when casein Received for publication January 19, 1953.