An Improved Nonselective Culture Medium for Ruminal Bacteria and Its Use in Determining Diurnal Variation in Numbers of Bacteria in the Rumen

Summary An improved rumen fluid agar medium was developed that permitted the growth of about double the numbers of bacteria from ruminal contents grown in the medium previously used. It allowed much larger numbers of colonies to be counted and isolated with greater ease from a given volume of medium and after longer incubation periods. The main modifications made were the use of rumen fluid clarified by centrifugation, reducing the concentration of the sugar energy sources (glucose and cellobiose) and inclusion of soluble starch as an additional energy source, and changing the reducing agent from cysteine to a combination of cysteine and sodium sulfide. Many other modifications in the medium were tested and found to be without effect or detrimental to colony counts. Experiments indicate that about the same range of species of bacteria are cultured in the improved medium as in the old, except that the improved medium allows the growth of bacteroides strains that utilize starch but not glucose or cellobiose. Significant differences were shown in nonselective viable counts of ruminal bacteria obtained from samples of rumen contents collected at different times during the day from a cow fed alfalfa hay or a grain mixture once a day, and there were also differences in counts due to the ration.