SUMMARY: Suspensions of the oligotrich protozoan, , were extracted from whole rumen contents by allowing the large protozoa to settle out from the bulk of the rumen liquor and concentrating by pouring off the supernatant. The settled protozoa were separated from the associated rumen debris by incubation with maltose, when a bacterial fermentation set in with a vigorous production of gas which carried most of the rumen debris to the surface for removal by gentle suction. After repeated washing of the protozoa by resuspension in buffer and removal of the supernatant, was cultivated on powdered cotton-wool (0·05%) and powdered hay (0·05%) in buffer at pH 6·8. These oligotrich protozoa remained alive and dividing for 12 days. A species of , concentrated from rumen liquor and washed by centrifugation, after the larger protozoa had settled out, was kept alive for 12-14 days under similar conditions in the presence of rice starch grains instead of powdered cotton-wool. Streptomycin (560 μg./ml.) was used to prepare cultures of protozoa which were bacteriologically sterile when tested by streaking agar plates containing yeast extract, starch and glucose or cellobiose. Under these conditions, spp. disappeared within 24 hr. and in 3 or 4 days. This suggests that these organisms are dependent on bacteria for some of their metabolic processes. It is concluded that the species of studied here contained symbiotic cellulolytic bacteria which were destroyed by streptomycin, but that the protozoa continued at least for some time to draw on their polysac-charide reserves after treatment with the antibiotic. Rumen oligotrichs may be of use to the sheep by acting as ‘reservoirs' of polysaccharide and so preventing rapid bacterial breakdown. The relationship between the protozoa and the ruminant is considered to be a true symbiosis.


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