Some factors influencing the chemical composition of mixed rumen bacteria.

I. Sheep, cows and calves fitted with rumen cannulas were given diets mostly containing 10-16 g nitrogen/kg dry matter and consisting of roughage and cereals. Mixed bacteria w-ere separated from samples of their lumen contents. 2. Bacteria taken 4-6 h after a feed from calves which were kept in an experimental calfhouse with no contact with adult animals (environment -4) contained more a-dextran, less total N and higher nucleic acid : total N ratios than similar bacteria from calves reared in contact with adult sheep (environment C ) but otherwise treated in an identical way. 3. Mixed bacteria taken 4-6 h after a feed from sheep and cows were similar in composition, with respect to nitrogenous components, to those from the ‘environment C’ calves. This composition did not vary significantly when diets containing differing proportions of roughage were given. 4. The ‘environment A’ calves were free of ciliate protozoa. When they were placed in contact with, and were inoculated with rumen contents from, adult cattle (environment B), they rapidly developed a normal protozoal population and the chemical composition of their rumen bacteria became like that of the bacteria from the ‘environment C’ calves. 5 . Mixed bacteria taken just before a feed, from either cows or ‘environment A’ calves, showed significantly lower RNA-N : total N ratios and slightly (but not usually significantly) higher DNA-N:total N ratios than bacteria taken 4-6 h after feeding. Total N contents of the bacteria did not change consistently with time after feeding. 6 . The possible significance of these differences in relation to the nutrition of the host animal is discussed.