Five species of rumen ciliate protozoa, isolated from sheep in which they were the only ciliate protozoon, and mixed rumen ciliates were incubated with labelled bacteria and the rate of uptake and digestion of the bacteria was followed. Evidence is presented that mixed rumen bacteria labelled by incubation with 14C-isoleucine, 14C-guanine or 35S-sulphate were engulfed more rapidly by the protozoa than those labelled with 14C-glucose but that, with Entodinium spp. at least, this apparent lower rate of uptake was associated with an increased rate of digestion. The rate of uptake of mixed rumen bacteria (at 109/ml) varied from 230 to 2670 bacteria/protozoon per hour, depending on the protozoal species, and increased with increasing bacterial population density. Starvation of the protozoa decreased the initial rate of uptake of bacteria by up to 60% and the length of time over which the protozoa would engulf bacteria. Entodinium spp. and Epidinium ecaudatum caudatum engulfed fewer bacteria when these bacteria were prepared from the same sheep as the protozoa than when they were prepared from sheep with no or other protozoal populations. Of the protozoa tested, only Entodinium spp. and the mixed large rumen protozoa digested mixed rumen bacteria, Eremoplastron bovisand Epidinium ecaudatum caudatum being inactive. It is suggested that previous estimates of the rate of uptake and digestion of bacteria based on studies in vitro were too high.
The non-rumen bacteria Eschericfoia coli and Klebsiella aerogenes and the rumen bacterium Megasphaera elsdenii were not or only poorly taken up by the protozoa. Entodinium spp. selectively engulfed bacteria of rumen origin and digested two cellulolytic bacteria rapidly. In contrast, Eremplastron bovis engulfed these cellulolytio bacteria more slowly than other bacteria. Entodinium spp. and the large rumen protozoa digested more bacterial species than the other protozoa but Butyrivibrio fibrisolvens was digested more rapidly than other bacteria by all the protozoa.