Electron microscopy of the microbial populations present and their modes of attack on various cellulosic substrates undergoing digestion in the sheep rumen.

Cotton fibers and various cell wall preparations from grass leaves and from the feces of sheep fed on dried grass were placed in the sheep rumen in bags made from 5-mum-mesh nylon cloth. After periods of from 3 to 48 h, bags were removed, and the contents were fixed, embedded, sectioned, and stained for electron microscopy. Some of the bacteria present were seen to be closely associated with the cell walls, either tunneling within them or making very close contact. Evidence was obtained for differential digestion of cell walls and of the layers within them. Distinct differences were noticed between bacterial populations attacking the more susceptible wall types and those attacking feces cell walls and cotton fibers. Among the latter, the dominant form was a long, thin rod with a typical gramnegative cell wall structure, different from that described for Bacteroides succinogenes S85 or for Butyrivibrio fibrisolvens.