Electron microscopy of bacteria involved in the digestion of plant cell walls

Abstract

The bacterial digestion of plant cell wall materials is the result of the complex processes of the microbial ecology of the rumen. As in any ecological system, bacteria are attracted to their specific nutrient substrates and, in this case, adhesion is the first specific event in the digestion of cell wall material. Plant cell walls are differentially colonized in rumen fluid, in that digestible parenchyma cell walls are heavily colonized and rapidly digested, whereas the thick recalcitrant cell walls of the vascular and sclerenchymal tissue of the same plant are sparsely colonized and no digestive pitting is seen. Cells of Bacteroides succinogenes adhere closely to plant cell wall material that is difficult to digest and form distinct digestive pits in this cellulose material. The surfaces of easily-digested plant cell walls are colonized by a more varied bacterial population. Physiologically related organisms are attracted to the adherent cellulolytic bacteria, and their juxtaposition and subsequent growth lead to the development of structured consortia of several organisms on the colonized surfaces. When barley straw is chemically treated with NaOH or NH 3, a larger number of colonizing bacteria are seen, including those recognized as important cellulolytic species, but there are no significant differences in the associated morphotypes. These observations raise the exciting possibility that simple tests of colonization and digestion by specific cellulolytic bacteria (e.g. Bacteroides succinogenes) may provide the basis for the selection and optimization of treatments to increase the digestibility of barley straw, and later, even to provide the basis for the selection of barley cultivars whose straw is more easily digestible.