Fibre-degrading microorganisms in the monogastric digestive tract

Abstract

The aim of this review is to take stock of our present knowledge of the qualitative and quantitative composition and activity of the microbial population responsible for the degradation of plant cell-wall polysaccharides in monogastric animals and humans. It focuses particularly on animals that may be used as an experimental model for humans. There is also a brief survey of the fibre breakdown in invertebrates. Although the effect of dietary fibres on the human and monogastric animal digestive physiology is generating more and more interest, the knowledge of the microorganisms involved in the fibre breakdown is still limited. Compared with the rumen, only a few studies concerning these micro-organisms in the intestinal tract of non-ruminant animals have been done. Except in humans, studies have essentially been focused on the cellulose-digesting bacteria. The hemicellulolytic and pectinolytic bacteria have received less attention. Bacteria are considered to be the major plant cell wall degrading agents. Little is known of the ability of intestinal protozoa to degrade complex polysaccharides; much work remains to be done on these microorganisms. The data reported in this review show that the numbers of fibre-degrading bacteria in the large intestine of monogastric animals are similar to those found in the rumen and suggest that the cellulolytic bacteria are largely similar in different gut regions of different animal species. Bacteroides succinogenes and Ruminococcus flavefaciens seem largely distributed in the higher animals. Similar strains of Bacteroides, especially B. ruminicola, are hemicellulolytic bacteria found in the digestive tract of numerous mammals, even in the human colon. In contrast, the fibre-degrading microorganisms in the digestive tract of insects or other invertebrates differ from those found in mammals. Flagellate protozoa are the main, if not the only, agents of degradation of plant cell-wall polysaccharides. In many species, the role of bacteria is not clear. This suggests a divergence in the development of microbial digestion of cellulose and hemicellulose in the gastrointestinal tract of invertebrate and higher animals. Much work remains to be done on the taxonomy, ecology, physiology and metabolic activities of the microorganisms capable of degrading plant cell walls. These works should also concern polysaccharide-degrading enzymes of the different microbial species. A better knowledge of the mechanisms involved in polysaccharide breakdown is also required for the optimization of the dietary fibre-digesting microorganisms in animals such as herbivorous birds, reptiles and fishes.