In vitro microbial digestion of straw cell wall polysaccharides in response to supplementation with different sources of carbohydrates

The effect of carbohydrate supplementation on microbial fibre digestion was studied in vitro, by measuring the disappearance of cell wall monosacchari des, bacterial adhesion (mu-mol purine bases per g residue), and total (per g residue) and bacterial (per mu-mol purine bases) polysaccharidase activi ty. Straw cell walls (CW, 0.5% w/v) were cultured in medium supplemented wi th (0.275% w/v) or without starch, a sugar mixture, or pectin. Supplementat ion with these constituents did not cause a drop in pH below 6.1, and incre ased all parameters investigated with the exception of bacterial polysaccha ridase activity, which was higher for CW cultures, suggesting a higher prop ortion of fibrolytic bacteria in the adherent population. By comparison wit h starch and sugar, pectin supplementation resulted in a lower proportion o f residual sugars remaining from cell walls after 60 and 72 h (P < 0.05), w hich resulted in greater bacterial adhesion after 8 and 12 h (P < 0.05) and higher total cellulase activity after 8 h (P < 0.01). This was perhaps bec ause pectin may cover particle surfaces, protecting the digestive area from external factors, or may act as a substrate for cellulolytic bacteria. The lack of differences in bacterial enzymatic activities suggests the absence of qualitative or quantitative differences in the adherent fibrolytic popu lation.