The degradation and utilization of structural polysaccharides of sorghum straw by defined ruminal bacteria

Abstract

Cell wall (CW) preparations of untreated sorghum straw (SORG) and sulfur dioxide-treated SORG (T-SORG) were used as substrates for the solubilization and utilization of CW carbohydrates by pure cultures or pair-combinations of defined rumen bacterial strains. Ruminococcus flavefaciens FD1 and R. albus 7 monocultures and cocultures with Butyrivibrio fibrisolvens D1 had very little solubilizing effect on SORG and even less on T-SORG CW. Fibrobacter succinogenes S85 was the best solubilizer of SORG CW components. The T-SORG CW was composed of fewer hemicellulose components and more cellulose and total CW polysaccharides than the SORG CW. Consequently, F. succinogenes S85 and BL2 monocultures and their cocultures with D1 degraded T-SORG CW polysaccharides by 21–33% units better than those of SORG CW, solubilizing 61–66% of T-SORG CW-glucose, 58–62% of CW-xylose, 58–63% of hemicellulose and 61–65% of total T-SORG structural polysaccharides. Complementary action between B. fibrisolvens D1 and the F. succinogenes strains was identified with respect to coculture growth and carbohydrate utilization, but not with respect to the extent of CW solubilization which was determined mainly by the F. succinogenes strains during 120 h of coculture incubation. In both CW substrates, utilization of solubilized cellulose by F. succinogenes S85 and BL2 monocultures was higher than that of CW-xylose and hemicellulose, being 97–98%, 53–69% and 54–75%, respectively. Under scanning electron microscopic visualization SORG CW particles were less colonized by attached bacterial cells than T-SORG. In both substrates attached F. succinogenes cells were characterized by the appearance of protuberant structures on their surface topology.