Untreated and delignified cotton stalks as model substrates for degradation and utilization of cell-wall monosaccharide components by defined ruminal cellulolytic bacteria

Abstract

Pure cultures of the defined ruminal bacterial strains Fibrobacter succinogenes S85 and BL2, Ruminococcus flavefaciens FD1, Ruminococcus albus 7 and Butyrivibrio fibrisolvens D1, and the pair combinations of strains S85 + D1, BL2 + D1, FD1 + D1 and 7 + D1, were grown on isolated cell walls (CW) of untreated cotton stalks (CS) or ozone-treated CS (OCS) as the sole added carbohydrate substrate. F. succinogenes S85 was the best CW hydrolyzer of CS, solubilizing 27·9% of CW-glucose polymers, 21·9% of xylan, 24·8% of hemicellulose and 26·9% of total CS-CW polysaccharides. The Ruminococci monocultures and cocultures had very little hydrolytic effect on CS but somewhat better hydrolytic effect on OCS-CW. Since OCS-CW contained less lignin and hemicellulose components and more cellulose and total CW polysaccharides than CS-CW all bacterial cultures and cocultures degraded OCS-CW components better than CS-CW polysaccharides. S85 was the best hydrolyzer of OCS residual CW, solubilizing 52·8% of CW-glucose, 51·2% xylan, 51·5% hemicellulose and 52·6% of total OCS-CW monosaccharides. There was complementary action between B. fibrosolvens D1 and the F. succinogenes strains S85 and BL2 with respect to coculture growth and carbohydrate utilization, but not to the extent of CW hydrolysis, which was determined mainly by the F. succinogenes strains during the 120 h of coculture incubations. Utilization of solubilized cellulose, xylan, hemicellulose sugars and total carbohydrates by S85 monoculture was 94·4%, 80·3%, 61·4% and 84·1%, respectively, with respect to CS components, and 96·7%, 64·8%, 62·1% and 89·6% with respect to OCS-CW carbohydrates. Under scanning electron microscopy (SEM) visualization, S85 formed the most dense layer of bacterial cell mass attached to, and colonized on, OCS-CW stems. CW particles of CS were less crowded with attached bacteria. The cell-surface topology of S85 cells attached to CW particles was specified by a coat of characteristic protuberant structures.